Fish 53 on Migration and River Flow Results of tracking radio tagged salmon in six rivers in South West England LIBRARY COP1

RESEARCH & DEVELOPMENT PUBLICATION 4

ENVIRONMENT AGENCY Salmon Migration and River Flow Results of tracking radio tagged salmon in six rivers in South West England

David J. Solomon Fisheries Consultant Hugh T. Sambrook South West Water Kelvin J. Broad Environment Agency

Research Contractor: Dr David I Solomon

Publisher: Environment Agency, Rio House Waterside Drive, Aztec West Almondsbury Bristol BS32 4UD Commissioning organisations : Environment Agency South West Water Rio House Peninsula House Waterside Drive Rydon Lane Aztec West Exeter Almondsbury EX2 7HR Bristol BS32 4UD Tel: 01 392 446688 Tel: 01454 624 400 Fax: 01 392 434966 Fax: 01454 624 409

© Environment Agency and South West Water 1999

First Published 1999 ISBN 1 873160 88 7 All rights reserved. No part of the document may be reproduced, stored on a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the Environment Agency and South West Water.

The views expressed in this document are not necessarily those of the Environment Agency or of South West Water. Their officers, servants or agents accept no liability whatsoever for any loss or damage arising from the interpretation or use of the information, or reliance on views contained herein. Dissemination status Internal: Released to Regions External: Released to Public Domain Statement of use The information contained in this report is for use by Environment Agency staff, water undertakers and others involved in managing salmon fisheries and water resources. It will also be of value to anyone interested in the migration of salmon into and within rivers. Research contractor This document was produced under Environment Agency R&D Project W2-021 by:

David Solomon Foundry Farm Kiln Lane Redlynch Salisbury Wiltshire SP5 2HT Tel: 01725 512523 Fax: 01725 512964 Funding

Production of this report was jointly funded by South West Water and the Environment Agency.

Project Manager The Environment Agency's Project Manager for Research and Development Project W2-021 was: Dr Kelvin Broad - Environment Agency, South West Region

Cover photograph courtesy of David Peake

This document was produced by Dr Y C Solomon and Dr D ] Solomon

Printed by Hobbs the Printers, Totton, Hampshire.

R&D Publication 4 Foreword

Rivers in the South West of England have a high quality status and as such are extremely valuable in terms of both Atlantic salmon and water resources. In general, the management of water resources, through the promotion of new developments and refinement in the operation of existing arrangements, is likely to have potential for conflict to arise between fisheries interests and those of water resource managers. Constructive resolution of such issues is dependent on the use of reliable scientific information on which to base sound decisions, develop optimal solutions and implement sustainable operating rules. Ultimately, the regulator and users of the river have shared responsibilities, and, by working together environmental benefits should result.

This publication deals with an area of critical importance to both the Environment Agency and for water undertakers, such as South West Water. The relationship between salmon migration and river flow has been a subject of intense interest and debate for many years. A full understanding of how salmon respond to different river flows is essential for the proper and responsible management of water resources to ensure both environmental protection and sustainable resource development. The report describes the application of radio tracking techniques on six rivers and presents a new analytical methodology. Most importantly, it then translates the results into pragmatic solutions and guidelines for the future management of water resources. The approach, application and results are of direct relevance to specialists and non-specialists operating in areas of water resources and fisheries management.

The report is a unique opportunity for the Environment Agency and South West Water to disseminate information gained by the respective organisations and their predecessors over a twenty-year period. This partnership has its origin on the River Fowey, when in 1977 the first field trials of radio tracking of fish in Europe were undertaken by Dr David Solomon. The three authors have been involved with each of the investigations described in this report from the earliest inception stages. Their individual backgrounds and experience have been important contributory factors in the success of this study, which maintains an applied perspective throughout.

Overall, this publication presents new information and ideas that together represent a significant advance in our understanding of salmon migration and river flow. Its production represents a first- class example of collaboration between our respective organisations.

R. J. Baty Dr]. Pentreath Chief Executive Chief Scientist South West Water Environment Agency

R&D Publication 4 CONTENTS

Page

Executive summary 1

Key Words 4

Acknowledgements 4

1 Introduction 5

1.1 Background and terms of reference 5

1.2 Presentation of results 6

1.3 Topography and hydrology of the rivers studied 6

2 Development of methodology 9

2.1 Radio tracking of fish 9

2.2 Development of the methodology and the River Fowey Study 9

2.3 Details of equipment used 9

2.4 Approach taken in these studies 10

2.5 Experimental design and approaches to data analysis 11

2.6 The flow-frequency analysis method 14

3 The Hampshire Avon Study 18

3.1 Introduction 18

3.2 Hydrometrie background 20

3.3 Movements into the river 20

3.4 Summer movements within the river (primary phase of migration) 24

3.5 The spawning migration (secondary phase of migration) 27

3.6 Fish ascending the River Stour 29

4 The River Tamar Study 31

4.1 Introduction 31

4.2 The radio tracking programme 31

4.3 The fish counter 35

4.4 Analysis of angling catch records 36

4.5 Fish trapping atGunnislake 37

4.6 Distribution of spawning 38

5 The River Exe Study 39

5.1 Introduction 39

5.2 Hydrometrie background 39

5.3 General patterns of movement 40

5.4 Influence of flow on fish migration 43

5.5 Obstructions to passage upstream 47

5.6 Distribution of spawning salmon 49

R&D Publication 4 6 The Taw and Torridge Study 50

6.1 Introduction 50

6.2 River Taw 51

6.3 River Torridge 53

7 The River Tavy Study 55

7.1 Introduction 55

7.2 Hydrometrie background 56

7.3 Influence of flow on migration 56

8 Effects of river flow and other environmental variables on migration and survival 59

8.1 Introduction 59

8.2 Entry to the river 59

8.3 Changes in migration index upstream 64

8.4 Effect of spates 67

8.5 Effect of obstructions 67

8.6 State of tide 68

8.7 Time of day 69

8.8 Spawning distribution 71

9 Other îishery management issues 73

9.1 Introduction 73

9.2 Fish going elsewhere 73

9.3 Fish caught by anglers, and population estimates 73

9.4 Fish recaptured in subsequent years 75

9.5 Angling success and river flow 75

10 Migration models 76

10.1 Introduction 76

10.2 Avon migration model 76

10.3 Tamar migration model 78

11 Impact and management of abstraction 81

11.1 Introduction 81

11.2 Avon 81

11.3 Tamar 82

11.4 Exe 83

11.5 Taw 89

11.6 Tavy 90

12 Guidelines for planning and managing abstraction 95

12.1 Introduction 95

12.2 Migration flows and operating rules 95

R&D Publication 4 98 12.3 Location ot ihe abstraction 98 12.4 Diurnal and tidal modulation of abstraction 99 12.5 The way ahead 100 13 Guidelines for salmon tracking studies 100 13.1 introduction 100 13.2 Numbers of fish 101 1 3.3 Number of years and distribution of tagging effort 101 13.4 Source of fish 101 13.5 Handling and tagging fish 102 13.6 Siting of automatic listening stations (ALS) 103 13.7 On-foot and aircraft tracking 103 13.8 Quality control and reporting 104 13.9 Radio tracking and fish counters compared 105 B.lOTracking fish in coastal waters 107 14 References 108 Appendix 1 Glossary 109 Appendix H inputs to the Tamar salmon migration model

RbD Publication 4 EXECUTIVE SUMMARY

1. This report describes the results obtained from catchment varied between the studies depending five separate studies involving tracking of 1,830 upon the individual terms of reference. In most radio tagged adult salmon into and through six of the studies only radio tracking was used to rivers (Hampshire Avon, Exe, Tamar, Tavy, Taw, study movements, but on the Tamar results from Torridge) in South West England over a 10-year trapping and an electronic counter were also period (1985-1994). Of the total of 1,830 fish, used. Angling success was also examined to 1,514 (82.7%) were netted in the estuary and determine its usefulness as an indicator of the 316 were trapped at the tidal limit. pattern of timing of fish migration (Sections 3 - Environmental factors that affect movement, 7). especially river flow, were examined and the patterns of relationships in space and time are 5. The results of the five studies were analysed to compared and contrasted. This allows examine the effect of river flow on the pattern of examination of the impact upon salmon movements. Of the 1,514 salmon tagged in the movement of water resource management and estuaries, a total of 1,012 (66.8%) were recorded obstructions such as weirs and mills. It also having entered the tidal or non-tidal freshwater allows the derivation of general relationships reaches of rivers. The "not seen again" fraction, between flows and salmon movement that can and the numbers of fish recorded only from tidal assist in examining the potential impact of water reaches of rivers, were markedly higher at times resource schemes elsewhere. of low flow. It is concluded that many of the fish delayed in entering the river by low flows die 2. Details of the topography and hydrology of without entering the river at any time. Half or the study rivers are provided. The rivers involved more of the years run of fish can be lost in this ranged from a high base-flow groundwater-fed way in a drought year (Section 8.2). river (Avon) to small spate rivers (Torridge and Tavy), with an eight-fold range of catchment 6. The flows at which the migration index sizes. The rivers exhibit a range of water resource reached 1.0 at the tidal limit varied from close to management issues including abstraction, Q95 on the Avon to several times the Q95 flow impoundments, regulation releases, hydro­ on the smaller surface-water fed rivers. The electric power schemes, inter-river transfers and pattern of variation is discussed (Section 8.2). winter pumped storage schemes (Section 1). 7. Threshold flows (flow at which migration 3. The development of the methodology used is index = 1.0) for summertime movements described (Section 2). This includes both the increased in an upstream direction. On the Avon fieldwork methods and the techniques developed the increase over a 60 km reach was only for analysts. Full details are given of a powerful modest, reflecting the lowland nature and low analytical technique based on the cumulative gradient of the river. On the other rivers the frequencies of available flows and of flows threshold flows increased several fold (in terms of actually utilised by salmon for migration. This multiples of the appropriate Q95) over the length allows the derivation of a migration index for of river studied. These observations mean that each flow; an index of 1.0 means that salmon the impact of a water resource scheme may be migration is occurring at the rate that would be heavily dependent upon its location within the expected if flow were having no influence i.e. at river, even over a matter of just a few kilometres. an average rate. An index of less than 1.0 means It also means that observations from a fixed that salmon migration is under-represented at location (e.g. counter or trap) cannot be simply that flow, and an index in excess of 1.0 means interpreted for other locations in the river that migration is over-rep resented. (Section 8.3).

4. Details of the execution, results and analysis of 8. The differences in migration indices for fish each set of river investigations are provided. The arriving at, and passing, potential obstructions approach and extent of coverage of the allow a clear quantitative analysis of the degree of potential delay to migration at different Hows.

R&D Publication 4 This allows identification of the most serious abstraction) on the pattern and numbers of fish passage problems in the river, and the range of entering and ascending the river (Section 10). flows over which ameliorative action is required (Section 8.4). 15. The impact of existing licensed public water supply abstractions on each river is examined, 9. While there are definitive patterns of timing of and possible approaches to amelioration are arrival at the tidal limit with respect to state of considered (Section 11 ). tide, these are different on each of the rivers (Section 8.5). 16. On the Avon, the migration model is used to estimate the impact on salmon movement as 10. The pattern of time of day of arrival at the residual flows fall through the season. The tidal limit was very similar on all the rivers, with impact is greatest as residual flows fall through strong peaks around dawn and in the early hours the 8 - 9 m3/sec range, especially if this coincides of darkness, steady movement at night but much with the peak of salmon migration in June and less during the day. However, these patterns July. Approaches to flow sparing on a weekly or were much weaker, lost or changed further diurnal basis are examined (Section 11.2) upstream. This is likely to reflect the tendency for migration further upstream to be associated with 17. On the Tamar, the results of the tracking and higher flows, and possibly with an unwillingness other studies have already been used to set by salmon to ascend difficult falls or weirs in operating rules that effectively avoid any impact darkness (Section 8.6). of abstraction at Gunnislake (Section 11.3).

11. On the Avon, where low flows may extend 18. On the Exe, the combined effects of the full well into the winter, the pattern of spawning licensed abstractions at North Bridge (9.4 km distribution was severely truncated in drought upstream of the tidal limit) and Bolham (32.3 km years. On the other rivers, spates occurred upstream of the tidal limit) are calculated to have sufficiently reliably in the autumn for there to be an impact upon migration of salmon when little gross difference in spawning distribution residual flows to the sea are between 3 and 6 between years (Section 8.7). mVsec, and a greater impact at flows between 6 and 10 mVsec. This is largely a matter of 12. The tracking studies provided a range of location of the intakes; the impact of the same information of general fisheries management abstracted volume at the tidal limit would be interest including patterns of recaptures in other negligible. Viable options for amelioration are rivers, the extent to which river stocks mix in limited, the most realistic being correction of estuaries, tagged fish caught by anglers and flow-sensitive obstructions to movement at the tagged fish recaptured in subsequent years tidal limit and upstream. The winter abstraction (Section 9). at Exebridge for pump storage at Wimbleball Reservoir is considered to be of negligible impact 1 3. Comparison of analysis of salmon angling (Section 11.4). success with the results of tracking indicates that they present somewhat different patterns in 19. The current take from the Taw is small and is relation to flow, but with care the former can be considered to be having negligible impact upon used as a surrogate for the latter in some salmon movement (Section 11.5). situations (Section 9.6). 20. Although the PWS abstraction on the Tavy is 14. Spreadsheet models for salmon migration situated at the tidal limit, on the evidence were developed for the Avon and the Tamar. available it would appear to be having an impact These incorporate a generalised seasonal upon fish movement at residual flows between availability curve as well as the relationship 0.8 and 1.9 m3/sec. Raising the pf to 1.9 mVsec between river flow and fish movement. Such is examined but may not be viable in cost and models can be used to estimate the effect of any resource loss terms. Diurnal modulation and pattern of change in flows (e.g. due to spate sparing are worthy of further evaluation (Section 11.6).

R&D Publication 4 21. The implication of the findings with respect provides a reasonable picture in both space and to general guidelines for planning future water time. A combination of the two techniques is resource schemes is discussed. From the analysis particularly powerful (Section 1 3.9). of the impact of existing schemes on the study rivers, it is clear that there is scope for major 24. The main recommendation is for a project to abstractions to have impacts that range from undertake the analysis of all available data on negligible to significant. Factors covered are movements of salmon in relation to river flow for prescribed flows, protected flow "windows", England and Wales using the methodology protection of certain flow events ("spate developed in this study. This would develop a sparing"), location of abstractions, and diurnal clearer picture of patterns of behaviour in and tidal modulation of take (Section 12). different rivers with the eventual aim of contributing to a predictive model 22. The experience gained in these (Section 8.2.3). As an adjunct to this, it is investigations is distilled to derive guidelines for recommended that situations where appropriate future salmon tracking studies (Section 13). detailed rod catch data and independent observations on migration (e.g. trap, tracking, 23. The usefulness and validity of observations fish counter) are examined to explore the on fish movement made by radio tracking and potential for fishing success to be used as a counters are compared. Counters provide a very surrogate for fish migration observations in good picture in time for a specific point on a assessing the impact of water resource river, but the results cannot be extrapolated management (Section 9.5). elsewhere in the catchment. Radio tracking

R&D Publication 4 KEYWORDS

Salmon, migration, river flow, abstraction, radio diurnal, tide, water resources management, tracking, telemetry, fish counters, angling catch, fisheries management,

ACKNOWLEDGEMENTS

This investigation, based upon work undertaken of gratitude to all who helped, including over many years by several teams, has been colleagues in the Environment Agency and Water heavily dependent upon the help and co­ Companies and their predecessor bodies, and the operation of people and organisations too netsmen and riparian owners on the individual numerous to mention individually. However, the rivers involved in these studies. authors would like to record a considerable debt

R&D Publication 4 1 INTRODUCTION

1.1 Background and terms of To report and publish the results of 10 years of reference salmon radio tracking studies on six rivers in South West England, with an output of a single The purpose of this report is to bring together R&D publication and present the results and conclusions from studies involving radio tracking of over 1,800 SPECIFIC OBJECTIVES salmon in six rivers (Figure 1.1 ) in South West England over a ten-year period. The migratory 1. To present the results of salmon radio behaviour of the adult fish throughout their stay tracking on a range of rivers in a format in the river is described and analysed with respect which allows comparisons and contrasts to to environmental factors such as river flow. Also be made. covered is an examination of the impact of major licensed abstractions. In addition to presenting 2. To relate the patterns of movement to the the results of each study to interested parties, it hydrometric parameters of each river and allows a critical analysis of the similarities and each river reach. differences between behaviour patterns in the 3. To describe and present the analytical different rivers with respect to the influence of methodology developed in these studies and river flow on migration. This in turn facilitates demonstrate its value in establishing the synthesis of general principles and guidelines operating rules for abstractions and flow for operating rules for water resource schemes regulations. that have an influence upon river flow. It also allows presentation of the methodology for 4. To demonstrate how the methodology can design and analysis of tracking investigations that be used to evaluate the level of obstruction has evolved during the separate studies. represented by weirs etc. and the effectiveness of fish passes. The terms of reference for this report are:

OVERALL OBJECTIVE

Bristol Channel

Torridge

To mar

Figure 1.1 South West England showing the location of the study rivers.

R&D Publication 4 Each study of course had specific objectives. though the general interest and value of the Those for the Avon and Exe investigations were: investigation in terms of salmon biology and other fishery management issues have been To undertake a programme of radio-tracking recognised throughout. This report results from fieldwork to provide data to contribute to the desire of all parties involved to maximise the answers to the following questions: dissemination of the non-confidential aspects of these investigations and the conclusions that can a) What is the pattern, in space and time, of be drawn from them. movements of adult salmon from the estuary, into the river and throughout the river What is presented is a description of the methods system? used for fieldwork and for analysis of results, (Section 2), followed by a concise description of b) How is this pattern of movements influenced each study and its findings (Sections 3 - 7 ). by river flow? Then follow a comparison and synthesis of the c) What is the spawning distribution of salmon results of each study to derive general and what factors influence this? conclusions regarding the relationship between d) To what extent do existing weirs and river flow and salmon migration, the impacts of obstructions represent impediments to free water resource management, and guidelines for movement at different river flows and the future (Sections 8-1 3). seasons? 1.3 Topography and hydrology of e) What are the potential implications of future the rivers studied proposals for water resource development on the movement of fish? Detailed statistics for the six rivers are presented f) Is it possible to propose acceptable in Table 1.1. The six rivers represent a fair cross- modifications to existing structures, existing section of small to medium-sized British salmon abstractions and proposed abstractions which rivers, with varied catchment areas, rainfall would improve conditions for salmon patterns, topography and underlying geology. migration in a cost-effective way? The Avon derives most of its flow from groundwater sources, reflected in the high g) Is it possible to formulate tentative proposals baseflow index value of 0.89. Its upper reaches for future water resource management and tributaries are mainly chalkstreams, but it strategy to take account of the requirements does receive some surface run-off from its lower of migratory fish? catchment and tributaries, The objectives for the other studies were generally more specifically targeted at existing or The Exe receives some.groundwater input, but proposed water resource schemes, and hence the flow is predominantly derived from run-off were limited typically to addressing items a, b from the upper catchment on Exmoor and the and e in the above list. lower catchment on Devonian sandstones and the Carboniferous Culm Measures. The flow of 1.2 Presentation of results the other four rivers is predominantly surface run­ off. The Tamar drains a range of Carboniferous Detailed reports have been prepared for each slates, shales, grits and volcanics, with some study, but some of these are not publicly tributaries draining moorland. The Tavy derives available as they contain information of its flow mainly from Dartmoor granite, and the commercial confidentiality concerning individual Taw from both Dartmoor and Exmoor as well as fisheries and water resource management. In Culm shales and sandstones. The Torridge drains most cases the primary justification for the two distinct areas, Dartmoor Granite (the River studies was the evaluation of the actual and Okement) and Carboniferous shales and potential impact of water abstraction schemes, sandstones (main river).

R&D Publication 4 Table 1.1 Hydrometric characteristics of the rivers studied.

River Units Hampshire Exe Tavy Tamar Torridge Taw Notes Avon a) Tidal limit SX 157 933 SX 930 909 SX 474 650 SX 436 709 SS 477 216 SS 570 282 Catchment area km* 1,706 1,195 210 928 722 916 Naturalised ADF mVs 19.85 24.61 7.25 22.45 16.82 19.52 Naturalised Q95 mVs 6.97 3.45 1.21 1.77 0.98 1.21 b) Lowermost GS Knapp Mill Thorverton Ludbrook Gunnislake Torring ton Umberleigh Catchment area km3 1,706 601 197 917 663 826 Mean rainfall 812 1,270 1,577 1,241 1,172 1,155 Base Flow Index 0.89 0.59 0.46 0.47 0.39 0.42 Q100 mVs 2.490 0.332 0.440 0.581 0.192 0.202 Q95 mVs 6.206 1.779 1.144 1.783 0.922 1.138 Q90 mVs 7.500 2.291 1.437 2.390 1.328 1.642 Q85 mVs 8.168 2.894 1.728 3.055 1.721 2.110 Q80 8.877 3.576 2.049 3.892 2.217 2.753 Q75 m3/s 9.500 4.302 2.340 4.808 2.824 3.494 Q70 mVs 10.298 5.095 2.657 5.910 3.516 4.328 Q65 mJ/s 11.204 5.963 3.036 6.980 4.314 5.288 Q60 mVs 12.381 6.886 3.435 8.334 5.256 6.365 Q55 nWs 13.800 8.053 3.893 9.949 6.451 7.646 Q50 mVs 15.128 9.322 4.408 11.801 7.772 9.072 Q45 mVs 16.616 10.840 5.026 13.861 9.250 ! 10.797 Q40 mVs 18.500 12.631 5.741 16.439 10.970 12.909 Q35 m3/s 20.428 14.737 6.564 19.458 12.951 15.503 Q30 m3/s 23.000 17.401 7.641 23.046 15.570 18.722 Q25 mJ/s 26.000 20.589 8.738 27.596 19.070 23.043 Q20 mVs 29.000 24.944 10.243 33.575 23.701 28.664 Q15 mVs 33.364 30.614 12.369 42.360 30.365 36.613 Q10 m Ys 38.000 38.671 15.447 55.666 40.258 47.847 Q5 m Ys 44.123 52.519 20.798 80.363 58.502 66.376 Period years 1975 - 97 1957- 96 1957 -96 1957-96 1957-96 1957-96 Notes

1. Source - Environment Agency

2. Source - Calculated by SWW: Water Strategy Group using naturalised gauge records to December 1996 (except Avon, source EA)

3. Source - Hydrometric Register and Statistics, 1986-90, IOH

There are also a number of human influences on abstractions in the lower reaches as described in the six rivers that have an impact upon their flow Section 11.2. regimes. There are extensive PWS groundwater abstractions from the aquifer upstream of The Tamar has a major surface water PWS Salisbury on the Avon. The total licensed abstraction just upstream of the tidal limit at quantity is 1.65 mVsec (142.2 Ml/d) but Gunnislake (Section 11.3). At low flows this currently not all of this is taken; in 1990 the total abstraction is supported entirely by regulation take averaged 1.05 m Vsec (90.5 Ml/d). Some of releases from Roadford Reservoir, on the Lyd sub- this volume is returned to the river via STW but catchment. At such times flows between the much is exported from the catchment. In reservoir and Gunnislake are enhanced. addition, there are major surface-water PWS

R&D Publication 4 The Exe too has a major river-regulating reservoir, station; the water is discharged to the Tamar Wimbleball on the Haddeo (Section 11.4). This estuary. Operating rules protect low flows. supports major PWS abstractions at Bolham (32.3 Finally, abstraction and phased release of water km above the tidal limit) and North Bridge (9.4 for the Mary Tavy HEP station can result in km) above the tidal limit at times of low flow. diurnal pulsing of flows throughout the middle Reservoir releases also support transfers to the and lower reaches of the river. Taw catchment, the water being pumped from Exebridge (47.9 km upstream of the tidal limit). In addition there are other minor industrial and The reservoir is refilled naturally from its own agricultural abstractions on all the rivers, and catchment with additional winter pumped effluent discharges from STW complicate the flow storage from Exebridge. patterns.

The Taw experiences only a minimal PWS All the rivers studied have important stocks of abstraction at Newbridge, about 2 km above the salmon and sea trout supporting both rod and tidal limit (Section 11.5). At times of low flow net fisheries. In common with most other this take is supported entirely by an inter- salmon rivers, grilse (one-sea-winter) fish catchment transfer from the Exe (see above). No currently dominate the adult runs, entering the large-scale PWS abstractions currently take place rivers between June and August. However, all the from the Torridge. rivers also have fair numbers of two-sea-winter fish that generally run from March to June, On the Tavy there is a major PWS abstraction though they may enter the river in almost any from Lopwell, at the tidal limit (Section 11.6). month of the year. This current domination of This is operated conjunctively with other sources summer running grilse tends to increase the and no water is taken at times of low flow. There vulnerability of runs to low-flow conditions. is a major total-loss abstraction from the middle reaches of the river at Abbey Weir in Tavistock, via the Tavistock Canal for Morwellham HEP

R&D Publication 4 2 DEVELOPMENT OF METHODOLOGY

2.1 Radio tracking of fish estuary. Some of the later salmon results have been published in the scientific literature The principles of tracking fish fitted with radio (Solomon and Potter 1988, Potter 1988) and transmitting tags are reviewed by Stasko and some sea trout tracks are described by Sambrook Pincock (1977), Solomon (1982) and Solomon (1987). and Storeton West (1983). Its use is limited to freshwater (saline or brackish water is effectively The Fowey study saw considerable development opaque to radio frequency (rf) signals) and of the methodology for tracking migratory fish relatively shallow rivers; attenuation of rf signals is and of the equipment for its execution, including high even in fresh waters, especially those of high reliable tags, automatic listening stations (ALS) conductivity. However, if the tag is within a few and an approach to tracking in both salt and metres of the surface a detectable signal will fresh water using combined acoustic and radio emerge from the surface directly above it. As tags. This work also identified the clear attenuation is very low in air, detection of the difference between primary and secondary signal may then be possible from a considerable migration patterns (see Section 2.5), and formed distance, typically 500 - 1,000 m using the the sound base for planning and execution of the equipment deployed in these studies. Fish may studies described in this report. be fitted with miniature transmitters with a characteristic radio frequency and pulse 2.3 Details of equipment used repetition rate so that many individuals can be identified and tracked using a range of detection All transmitting tags and receiving equipment and recording equipment; up to 100 tags per used in these studies were designed and year were used in each river in these studies. developed by the MAFF Fisheries Laboratory at Details of the equipment used in these studies are Lowestoft (Solomon and Storeton West 1983) given in Section 2.3 below. and were manufactured or supplied by HS Electronics (tags) and Argus Electronics (tags and 2.2 Development of the receiving equipment) under licence from MAFF. methodology and the River Fowey Study The transmitting tags used were designated type SAL3 by MAFF (very similar to the Mk IIA Although tracking offish implanted with radio described by Solomon and Storeton West 1983) transmitting tags had been undertaken in North and were placed in the stomach of the fish. Until America for a number of years (Stasko and 1988 these operated within the band 104.70 to Pincock 1977), the method was not used in the 104.75 MHz, and from 1989 onwards on 173.0 UK until equipment was developed by the to 173.5 MHz. Ten frequency "channels" were Lowestoft Fisheries Laboratory of the Ministry of used at 5 kHz intervals, with up to 12 different Agriculture, Fisheries and Food (MAFF) in the late pulse rates (25 - 120 pulses per minute) allowing 1970's. This was undertaken to provide a up to 1 20 unique tag "codes" to be used in each method for investigating the migration of salmon river each year. Occasional drifting in radio and sea trout on the River Fowey in Cornwall, frequency (rf) or pulse rate caused some and is described by Solomon (1 982) and confusion but generally individual fish could be Solomon and Storeton West (1983). The first identified throughout their stay in freshwater. salmon to be radio tracked in the UK was tagged by two of the authors of this report (D Solomon On-foot and spot check tracking was undertaken and H Sambrook) in October 1978 on the River using a Yaesu FT290R transceiver, modified to Fowey near Restormel. Between 1978 and 1985 "receive-only" on the appropriate frequency about 40 salmon and over 100 sea trout were band, and a H-Adcock aerial of suitable radio tagged on the Fowey, mainly being dimensions for the radio frequency. Depending obtained by electric fishing, trapping near the upon local conditions and the depth at which fish tidal limit and later by seine-netting in the were lying in the water this arrangement gave a detection range from 100 m to over a kilometre.

R&D Publication 4 Using the directional characteristics of the aerial mud bank with minimum delay. Most fish were and variation in signal strength with distance caught by being encircled by the net and did not from the tag, the location of fish could generally become enmeshed in the net itself - any which be determined to within about 50 m or less. On did were usually rejected for tagging. As the net occasions this equipment was also used from a and the fish were drawn into shallow water and moving or stationary vehicle or from an aircraft. almost ashore, the fish was guided or scooped into a specially-constructed canvas bag Most of the results used in this analysis however containing clean water. Anaesthetic was then were obtained using automatic listening stations added to the bag to sedate the fish. The radio (ALS). These are based on the modified Yaesu tag was then inserted into the stomach via the FT290R receiver described above and "listen" to oesophagus using a plastic plunger system each rf channel at regular intervals (typically {Solomon and Storeton West 1983), an external every five minutes) for four seconds. If a signal is anchor tag applied to the fish's back, and the detected within that period, a further four-second fork-length of the fish measured with a metre period is sampled to validate the signal. A 12 rule. These operations were conducted without second period of signal is then recorded onto removing the fish from the anaesthetised water in magnetic tape (for further verification and later the canvas bag. Next the fish was carefully slid determination of pulse rate) and a paper print­ into a large polythene sack for weighing and out made of the date, time and radio frequency then quickly transferred into a second canvas bag channel. A simple bipolar aerial is used to limit containing clean water to recover from the the range of each station so that it effectively acts anaesthetic. Generally the fish was then as a "gate" recording passage time. Fast-moving transported a short distance from the nets for fish may be recorded just once or a few times for release to minimise the risk of immediate adjacent 5-minute periods, while other fish might recapture. In the Exe study each fish was kept for remain in the area for some time. up to three hours in a keep-box moored in the estuary until the netting session on that tide had 2.4 Approach taken in these studies finished, before they were released. The external "anchor" tags were used so that radio-tagged A basically similar methodology was used in all fish could be identified and reported if recaptured the studies considered here, with minor variations subsequently - the radio tag itself is visually due to logistic considerations or the specific aims undetectable unless the gut is dissected. of the individual project. The general approach was to purchase fish from the licensed nets in the A great deal of attention was paid to obtaining estuary of the river concerned, fit them with radio and releasing the fish with minimal damage and tags and release them nearby. In some cases trauma. Only undamaged fish or those with negotiations,were undertaken, with individual minimal.damage were selected from, the net netsmen to operate outside their normal fishing catch. All possible processing was undertaken periods or seasons or when their activities were with the fish still in water, including extraction otherwise curtailed by buy-backs or closures. In from the net, anaesthetising, tagging and the Tamar study, some fish were obtained by measuring. The fish were almost untouched by trapping at the tidal limit; most were released hand, and remained in water for all but a few nearby after tagging, while others were displaced seconds while they were being weighed in a about 1.8 km seawards. On all the rivers, most heavy-duty black polythene sack. fish were tagged and released between May and August. This period was selected because the All the net fisheries used to obtain fish took place majority of salmon first enter the estuaries of the in areas where the water was saline or brackish rivers concerned in these months, and this period under most conditions, and the tagged fish were also covers the most critical time for low flows. therefore undetectable immediately after release. Details are given for each river in Sections 3-7. Generally the next time they were recorded was arriving at the lowermost ALS (Section 2.3) Seine nets are used in all the fisheries employed situated near the saline limit near or a little to obtain fish for tagging, with the net being shot seawards of the tidal limit. On occasions, from a rowing boat and hauled onto a beach or searches downstream of the lowermost ALS were

R&D Publication 4 10 made at low tide using portable equipment, to environmental factors it is important to avoid any locate fish lying in the tidal reaches. bias with respect to these factors in the timing of tagging of fish. If all or most fish were tagged on The numbers of ALS deployed within the neap tides or at times of low flow, for example, catchment varied between studies and years, one might expect most of the tagged fish to from around six to 15 or more. They were approach or pass the tidal limit under such generally concentrated in the lowermost reaches conditions. It would be quite wrong to conclude where the greatest impact of water resource that stock as a whole behaved in this way. management was apparent, with sites more spread-out in the middle and upper reaches. The simplest way to avoid bias is to tag fish on a Details of the sites used in each study are given in large number of occasions throughout the the relevant sections later in this report. Most season, in numbers that are proportional to the stations were deployed from the time that the numbers of fish available at the time. As the net first tagged fish was released until about the end catch on any day is, generally speaking, of the calendar year, but in some situations proportional to the numbers of fish present in the equipment was moved upstream as the bulk of estuary the latter condition is readily applied; the stock migrated, "thinning out" the more fish are available for tagging when the concentration of equipment in the lowermost catches are large. Spreading the tagging sample reaches during the autumn. through the season can be achieved by mounting a fixed number of tagging trips each week, and The information most heavily used for the avoiding the temptation to target "peak weeks" analysis of patterns of migration in this study was to achieve a large number of tagged fish. In the time/date of the first valid record of each some cases some bias was inevitable, but care is tagged fish at each ALS. In some situations, such used in analysing results where this is thought to as where delay to migration was apparent e.g. at have occurred. The most obvious bias concerns a weir, the last record is also used as an indicator that part of the run which occurs outside the of the time spent in the area. netting season, or before tagging commenced (typically in May). It is stressed that the results Flow values for analysis of migration patterns strictly apply only to fish that arrive in the estuary were generally obtained from the records for during the period of tagging, but in most cases gauging stations (GS) maintained by the Agency this comprises the great majority of the run. In and its predecessors. In most situations the any event, it is the summer period which is most values from the nearest station were used, either vulnerable to low flows and the impact of the most recent 15-minute or hourly reading. In abstraction, so this bias is not inappropriate for some cases a time correction factor was applied if studies of this type. In the first table in each of the station was some distance away. On the Exe the descriptions of individual studies (Sections 3 - and Tavy, where the lowermost GS records were 7), the monthly totals of fish tagged and declared not appropriate for some of the furthest net catches for the years of the study are downstream sites because of tributaries or presented, which indicate that the catch was abstractions between the GS and the ALS site, an generally well represented. No such catch figures hourly or daily mean flow was derived by are available in the Taw/Torridge study as there calculation. Details of the approach used in each was a temporary closure of the net fishery at the study are given in Sections 3-7. time.

Tide times and heights for analysis of conditions As already discussed, most analysis was based prevailing during migration in tidal reaches, were upon the time of the first valid record of each obtained from published tide tables. tagged fish at each ALS. Early tracking studies often presented results as a series of "tracks" on a 2.5 Experimental design and time/distance graph, often with flow data approaches to data analysis superimposed; an example of such a presentation for some of the data is shown in Figure 2.1. This When undertaking a tracking study to investigate allows a visual examination of the pattern of the influence of flow, season, tide and other movement in relation to flow, but presentation of

R&D Publication 4 11 results based on large numbers of fish, statistical elevated flows during the summer are also analysis and derivation of flow/migration models effective at stimulating migration, this is mainly is difficult or impossible. Three different of fish fairly recently arrived in the river and thus approaches to presentation and analysis of results still in their primary phase of migration. The were therefore developed during these separation is not always clear-cut, especially in investigations. First, where sufficient results were years with high flows during August and obtained, a histogram of daily numbers of tagged September, but the difference is important as the fish arriving at each ALS through the season was relationship between movement and flow may be prepared. Results from a selection of key stations different; fish are more readily stimulated by can be presented in a single figure for each year, minor increases in flow during the secondary along with flow data. An example for the River phase of migration. In general the end of Tamar is presented in Figure 2.2. Others are September is regarded as the transition between presented in the descriptions of each study, but the two phases of migration. space precludes the inclusion of all 30 river-years involved. These presentations allow an The second approach to analysis of results was immediate appreciation of the overall pattern of used only for the Avon study as it is appropriate movement in space and time, and how, in only for rivers with a very stable flow regime. general terms, this pattern is related to river flow. Nearly all fish entering the river did so either within a matter of days of being tagged in the Examination of this type of figure indicates that estuary, or waited until elevated flows from migration generally occurs in two or more September onwards again stimulated migration. distinct phases. First, fish may enter the river Summer entrants in the Avon thus entered the within a few days of tagging and continue river at a very similar flow to when they were migrating upstream for a period of days or a tagged. It was therefore possible to identify week or two. This is referred to as the primary patterns of movement which were associated phase of migration and is generally followed by a with particular levels of flow, and to develop a period of quiescence. Second, there is a major simple model based on this. The development of movement of all fish in the autumn in response the methodology for the Avon is described in to elevated flows. This is referred to as the Section 10.2. secondary phase or spawning migration, While

Figure 2.1 Time-distance plots for four tagged fish on the River Exe in 1991, with dmf for Thorverton CS.

R&D Publication 4 12 Beals Mill (R. Inny) 19.7 km

Lifton (R. Lyd) 2^.6 krfi

Poison G.S. 28.8 km

Cfeystone 21 .9 km

d/s Endsleigh 1 3.8 km

Lamerhooe 7.05 km JJUM JJJL

Weir Cottage 4.95 km

10 -

Trap release Cunnislake 0.0 km 5 -

5 H ni Cunnisiake Weir 0.0 km

Impham meadow -1.0 km ji, un H n i H Net release estuary -9.7 to -22.7 km „n „ ,nfi n il n l-Aor 1-Mav 1-lun l-|ul 1-Aua l-Seo 1-Oct 1-Nov 1-Dec

Figure 2.2 Daily records of fish tagged and passage of tagged fish past recording stations in 1987. Also shown is the dmf for Cunnislake Gauging Station. Distances shown are from the tidal limit at Gunnislake Weir.

R&D Publication 4 13 This approach is not appropriate for surface- use of dmf values instead of instantaneous flows water-fed rivers due to short-term fluctuations in made little difference to the analyses. On the flow, where a fish may experience a many-fold Exe, no reliable measure of instantaneous flow in change in flow between being tagged and the lowermost river was available and daily mean entering the river. flows for the tidal limit were estimated from gauging results on the main river and lower The third approach allows a more thorough tributaries. These values were used for ALS sites statistical examination which in turn allows the in the lower 10 km of the river. While this potential impact of changes in flow regime to be doubtless blunted the analyses, it is likely to have quantified, and involves comparing the flows made little difference at lower flows, the most actually used by fish with those "available" critical part of the overall interpretation, as during the migration season. In addition to fish discharge changes only slowly at such times. tracking records, the method is equally well suited for analysis of data from fish counters and For each ALS site a list of flows "used" by fish (a catch statistics. The methodology is described in single value for the time of the first record of detail below. each fish) is prepared, and this is then sorted in rank order by flow magnitude, the lowest 2.6 The flow-frequency analysis through to the highest. Each instantaneous flow method value is then given a cumulative percentage figure; thus if there are, say, 100 values in the list, 2.6.1 The basis of the method the first and smallest would be 1 %, the second 2% and so on; the largest value would be given The basis of this approach is the construction and 100%. comparison of cumulative frequency curves of-. 2.6.2 Selection of periods for analysis • the flows prevailing when fish were recorded migrating ("fish flows") and An important consideration at this point is the selection of the periods for inclusion in the • flows prevailing throughout the period under analyses. Ideally one might consider months, for consideration ("all flows"). example, separately, or perhaps combine the same month each year together (e.g. all July's for The flow record used for each analysis is based the period of study). However, numbers of fish upon the nearest gauging station with observations in tracking studies are rarely appropriate records. Whenever possible adequate to allow such an approach, and it is instantaneous flow values are used, typically the usually necessary to combine the data from most recent 15 minute reading. If the station is longer periods. This raises potential problems if close, the figures are used without modification; the period includes significant time when there if it is more distant, the figures are modified to are few or no fish available to migrate. For take account of the time-lag for passage of flow example, including the months January to April events down the river. For the all-flows analysis in an analysis of a study when fish were tagged either the full record of 15-minute readings is only from May onwards is likely to introduce bias, used, or, if this is in excess of 10,000 readings, erroneously indicating that fish tended to avoid hourly readings. migrating on higher flows (which generally predominate in the early months of the year). In some situations, appropriate instantaneous The start of the annual period selected was readings are not available and daily mean flows therefore generally the first month in which have to be substituted. On the Avon, at the time reasonable numbers of fish were tagged. The of the fieldwork, consistent problems with the end of the selected annual period is more Knapp Mill GS meant that the instantaneous problematic, as fish may migrate upstream for record was incomplete. Fortunately, the nature many months after tagging ceases. On the Avon, of the river (high baseflow index and thus very where the range of flows was relatively much less consistent flows) meant that a reliable series of than on the other rivers the period selected daily mean flows could be constructed, and that covered all months when more than the odd fish

R&D Publication 4 14 migrated; typically April to November in the lower reaches. For the lower reaches of the other / rivers the selected period covered mainly that considered to represent the primary migration

period (see Section 2.5), typically May to — liih- flows September. The analyses presented here for the ET c so — atuiowi Tamar, Taw/Torridge and Tavy are limited to the n = 36 lower reaches, and a single annual period is used for each river. On the Exe, however, the analyses include ALS sites throughout the river, and for the more upstream sites a later annual period is used (June to November at the uppermost sites) to reflect the later pattern of migration there. Although limiting the annual period in this way means that some data are not used in the 2.3b flow/frequency analysis, it does largely remove a potential bias.

2.6.3 An example of the method. f 40

To illustrate this approach, the results for the station situated at Ludbrook C.S., 3.25 km upstream of the tidal limit on the Tavy, are shown in Figure 2.3. Figure 2.3(a) shows the cumulative frequency % of all-flows and flows used by fish. The all-flows line is fairly smooth as it is based upon over 7,000 data points (two-

hourly values for four months in each of five Figure 2.3 Analytical methodology - salmon migration at Ludbrook on the Tavy. years) The fish-flow line is more stepped, as the a) Cumulative frequency of daily mean flows (all-flows) and of instantaneous flows used by fish (fish-flows), b) Difference between cumulative frequencies of all-flows number of observations is small (36 fish) and and fish-flows. Ludbrook CS. flows used. each fish record therefore represents an increment of about 2.8%. The difference lesser rate than expected, there will also be a flow between the two lines is clear, however, with the range where they do so at a higher rate, as both fish-flow line running well below the all-flow line. "fish flow" and "all flow" lines are bound to If fish passage occurred without reference to river reach 100%. flow, the two lines would tend to be identical; for example, if river flows were below 3 mVsec for 2.6.4 Derivation of the migration index 70% of the time, then one would expect that about 70% of fish passage events would occur at The next stage of the analysis used here involves such flows. Any real difference between the lines consideration of the slopes of the all-flow and indicates that flow does have an impact upon fish fish-flow lines, as indices of the rate at which passage. In Figure 2.3(b) the difference between each flow occurs and the rate at which fish the two lines is shown, peaking at around 3 migration occurs at such flows. In order to mVsec. This means that at flows below that obtain values for the instantaneous slope of the level, available flows are accumulating at a lines it is necessary both to smooth the steps and greater rate than are fish passage events, i.e. fish to derive a formula for the curve so derived. This are tending not to use low flows for migration at was done using a least-squares polynomial this location. Fish passage events accumulate at regression analysis, generally sixth order. The a greater rate than flow availability between fitted lines for all-flows and fish-flows at Ludbrook about 3 and 25 m'/sec. It is of course a truism are shown in Figure 2.3(c) and Figure 2.3(d) that, in any one period when there is part of the along with the formulae for the lines. It was flow range where fish are migrating at a necessary to use two polynomial lines to obtain a close fit for the all-flows data, with a switch-over

R&D Publication 4 15 values are involved, the complex shape of one or both lines precludes reliable curve fitting, and the quality control check shows an anomaly. On the , - -LirXZW-Ob''- L9J6î(ÔE**t*.i.!J!ilSE-0J«* • i.iOMtWF-01« -2.SIMIOE-O0v,-2.0S6ÛMEHili < :jW|1>li-01 lower Tamar, for example the shape of the "all- R* - « iSSIOÎiMIl (4 3ft - 30.0 mWi flows" line was complicated in the critical area of low flows by the pattern of abstraction and no suitable line fit could be achieved. --î,7bs2j:i;«3.' • I.JOWIOË^II' - iowfjTE-oo.' - i.ttimjüi'.'Oi• • ip.WiliJF.I».1 ' t,J3t>S<»Ë'02i - 7.M50771 i-ûl K'-o.9742]ltlMll 10 • 4 2! m'i'SKl A series of values of instantaneous slope of each line is then determined by differentiating the polynomial regression equations and calculating the slope at 0.01 m3/sec intervals. If the flow m Vi« instantaneous slope of the fish-flows line is divided by the instantaneous slope of the all- flows line for each flow increment, an "index of 2.3d fish migration" is derived. A value of 1.0 indicates that the slopes are the same and thus all-flows and fish-flows are accumulating at the same percentage rate at that flow value. A value ÏT - SO of less than 1.0 indicates that fish migration is

- under-represented, and a value in excess of 1.0 ï-J-WJJftE^.' • 3.«t7J)i.

, .:.:iii33E-oi<' • t.:its*)E-oi.-;j '«3üE-oi indicates that fish migration is over-represented. The result of doing this for the Ludbrook data is shown in Figure 2.3(f). A number of points are worthy of consideration. First, up to a flow of 10 IS ÎO flowmVlec about 2.2 mVsec the migration index is zero; no fish were recorded at such flows, even though flows below this level occurred for more than 50% of the time. Second, between about 2.2 mVsec and 2.69 m Vsec the index is below 1 ; this corresponds to the rising limb of the curves in Figure 2.3(b) Figure 2.3(e). From 2.69 mVsec to 27.7 mVsec the index is in excess of 1. The narrow range of low flows for which the index is less than 1 (0 - 2.69 mVsec) compared to the wide range of flows for which it is in excess of Î (2.69 - 27.7 mVsec) is perhaps surprising when one considers that, over the period as a whole, the index must by definition average 1. The How m1 lac explanation of course lies in the length of time for which these flow ranges prevail; for about Figure 2.3 continued. Analytical methodology, c) Fitting a polynomial line to the all- 68% of the time the flow was less than 2.69 flows data, d) Fitting a polynomial line to the fish flows data, e) Comparison between the "difference" Unes using the raw data and the fitted curves. m Vsec, whereas it iay between 2.69 m Vsec and 27.7 m Vsec for only 32% of the time.

at 4.25 m Vsec. As a quality control check, in the 2.6.5 Examining the impact of Figure 2.3(e) the difference between the abstraction calculated lines is plotted superimposed on the "difference" line derived from the raw data; a We can now consider directly the impact on close fit, such as is apparent here, indicates that migration of a reduction in flow due to the line-smoothing process has not significantly abstraction, by comparing the migration index changed any critical factors of the relationships. for each natural flow with the migration index for In some cases, especially where low numbers of the equivalent residual flow left after abstraction.

R&D Publication 4 16 In Figure 2.3(g) the migration index at Ludbrook conditions are still considered to be having a is shown, with and without the influence of the stimulating effect upon movement. full licensed abstraction at Abbey Weir (see Section 11.6). As an example, consider the This type of analysis makes the assumption that impact of the full abstraction of 1.23 m Vsec, the relationship between flow and migration is upon movement of fish at a natural flow of driven directly by the value of discharge perse. 5.5 m Vsec. From Figure 2.3g we can see that In practice, at many sites at least part of the effect the migration index for a flow of 5.5 m Vsec is is likely to be due to factors associated with 2.1 6, and for 4.27 mVsec (the residual flow) it is discharge, such as turbidity and other physico- 1.6. Thus this model suggests that the number chemical parameters, or rate of change in of fish migrating would be reduced to 74% of discharge. These will be less affected by that which would have run without the abstraction than is the absolute level of abstraction, at any time that this particular discharge. The calculated impact of abstraction natural flow occurs. on fish migration produced by this methodology should therefore be considered as a maximum It must be stressed that impact of reduced flows value. Migration past weirs and obstructions is in most cases represents only a delay in migration likely to be truly flow-dependent to a greater and not a loss in terms of stocks of fish. extent than movement in the open river. However, there are two distinct situations where delays can have significant impacts upon stock wellbeing. First, fish delayed reaching and passing the tidal limit may experience an increased mortality compared to fish which achieve prompt passage past the tidal limit. Second, fish delayed near spawning time may not have the opportunity to reach the optimal spawning grounds. These important aspects are discussed in detail later in this report. Further, delay in passing particular points means that the pattern of distribution of fish during the angling season will be modified, to the detriment of upstream fisheries and perhaps to the benefit of those immediately downstream.

In considering the impact of abstraction it is appropriate to consider whether the residual-flow migration index value lies below or above a value of 1. It will be recalled that, at a value of 1.0, fish are arriving at the "expected" rate i.e. flow is having an entirely neutral effect upon movement. At a value of less than one, flow is having a negative impact on migration, and when the value is in excess of 1.0 it is having a stimulating effect.

For the purposes of this investigation an adverse impact is assumed if the abstraction is associated Figure 2.3 continued. Analytical methodology. 0 Migration index derived from the with a reduction in the migration index with a slopes of the fitted lines, g) Migration indices with and without the Abbey Weir abstraction. resulting value of less than 1.0. A reduction in migration index of (say) 3.0 down to 2.0 is not considered as an adverse impact, as the resulting

R&D Publication 4 17 3 THE HAMPSHIRE AVON STUDY

3.1 Introduction purchased from commercial netsmen operating in the narrow channel (The Run) at the harbour This work was undertaken by Dr D Solomon as an mouth at Mudeford and were released independent fisheries consultant under contract immediately after tagging a few hundred metres to the Wessex Water Authority and subsequently upstream. The monthly distribution of fish the NRA. The need was stimulated by water tagged throughout the study is shown in Table resource development (the Blashford Lakes 3.1. Also shown is the monthly distribution of Scheme) and a realisation that the level of the declared net catch. understanding of the relationship between salmon migration and flow in the river was About forty of the fish were tagged with inadequate to assess the potential impact of both combined acoustic and radio tags (CARTs) to this and any future resource development allow tracking within the saline and brackish proposal. estuary as well as in the river (Solomon and Potter, 1988). The results of the estuary tracking The work was undertaken over a period of five have been published elsewhere (Potter, Solomon years (1986 - 1990) during which 437 salmon and Buckley 1992) and only the radio tracking were tagged and released at the mouth of results of the fish entering and ascending the Christchurch Harbour, which forms the estuary of river are used here. the Avon and Stour (Figure 3.1 ). The fish were

River Avon Knapp Mill 65 ALS (1.4 km) x

Tidal limit KnCpPWi

If ord ALS

River Stour

Netting site - The Run

Figure 3.1 Lowermost reaches of the Rivers Avon and Stour showing location of the tagging site and automatic listening stations.

R&D Publication 4 1.4 km & 0.0 km Iford X^-v^/v-l.Z km Christchurch Harbour

Figure 3.2 Middle and lower reaches of the Rivers Avon and Stour and their tributaries showing location of automatic listening stations, with distances from the tidal limit.

R&D Publication 4 19 Table 3.1 Monthly distribution of tagged fish and ALS records.

Net Fish Tidal limit Knapp Mill Causeway Ringwood F'bridge Breamore Salisbury catch tagged CS

Distance km -4.6 -4.6 0.0 1.4 9.1 20.2 33.4 41.6 60.4

Years 1986-90 1986-90 1986-90 1986-90 1986-90 1986-90 1986-90 1986-90 1986-90 no. tagged 437 437 437 437 437 437 437 437

March 10 0 0 0 0 0 0 0

April 49 10 4 1 2 0 0 0

May 202 41 38 13 11 5 2 1 0

June 693 179 76 66 50 23 5 2 0 July 1,460 205 83 63 56 29 5 6 0 August 0 2 8 6 2 1 1 2 0

September 0 0 2 3 5 3 2 2 0

October 0 0 33 46 47 52 44 44 16

November 0 0 12 13 18 17 13 19 12

December 0 0 2 4 5 7 9 12 2

Total 2,414 437 262 218 195 139 81 88 30

Many sites were used for ALS deployment at tracking period 1986 and 1987 flows were close different times to examine specific aspects of fish to or a little above average. Flows during 1 988 behaviour. Long-term sites used for the analyses were below average for much of the summer. in this report are indicated in Figure 3.1 and 3.2. From June onwards in 1989 flows were well The monthly pattern of tagged fish recorded below average, while flows in 1 990 were very low passing strategic ALS sites is indicated in Table indeed from April onwards, with levels in August 3.1. being the lowest since 1976.

3.2 Hydrometric background 3.3 Movements into the river

As is apparent from the hydrometric statistics 3.3.1 General description provided in Table 1.1, the Avon has a very high baseflow relative to the ADF due to the fact that Of the 437 fish tagged, 296 (67.7%) were its flowis predominantly derived from recorded entering-the Avon and 14 (3.2%) the groundwater springs rather than surface run-off. non-tidal Stour. The fate of the other 127 fish is Run-off following heavy rainfall does of course discussed in Sections 3.3 and 8.2. influence the river flow, especially in the winter, but throughout most of the summer and early Entry to the river generally occurred within 10 autumn the river flow gradually falls as days of tagging, most commonly within 3 days. groundwater levels decline (see for example the However, a small number of fish (29, hydrographs in Figures 3.3 and 3.4). This pattern representing 9.8% of fish recorded entering the contrasts with that on the other rivers covered by river) spent from 3 to 4 months in the harbour this report (for example see Figures 5.3 - 5.4 for area (or out at sea) before entering the river in the River Exe). the autumn. This pattern of behaviour was associated with low river flows at the time of

The monthly mean flows (Knapp Mill GS) tagging, and is discussed further in Section 3.3.2. throughout the study period are shown in Figure 3.5, with the long-term average flow The influence of state of tide and time of day on superimposed. It is clear that the period covered movement into the river is discussed with the a range of flow conditions. For much of the results for other rivers in Sections 8.6 and 8.7.

R&D Publication 4 20 Wilton 66.7 km

5 -

Salisbury 60.4 km i ,,,11, 5

Bream ore 4 1.6 km 0 -

1-Apr 1-May 1-Jun 1 -Jul -Aug 1-Sep 1-Oct 1-Nov l-Dec

5 - Tidal limit 0.0 km n J.njinJ.j LkA

5 - Fish tagged -4.6 km o unLn 1 - Apr 1-Mav 1 -lun 1 - lui 1 - Aua 1 - Sep 1 -Oct 1 -N ov 1-Dec

Figure 3.3 Daily records of fish tagged and passage of tagged fish past recording stations on the Avon in 1987.

Also shown is the dmf for Knapp Mill Gauging Station. Distances shown are from the tidal limit.

R&D Publication 4 21 W ilton 66.7 km

Salisbury 60.4 km

ifeamore 41.6 km

F o rd in g b rid g e 33.4 km

1-Apr l-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct 1-Nov l-Dec

« - TidallimitO.Okm nMn nnnnnn m n un

10 -

Fish tagged -4,6 km o 1 -Aor 1 -M av 1 -|un 1 -lui 1 -Aua -Sen 1 -Oct 1-Nov 1 -Dec

Figure 3.4 Daily records of fish tagged and passage of tagged fish past recording stations on the Avon in 1990. Also shown is the dmf for Knapp Mill Gauging Station (out of action from late November). Distances shown are from the tidal limit.

R&D Publication 4 22 3.3.2 The influence of river discharge

The influence of river flow on the likelihood and timing of entry to the river is indicated in Figure 3.6. The flow figure used here is the residual flow, being the flow gauged at Knapp Mill less the abstraction for Fawley Refinery and domestic supply by the West Hants Water Company just downstream of the gauging station. This figure is considered to be the freshwater flow at the tidal limit.

At residual flows above 9 mVsec, most tagged fish entered the river within 10 days. Of the 281 fish tagged at such flows, 214 (76.2%) entered Figure 3.5 Monthly mean flows for Knapp Mill CS (River Avon) 1986- 1990 with long term average. the Avon within this time. A further 6 (2.1 %) entered the river on elevated flows later in the year (October/November). • Failed to enter

DEntered river in Autumn Although there are fluctuations in the proportion O Entered river within 10 days of tagged fish entering the river at flows above 9 m Vsec, no clear trends are apparent (Figure 3.6). However, at residual flows below 9m3/sec the z1 ^ proportion of fish entering the river within 10 3rj days of tagging fell away sharply. Increased 20 numbers entered the river in the autumn, but this 10 increase did not make up for the decline in immediate entrants, and the total river entry 0-1 J.i 5-6 D-8 S-y 9-10 10-11 M-IJ IM3 I J-M U-Lû «-20 ÎOJO -IB- Residual flow m3/sec proportion fell. Of the 156 fish tagged at flows below 9mVsec, 55 (35.3%) entered the river within ten days. A further 23 fish (14.7%) entered the river in the autumn. Thus a total of 50% of the fish tagged at flows below 9 m'/sec eventually entered the river, compared to 78.3% tagged at higher flows. At a residual flow of less than 5 m Vsec, of the 25 fish tagged only 6 (24%) entered the Avon at any time. • Failed to enier • Entered river in Autumn It is assumed that the "disappearance" rate for • Entered river within 10 days fish tagged above 9mVsec represents the rate at .•.•.•.EJ.nn 0-1 4-5 S-6 6-ï S-1) 0-10 10-11 11-12 12-13 13-14 14-16 |(**0 2D.40 4D- which tags fail, fish die as a result of capture and Residual (low m'/set handling, and fish go to rivers other than the Avon. As there is no obvious reason why these rates should increase at low river discharges, it is Figure 3.6 Influence of residual flow at time of tagging upon entry to the River Avon (passage past Island ALS), All years combined. Upper - reasonable to suggest that the increased loss rate numbers. Lower - %. at lower discharges is due to mortality of fish 30.7% respectively. To put it another way, over while in the harbour or at sea between tagging a third of Avon fish entering Christchurch and river entry, due to factors other than tagging Harbour at residual freshwater flows of less than 9 and handling. Thus if we propose that all mVsec fail to enter the river, and over two thirds untagged Avon fish approaching the river at a at flows of less than 5m3/sec. This phenomenon residual flow in excess of 9 mVsec survive to of "lost" fish at low flows is discussed further in enter the river, the equivalent figures for flows Section 8.2.2. less than 9 m Vsec and 5 m Vsec are 63.9% and

R&D Publication 4 23 As flows generally fall steadily through the Fish homing to other rivers e.g. the Test and the summer, it might be considered that the gradual Frome, are to some extent "off course", though change in patterns of fish behaviour could be a tagging studies are increasingly indicating that seasonal phenomenon rather than being directly such "searching and overshooting" behaviour is affected by river discharge. However, the few widespread and does not indicate that the fish occasions on which sufficient rainfall occurred in are necessarily lost or confused. These fish are the summer months to significantly increase discussed further in Section 8.2. discharge indicated that direct influence of river flow perse. Fish behaviour observed at such At times, a high proportion of the fish tagged at times was consistent with that expected at the Mudeford entered the tidal reaches of the Stour, discharge prevailing, with faster estuary passage often penetrating as far as the tidal limit at Iford times and greater penetration of the river than Bridge, about 3.5 km from the confluence with fish tagged, for example, during the preceding the Avon estuary at Clay Pool (Figure 3.1 ). Many weeks at lower discharges. of these fish subsequently returned downstream and entered the Avon, or failed to ascend either 3.3.3 Fish entering other rivers river (see Sections 3.3.1 and 3.3.2). Some fish ascended the freshwater Stour, however, and Fish returning to rivers other than the Avon, were recorded by the ALS network there or by which are caught and tagged at Mudeford, are of spot checks. These fish are discussed in Section two distinct groups. First, fish homing to the 3.6. Stour are "on course" as the river shares its lower estuary with the Avon. 3.4 Summer movements within the river (primary phase of migration) • u/s Knapp Mill • Lower river • Tidal river 3.4.1 Baseflow movements

In this section, the movements within the river of the fish entering the river within 10 days of tagging are considered. Almost no fish entered the river more than 10 days after tagging except in the autumn in response to increased discharge.

Fish entering the river in the summer generally 9-10 10-11 WAX 12-1.1 IJ-M 14-16 16-20 20-10 JO- Residual flow m3/jec continued their migration for up to three weeks or so on a discontinuous basis, before laying-up for several months until the autumn. This pattern 100 is apparent from the almost total lack of ALS 90 • u/s Knapp Mil records of migrating fish between mid-August so • Lower river and mid-October in Figures 3.3 and 3.4. Some 70 • Tidal river fish remained in tidal water within the river, while 60 c the primary migration phase (the three weeks or S 50 so) of more active fish took some of them 70 km a!

40 or more upstream.

30

20 At residual flows in excess of 20 mVsec, most

10 fish, having entered the river very shortly after

0 S-9 9-10 10-11 11-12 12-13 13-14 L4-I6 16-20 20^0 40- tagging, continued on upstream beyond Knapp Residual flow m'/sec Mill (i.e. the abstraction point). As flows fell, however, an increasing proportion laid-up between the Knapp Mill weirs (1.1 km upstream Figure 3.7 Influence of residual flow at time of tagging upon summer destination of fish entering the River Avon before the autumn, all years combined. Upper - of the tidal limit) and Island ALS (-1.2 km); at numbers. Lower - % 9 m Vsec, about half the fish entering the river

R&D Publication 4 24 remained in this zone until autumn (Figure 3.7). Table 3.2 Months included and flows used in analyses. At residual flows below 8 m Vsec, only one of the 21 fish entering the river proceeded beyond Station Km Period covered Flows used Knapp Mill before the autumn. As the by analysis proportion of fish remaining downstream of Tidal limit 0.0 April - Nov Residual dmf Knapp Mill increased, there was also an Knapp Mill GS 1.4 April - Nov Residual dmf increasing tendency for fish to remain Causeway 9.1 May - Nov Knapp Mill gauged dmf downstream of the tidal limit. From Figure 3.7 it Ringwood 20.2 May - Nov Knapp Mill gauged dmf can be seen that, at flows below 8 m Vsec, about Fordingbridge 33.4 |une - Dec E. Mills dmf three-quarters of fish entering the river remained until the autumn in the 1.2 km reach between Breamore 41.6 June - Dec E. Mills dmf the tidal limit and the Island ALS. Salisbury 60.4 |une - Dec E. Mills dmf

As already described, virtually no fish ascended beyond Knapp Mill abstraction point at residual flows below 8 m Vsec. In Figure 3.8, the summer • u/s Bickton destination (= laying-up location) of fish • Ringwood to Bickton ascending beyond Knapp Mill is indicated. It • Knapp Mill to Ringwood appears that the distance travelled upstream by those fish ascending beyond Knapp Mill is largely Ï I* unaffected by river flow within the range that fish E 1 10 utilise to enter this zone of the river. It should be noted that Knapp Mill gauged flow is used in this m figure as this is upstream of the abstraction point.

T-V 9-11) 10.11 n-12 12-13 13-14 14.1S 15-17 17-ÏI 21-10 40* The migration indices for each ALS from the tidal Gauged flow m3/sec limit to Salisbury were determined using cumulative frequency methodology described in Section 2.5. The months and flow figures used in these analyses are shown in Table 3.2, and the results are shown in Figure 3.9.

It can be seen that the first fish pass the Knapp Mill CS ALS at a higher flow than those used to reach the tidal limit, but their migration indices • u/s Bickton reach 1.0 at about the same flow. The remaining • Ringwood to Bickton • Knapp Mill to Ringwood indices are basically rather similar, indicating that flow has little influence on the distance migrated upstream. This is consistent with the conclusion O-S 5-6 &-7 7-9 9-10 10-11 11-13 12-13 13.14 14-13 15-17 17-21 21-10 40- already drawn, and contrasts markedly with the Gauged flow m Vsec situation on other rivers studied. This [s discussed further in Section 8.3. Figure 3.8 Influence of gauged flow at time of tagging upon summer destinations of fish migrating beyond the Knapp Mill ALS on the River Avon, all years combined. Upper - numbers, tower - % . These observations are used to develop a simple model linking discharge and migration in the 3.4.2 The influence of spates lower river in Section 10.2. On four occasions during the five-year study, The influence of time of day on passage up river once in each of the first four years, enough is discussed in Section 8.7. rainfall occurred in a short period at a time of otherwise low flows to have a significant effect upon river flow.

R&O Publication 4 25 100 12S 150 250 Flow as % of Q95

Figure 3.9. Migration indices (or ALS stations on [he Avon.

As the effects on fish were somewhat different it Over 25 mm of rain was recorded at Salisbury on is useful to consider each in some detail. The July 1 7 - 18 1987. The 09.00 hour flow reading events here described are all during the period at Knapp Mill rose from 9.5 mVsec on July 17 to July to early September; the effects of heavy 19.3 mVsec on July 19, staying above 10 mVsec rainfall in October are described in Section 3.5. for a further week. Although no fish that had been in the river for more than a week or so were On August 25 1986, a total of 46.4 mm of rain stimulated to move, the increased discharge had was recorded at Salisbury. This led to increased exactly the effect of a higher baseflow on fish discharge throughout the catchment, The single entering the river at the time. There was thus a 09.00 hours reading at Knapp Mill rose from greatly reduced tendency for new entrants to lie- 9.0 mVsec on the 25th to 23.9 mVsec on the up in the tidal and lowermost non-tidal reaches 27th. As this event took place more than three of the river, and an increased tendency for fish to weeks after the last fish was tagged, none were migrate for some distance up river. still in the "primary migration" phase of migration described in Sections 2.5 and 3.4.1, and there Regrettably, due to equipment failure, neither had been virtually no activity recorded at any of rainfall records and flow records are available in the ALS during the previous two weeks. detail for a rainfall "event" on August 30 - However, five fish which had been "laid up" for September 1 1988. On the Frome, a river with some time were stimulated to migrate upstream • broadly similar characteristics to the Avon, the by this event. Three of these fish were flow was doubled. As no fish had been tagged downstream of the Knapp Mill abstraction point for over a month, all had finished their initial and migrated between 6.5 and 20.0 km over the migration stage. Five "laid up" fish were next two days. The other two stimulated fish stimulated to migrate, however. Four moved were downstream of Ringwood, but as their upstream, having been located at approximately exact starting points were not known the -0.6, 4.4, 25.4 and 25.5 km from the tidal limit. distances migrated cannot be calculated. The The fifth fish, located near the tidal limit, five fish represented a wide range of tagging returned seawards and was recorded ascending da*es from June 25 to July 30, indicating that the the River Piddle. The fish had been tagged latter half of the tagged population was well between April 20 and July 26, a fairly represented but not the first half. representative sample of the tagged population.

R&D Publication 4 26 A series of thunderstorms on the evening of July 6 pattern of behaviour therefore appears to be 1989 resulted in rainfall of the order of 25 mm linked to the physiological state of the fish, and over much of the catchment. Unfortunately, may be considered a precursor of the spawning both the Salisbury rain gauge and the Knapp Mill migration phase described in Section 3.5. flow gauge again were not operational at the time, so the information is incomplete. The dmf 3.5 The spawning migration estimated at Knapp Mill rose from 9.6 m Vsec on (secondary phase of migration) july 5 (i.e. residual flow about 8.6) to 10.8 m Vsec (residual flow 9.8) on july 7, but it is suggested 3.5.1 General description that the real change may have been greater. The flow before this event was falling through the In each of the five years studied, rainfall during critical zone for fish migration (see Section 3.2.2) October leading to a significant increase in and numbers passing Knapp Mill were falling. discharge triggered a major movement of fish The storms kept the residual flow above 9 mVsec throughout the river, which then continued on for several days, and eleven tagged fish passed and off until the fish spawned at the end of the the Knapp Mill ALS between july 6 and July 10. year. The general level of activity through this All these fish had been tagged within the period, and the distribution of spawning was previous 17 days and are therefore considered to considerably influenced by discharge, with long have been still in the primary migration phase. dry periods leading to reversion almost to the These eleven fish represented 44% of the 25 quiescent state that predominated in August and tagged fish passing Knapp Mill before October in September. that year. Flows fell again quickly, and only one more tagged fish passed this point during the In 1986, many fish were stimulated to move by summer in spite of 41 being tagged between july an elevation in gauged flow from 8.5 mVsec to 10 and 31. This highlights the tremendous 1 3 m Vsec in the latter half of October. Some importance of spates at times of otherwise low were stimulated very early on the rising flow. hydrograph and indeed some fish in the lower reaches started to become "restless" before the Summer spates thus appear to affect fish river level rose. This supports the suggestion in differently according to the timing of the event Section 3.4.3 that increasing physiological and the length of time fish have spent in the readiness towards spawning time means that fish river. First, they operate in the manner of raised respond more positively and to smaller flow baseflow in influencing the pattern of events than in earlier months. movements of fish still in their primary phase of migration, i.e. within about three weeks of Activity fell away somewhat in late October as entering the river. The July flow events described flows fell, but increased again on much greater above operated in exactly this manner. flows in mid-November. This resulted in effective dispersion of fish to the general spawning areas, Second, the increased flow may stimulate with little long-distance movement being movement of fish that had been in the river for recorded after mid-November despite high flows some time, and were thus considered to have prevailing. ended their primary phase of migration. The Elevated discharges occurred earlier in October in timing of the event appears to have a bearing 1987, with much of the spawning dispersion here; while events in late August stimulated the being effected between October 7 and 20 (Figure movement of small numbers of laid-up fish, those 3.3). A period of low flows in late October was in July, although of similar hydrological associated with virtually zero activity, with two magnitude, did not. In turn, the flow events spells of elevated discharge triggering further which stimulated large-scale movements in activity, especially in the higher reaches, during October (see Section 3.5) were again of similar or November. As in 1986, little long-distance lesser magnitude hydrologically to those movement was recorded after mid-November. initiating only a minor response in late August. Thus a similar flow event has an increasing effect on fish behaviour as the season progresses. This

R&D Publication 4 27 The pattern was again similar in 1988, with a with few spates and a rapid return to base flows, spell of elevated flows between October 6 and 24 appear to be associated with severe truncation of resulting in widespread migration throughout the the spawning distribution. river system. A long period of low flows then ensued, with relatively little activity. However, it The level of flows required to stimulate appears that most fish had reached their target movement is not high. Major migration was destination during the October spates and a triggered in October 1 989 by a flow event with a further spell of elevated flows in early December peak DMF (measured at Knapp Mill) of only 12,8 was associated with minimal further migration. mVsec. Fully effective dispersion of spawners appears to require somewhat higher flows or at While heavy rainfall causing an increase in least flows maintained for many days. Sub- discharge in mid-October 1989 once again optimal dispersion may be associated with triggered large-scale movement of fish, the particular problem obstructions however, rather situation was rather different to earlier years. than a lack of stimulating flows. First, because the flows during the summer had been very low, the fish were generally distributed 3.5.2 Spawning distribution further downstream in early October than in earlier years. Second, flows did not exceed about Numbers of tagged fish still being followed at 1 5 m Vsec before falling back to almost summer spawning time (late December - early January) levels for several more weeks. Thus while large- were generally considerably reduced from the scale activity was triggered in October, relatively numbers tagged. Fish were lost to legal and few fish dispersed into the upper river (e.g. above illegal capture, and increasing numbers of Fordingbhdge) at this time. High flows then transmitting tags were failing as the year occurred again from mid-December, once more progressed. Nevertheless, numbers tracked each triggering activity. However, few fish penetrated year gave a good indication of the general beyond Breamore and the distribution of pattern of spawning distribution. spawning appears to have been severely truncated by the low flows. The first three years of the study represented "unremarkable" years in terms of river flow The exact mechanism of this reduced spawning (Section 3.2), and showed similar patterns of dispersion is not known. However, a major build­ spawning distribution. Table 3.3 shows the up of tagged fish immediately below Bickton in distribution of the most upstream record of each early December and below Breamore, in late fish being tracked near spawning time each year. December suggests that fish may have had difficulty in gaining access past these mill weirs at Combining the figures for 1986-88 indicates that the flows-prevailing. 69.6% of spawning fish ascended beyond Breamore Mill. In the two years of low flow, The final year of study was similar to 1989. Once 1989 and 1990 (see Section 3.2), less than 19% again, elevated flows in late October initiated a of spawning fish passed Breamore, with all zones major movement of fish, but as in the previous downstream more heavily populated than in the year this was mainly centred in the lower river for earlier years. the same reasons. A very protracted period of low flows then persisted until late December. It was hoped that the tracking study might Again, large numbers of fish were gathered indicate any differential spawning distribution of below Bickton, and few eventually penetrated the various age classes of fish. In fact there beyond Breamore (Figure 3.4). proved to be no discernible pattern. One possible exception is that the small numbers of 3- It thus appears that elevated flows are necessary sea-winter fish tracked appeared not to migrate to trigger large-scale movement towards the beyond the Fordingbridge area, but it is stressed spawning grounds in the autumn. It is also that the numbers are too small for this to be a apparent that fairly high flows or regular flow firm conclusion. The largest fish tagged, a 31 lb elevations are required throughout the autumn male, appeared to spawn in the Avon Tyrell area, for optimal dispersion of spawning. Dry years about 12 km above the tidal limit.

R&D Publication 4 28 Table Î.3 Distribution of tagged fish at spawning time.

Area Location km from 1986 1987 1988 1989 1990 tidal limit R. Wylye >65.9 R. Nadder u/s Wilton > 65.9 R. Nadder d/s Wilton 60.4- 65.9

Avon u/s Salisbury > 60.4

Breamore-Salisbury 41.6 - 60.4 10 11 11

Fordingbridge-BreamQre 33.4-41.6

Ringwood-Fordrngbricjge 20.2 - 33.4 16 16

Causeway-Ringwood 9.1 -20.2 10

d/s Causeway 0-9,1

Totals 36 , 26 32 43 31

The furthest upstream that tagged fish were The low numbers of fish entering the river in the found were West Amesbury on the Avon, Steeple dry years of 1989 and 1990 were, it is suggested, Langford on the Wylye and downstream of as a result of virtually the whole run being denied Dinton Mill on the Nadder. access to the river during the summer. Of the two ascending in 1989, one had in fact spent the 3.6 Fish ascending the River Stour summer/early autumn in the Avon, upstream of Knapp Mill. In October it continued up the Avon In 1 986, three tagged fish were located well up to Ringwood area, before returning seawards and the Stour system by spot checks. Five others ascending the Stour, where it was recorded by were recorded at or just seawards of Iford Bridge, successive ALSs up to Shapwick. and in the absence of an ALS network upstream of Iford in that year it is not possible to say It appears that the potential run up the Stour in whether any or all ascended the river further. the years of the study was of the order of a few Given the limited scale of the search that located hundred fish per annum. In average to wet' three fish, it is likely that more had in fact years, many of these fish ascend the river in the ascended the river. In 1987, five tagged fish summer, and exhibit a high survival. At lower were recorded in the river by the five ALS located flows, an increasing proportion appear to remain there. Again in 1988, five fish were recorded in tidal water through the summer, where they having penetrated well into the system. In 1989, experience a high mortality in the same manner only two were recorded well up the Stour, both as the Avon fish exhibiting the same behaviour having first ascended some way up the Avon. No (Section 3.3.2). However, as the Stour is more fish were recorded in the river above the tidal prone to low flows in dry years, the impact on limit in 1990, though many fish were recorded in the well-being of the stock appears to be greater, the tidal reaches from Iford Bridge downstream. with an overall very low survival in 1990. Summer passage past Throop gauging station The small numbers of observations preclude was associated with flows in excess of 5m'/sec. It detailed analysis, though generally the pattern of is stressed that these observations are based on movements in the Stour appears to be broadly relatively few data. similar to that on the Avon. Some fish ascend the river to the Wimborne area in their initial Although the ALS array gave a fair picture of the migration in the summer, but many do not enter dispersion of fish through the Stour system, the the river until the autumn. The river has a lower small numbers involved did not justify extensive base flow than the Avon, and Stour fish appear foot searches to locate spawning areas. Of the even more vulnerable to the problems of low fifteen fish, five were recorded passing/having flow and associated poor survival in the estuary. passed upstream of Shapwick, five more passed

R&D Publication 4 29 Canford, four more passed Throop, and one Shapwick - two tagged fish were located here in remained downstream of Throop. In addition to 1986), the Allen (between Canford and Shapwick the main river, the main spawning tributaries are - one tagged fish was located here in 1986) and considered to be the Tarrant (upstream of the Moors River (downstream of Throop).

R&D Publication 4 30 THE RIVER TAMAR STUDY

4.1 Introduction

This study formed part of a major fisheries and environmental investigation of the potential impact of the Roadford water resources scheme, involving six rivers and three reservoirs. This included the effect of abstraction from and regulation of the River Tamar. The work was undertaken by staff of the South West Water R. Walkham Authority and subsequently the NRA.

The overall study involved many aspects of the fisheries, macro-invertebrates and water chemistry of the Tamar system. Fisheries studies included radio tracking of salmon, trapping of upstream migrating salmonids at three sites and downstream migrants at two, extensive electric fishing surveys, redd counts, installation and commissioning of a head-of-tide fish counter, and analysis of angling records. The overall study was the most comprehensive and wide ranging T.L.^TV2 Lopwell Dam environmental investigation of a water resource TV1 scheme ever undertaken in the UK. Maristow The main area of the overall programme discussed here is the radio tracking, but aspects N = Netting site of the fish counter results and the analysis of T.L. = Tidal limit 6.5. = Gauging station angling records are integrated into the overall X = Automatic listening assessment. Comparison of the results obtained station by radio tracking and by the counter allows an Warleigh Point assessment of the relative merits of the two approaches, and of exploring the potential for Figure 4.1 The estuaries and lower catchment of the Tamar and Tavy, showing the integrating the two approaches (see Section location of automatic listening stations, netting sites and other sites mentioned in the text. 1 3.9). The extent to which salmon angling success can be interpreted as an indicator of fish reaches of the river, are shown in Figure 4.1. The migration is also examined. sites throughout the middle and lower river are shown in Figure 4.2. The monthly distributions 4.2 The radio tracking programme of fish tagged, and of tagged fish passing selected ALS stations are shown in Table 4.1. 4.2.1 Description Also shown is the monthly distribution of net catches, indicating that the tagged fish were a Fish were tagged and released each year from fair representation of the catch as a whole. 1 986 to 1995 at one or more of four sites; three commercial netting areas in the Tamar and Tavy In the ten years of the study (1986-95), a total of estuaries, and a trap at the tidal limit of the 330 fish were tagged and released from the nets Tamar. They were tracked by an array of ALS operating on the Tamar estuary at Cotehele (9.7 equipment throughout the Tamar catchment, km seawards of the tidal limit, 161 fish) and Weir and in later years on the Tavy catchment (see Quay (14.7 km seawards of the tidal limit, 169 Section 7). The locations of the netting and fish). Of these, 100 of the Cotehele fish (62% of trapping sites, and of the ALS in the lowermost those tagged) and 80 of the Weir Quay fish

R&D Publication 4 31 (47.3%) were detected by ALS equipment in the The numbers of tracked fish were considerably tidal or non-tidal Tamar. Of the remaining 150 boosted in two ways. First, 24 fish tagged in the fish, 103 (31.2% of the fish tagged) were not Tavy estuary at Maristow in connection with the seen again; 20 (6.1 %) were reported caught by study there (see Section 7) were recorded by ALS the Tamar estuary nets and two by the Lynher equipment on the Tamar. Second, a total of 316 estuary nets; 22 (6.7%) were recorded by ALS salmon were trapped and tagged at a trap at the equipment in the tidal or non-tidal Tavy; and tidal limit at Gunnislake Weir. Most (270 = three were recovered from fish found dead in the 85.4%) were released immediately upstream, but estuary or were reported from unrecorded net- 46 were displaced 1.8 km seawards for release. caught fish. Three of the trap-tagged fish subsequently entered the Tavy.

The fish listed in Table 4.1 as passing the tidal Poison R. Thrushel limit include only those tagged and released Bridge seawards of there i.e. those tagged at the netting .28.8 km sites, those coming from the Tavy, and those ft. Tamor trapped at the tidal limit and displaced seawards for release. The numbers for the sites upstream of the tidal limit include all tagged fish from all sources.

4.2.2 Hydrometric background

The tracking study spanned a ten-year period and thus a wide range of flow conditions was covered. Several years of very dry summers with low flows were included (e.g. 1989 and 1990) and several wet summer with frequent major 19.7 km spates (e.g. 1986, 1988 and 1993). The x- --'R. Inny hydrograph for a wet summer (1988) and for a dry summer with some minor spates (1987) are included in Figures 4.3 and 2.2.

4.2.3 General pattern of movements

The daily numbers of fish tagged and recorded at Sunnislake G.5. 3.0 km selected ALS stations on the main river and tributaries are shown for 1987 (Figure 2.2) and 1988 (Figure 4.4). These represent years of contrasting summer flow patterns as discussed in Section 4.2.2. * = Automatic listening station The totals for Impham Meadow and Cunnislake Weir only include fish which were tagged seawards of these points, i.e. they do not include fish released there. The daily mean flow (dmf) for Gunnislake CS is also shown. It is clear that the pattern of migration into and through the lowermost reaches is similar to the timing of

Figure 4.2 Middle and lower reaches of the River Tamar showing the location of release of tagged fish. automatic listening stations, with distances from the tidal limit.

R&O Publication 4 32 Table 4.1 Monthly distribution of tagged fish and ALS records (1986-1994).

Net catch Fish tagged Fish tagged Pass tidal] d/s i Gauging Lamer- Leigh Greystone (nets) (trap) limit I abstract, station hooe Wood ,

Distance km -9.7 to-14.7'.9.7 to-14.7 0.0 0.0 1.2 3.0 7.05 13.8 21.9

Years 1986-94 1986-94 1986-94 11986-94 1 1986-94 1986-94 11986-94 1 986-94 1986-94

March 58

April 183 3 14 12 11 10 4 3 May 768 9 28 22 15 16 10 7

June 2,663 49 83 30 55 34 54 32 21 July 6,543 98 72 65 64 54 39 34 17 August 3,974 106 78 53 57 47 35 19 11 September 59 23 31 32 27 19 21 11 October 4 13 22 26 46 60 56 56

November 5 3 8 13 26 33

December 2 13 4 5

Total 14,189 330 316 240 231 242 234 206 164

Further upstream, migration becomes abstraction (dma) figure. The abstraction pumps increasingly associated with elevated flows, with are generally adjusted at about 09.00 hours each little movement in the dry periods. Detailed morning, and are then run at that rate for 24 analysis of the impact of flow variation on hours. The abstraction return gives a dma figure movement is described in Section 4.2.4. which is thus appropriate for the period 09:00 to 09:00 the next day. Also included in the analysis 4.2.4 The relationship between flow and is the lowermost ALS site on the Lyd. Here the migration months June to December were covered, as the great majority of migration takes place in the The data for selected ALS sites on the lower river autumn. Flows used for this analysis were the 15- were analysed using the flow frequency method minute recordings at the Lifton Park Gauging (Section 2.6). Only fish passage events occurring Station. between May 1 and September 30 were included in the analysis, as discussed in Section 2.6.2. At The analyses presented here are limited to the Cunnislake Gauging Station and upstream, the primary migration period (May to September) on flow values used were the most recent 15-minute the main river, plus observations made on the readings from the CS record. This will clearly be Lyd sub-catchment. Data covering other periods accurate for the station situated at the CS itself, and parts of the catchment were collected but but increasingly less so further upstream due to were not analysed at the time. the time-lag of flow events. It is likely that the flow figures ascribed to Greystone, for example The analyses used here were conducted by Dr (18.9 km upstream of the CS) may be up to Solomon under two specific contracts limited to several hours too "early", but it is felt that the an independent analysis and assessment of error so introduced, especially at times of low aspects of direct relevance to the operation of flows, is negligible. Roadford-Gunnislake water resource scheme.

For sites situated downstream of the abstraction Efforts were made to utilise the cumulative point and for Green Stream, immediately frequency analysis approach on these data upstream of the abstraction, an estimated (Section 2.6). Unfortunately, the complex shape residual flow to the estuary is used. This is the of the cumulative frequency lines, especially in most recent 15-minute reading at Cunnislake the critical area when the migration index was Gauging Station, less the appropriate daily mean

R&D Publication 4 33 Liiton (R - Lyd ) 2 7.6 km JL D. .n n

Beals M ill (R . In n y) 19.7 km

Poison Bridge 28.8 km

Greys to ne 21.9 km

JUDIL ni.,in)fl i n, h,, : : , n—

d/s Endsleigh 1 3.8 km " C ". ... » " m .nHi Ih nn n nrm n nu

Lamefhooe 7.05 km n ni n .n n R, n,fl „,,B iïin .ni! n JL

Weir Cottage 4.95 km. " " M™ llniflV rfl JinmIflmnnui!

30 - - 30

25 - - 25

£ 20 - 20 E a £ Z Gauging Station 3.0 km - 15

d/sabstraction 1.2 km Jil.nfl ,IIH„, _n_jil]iiuiuiD_n kdnA. lu flifl.J,.,lkiii,,.,n„,. , [I n H-JJLJI J JU

Trap release Gunnislake 0.0 km

JUtt JiLAmU^JU^ ,1,1 kkUj llinun n 1il n n

Impham meadow -1.0 km JLEL.

Net release estuary -9.7 to -22.7 kri

I-Aor 1 • M a v 1-lun l-lul 1-Aua l-Sto 1-Oct 1 -N o i/ 1-Dec

Figure 4.3 Daily records of fish tagged and passage of tagged fish past automatic recording stations in 1988. Also shown is the dmf for Gunnislake Gauging Station. Distances shown are from the tidal limit at Gunnislake Weir.

R&D Publication 4 34 around a value of 1.0, precluded fitting reliable Table 4.2 Threshold flows (migration index equals 1) at ALS sites on the Tamar. polynomial equation lines. This complexity was mainly due to the pattern of abstraction and Site Migration index %Q95 regulation releases around the prescribed flow. =1 (mVsec) Analysis was therefore limited to deriving a) May to September threshold flows (i.e. when the index reaches 1.0) Impham Meadow (-1.0 km) 2.4 135.6 from the "difference" between the raw data lines Cunnislake Weir (0.0 km) 2.3 129.9 for cumulative frequency of all-flows and fish- Hatch Wood (0.3 km) 2.3 129.9 flows (see example in Figure 2.3 and description New Bridge (d/s abstraction, 1.2 km) 2.4 135.6 in Section 2.6). Green Stream (u/s abstraction, 1.5 km) 2.4 135.6 Cunnislake GS (3.0 km) 3.8 214.7 The mechanism of deciding the peak of the Weir Cottage (4.95 km) 4.6 259.9 cumulative impact, i.e. when the migration index Lamerhooe (7.05 km) 4.5 254.2 reached 1.0 and thus the flow above which there Leigh Wood (13.8 km) 4.8 271.2 is no adverse impact on movement, is clearly Greystone Bridge (21.9 km) 4.1 231.6 critical in this analysis. In most cases the peak b) June to December was clear and easily identified (Impham Meadow; Lifton Park, Lyd (26.7 km) 2.1 538.5 Hatch Wood; New Bridge; Weir Cottage; Lamerhooe; Leigh Wood and Greystone Bridge).

In the other cases, multiple "peaks" along a operate to a pf of 5.52 mVsec (477 Ml/d) until plateau make clear interpretation more difficult. the year 2002 should be dropped, and the pf on The left-hand end of any true plateau is the the licence of 2.85 mVsec (246 Ml/d) be appropriate point to be considered as the adopted from 1996 onwards. In return, SWW threshold. Therefore the first peak that reaches undertook not to seek a further reduction in pf 90% of the maximum is considered to be the before the year 2015 without the prior threshold in this analysis. The threshold points agreement of TTFA. thus identified are shown in Table 4.2. The radio tracking observations were also used as It is clear that the flow for which the migration an input to a model of fish migration at the tidal index = 1.0 has a similar value of 2.3 - 2.4 m Vsec limit. Development and evaluation of this model for the five stations situated in the zone up to 1.5 is described in Section 10.3. km upstream of the tidal limit. It is higher at 3.8 m Vsec at the Gauging Station (3.0 km), and over The tidal and diurnal timing of migration is 4 mVsec at the more upstream station. These discussed in Sections 8.6 and 8.7 respectively. values compare to the different physical habitat of the different zones, with the lowermost 2 km 4.3 The fish counter above of the tidal limit being a deep, slow flowing "ponded" section upstream of The counter is of the resistivity type and is Gunnislake Weir. These observations fit closely installed within the fish pass on the Cornwall the pattern of migration apparent in Figures 4.3 bank at Gunnislake Weir, the tidal limit. It was and 2.2, and discussed above. The phenomenon commissioned in July 1992 and has operated, of threshold flows increasing upstream is with breaks, since then. examined further in Section 8.3. The counter operates by detecting changes in the These results, supported by those from the resistance of the water column caused by a fish counter (Section 4.5) were used to recommend (Aprahamian et al 1997). Upstream migrants can that a prescribed flow of 2.5 m Vsec could be be differentiated from downstream targets. adopted for the Cunnislake abstraction. In view Although the counter cannot differentiate of this, SWW and the Tamar and Tributaries between species (e.g. salmon and sea trout), the Fisheries Association (TTFA) agreed that the size of the signal is proportional to the size of the original undertaking that the scheme would fish and this can give a fair indication of species.

R&D Publication 4 35 results of analysis for July 1995 is shown in Figure 4.5.

In July 1994, fish movement was under- represented at the lowest flows, with a threshold flow of around 2.5 m Vsec. In August 1994, it to "E was also apparent that fish were under- represented at low flows, but a series of minor spates complicated the picture and it is not Datei of non operation: May 24, 26: [uly 21; August IB; Otlober 24; possible to determine the threshold flow reliably. November 7; December 29-31. In July 1995, when flows below 2 mVsec occurred, the threshold flow was clear and was 01-Jun 01-FaD 01-Mar 01-AIM Qt-Usy Q\~Jun Ol-Jul 01-Aug Dl-5ep 01-Oc! 01

R&D Publication 4 36 the observation on the pattern of anglers' catches of radio-tagged salmon; in Section 9.3 it is noted that 62% of radio tagged fish reported by anglers on the Tamar were caught within three days of passing an ALS situated downstream.

•= 50

Analysis of the Endsleigh catches for September Î '0 and October give a higher threshold flow of about 4.8 m3/sec, the same as that for migration at the Leigh Wood ALS site (Figure 4.6), This may indicate that the salmon caught by anglers in these months are behaving differently from those caught in the summer. The most likely explanation for this apparent different behaviour lies in the pattern of flows in the early autumn. Once the dry weather that tends to predominate in August and early September has "broken", flows tend to stay somewhat elevated and tend not to fall so far or so fast as following a summer spate (see Figures 2.1, 2.2 and 4.3). Thus fish caught within a few days of arrival on the beat are generally associated with higher flows than in the earlier months.

Turning to the Arundell Arms Fishery on the Lyd, the threshold flow of salmon catches in September - October is about 2.1 m3/sec. This is the same value as for migration between June Figure 4.5 Flows (residual) used by salmon counted ascending the pass at and December. Similar arguments apply here as Gunnislake Weir in July 1995. Upper: Cumulative frequency of hourly flow values (all-flows) and of flows used by fish (fish-flows). Lower: Difference in cumulative for Endsleigh catches at the same time of year. frequency of all-flows and fish-flows. Where the "difference" line Is below zero, the value of the "fish-flow" line exceeds that of the "all-flow" line; hence fish migration is enhanced. 4.5 Fish trapping at Gunnislake However, this initiative did provide considerable A fish trap facility exists in the Cornwall fish pass information on the biological characteristics of at Gunnislake Weir, and regular sample trapping the run throughout the year, and also some data was carried out between March 1987 and August of relevance to this study. The general patterns 1991. The trap was operated for two 24-hour of numbers of salmon trapped at various flows periods per week from mid-March to mid- fitted closely with the findings of the tracking November whenever river flow allowed - very study. One event in particular highlighted the high flows precluded operation. This operation importance of minor spates in dry years. In total was of limited value in terms of gaining an of 112 salmon were trapped on August 10 1989, understanding of the factors affecting migration, when a spate caused the dmf to rise from less for the following reasons: than 1 mVsec two days earlier to 6.8 mVsec. The dmf had been below 2.5 m3/sec for a month • the observations were discontinuous beforehand, and trap catches during these weeks • very high flows could not be covered had been of none or a very few salmon per day.

• an unknown and probably variable proportion of the run ascended the weir by routes other than the Cornwall pass (but see Section 4.3).

R&D Publication 4 3? 4.6 Distribution of spawning

Examination of the ALS records for the lowermost sites on the River Tamar above the Lyd confluence, on the river Lyd and on the River Inny, allows comparison of the relative importance of these three sub-catchments.

E u *0 Eighty-four radio tagged fish ascended the Lyd, 47 the Inny and 72 the Tamar above the Lyd confluence. A further 39 fish were recorded ascending beyond the ponded lowermost reaches (i.e. passed the ALS at Gunnislake GS, Table 4.1). Several were reported caught by anglers in the zone between Gunnislake and Greystone Bridge, and others may have been caught and not recorded. There is also likely to have been some spawning in the lower Tamar and other tributaries downstream of the Lyd confluence.

r

Figure 4.6 Upper: Cumulative frequency of daily mean flows at Cunnisiake GS (all flows) and dally mean flows when salmon were caught by anglers on the Endsleigh Fishery in September - October, 1989-95 (fish flows). Lower: Difference between cumulative frequency of all-flows and fish-flows.

R&D Publication 4 38 S THE RIVER EXE STUDY

5.1 Introduction R. Exe The need for an investigation of the relationship between river flow and salmon migration in the Exe was identified by the River Exe Water Wimbleball Resources Study conducted for the South West Reservoir Water Authority by Sir William Halcrow and Partners. The tracking work was conducted by Dr D ) Solomon under contract to the NRA and was funded by SWW. The objectives are listed in Section 1.1 of this report.

Over the four years of the project (1991-94), a total of 318 fish were purchased from the commercial netsmen operating in the Topsham area of the estuary (Figure 5.1 ), tagged and released. All tagging took place during the commercial netting season, and all fish were released between May 11 and August 1 6. The monthly distributions of fish tagged and of the declared net catch are shown in Table 5.1.

Although the netting season opens on February 14, few fish are nowadays caught before May. Exeter x = Automatic The tagged fish represented well the distribution Listening Tidal limit 0 km of fish caught throughout the netting season. sham Station The fact that few fish are caught before May, and N = Netting sites that the net catch is falling quickly by the end of the season, suggests that the tagged fish are also a fair representation of the main part of the run of fish.

Most tracking was undertaken using a network of ALS units deployed throughout much of the Figure 5.1 The River Exe, showing the location of automatic listening stations, with catchment. The lowermost was situated at St. distances from the tidal limit. James Weir, at the tidal limit. The locations of the main ALS sites used in this analysis are shown monthly dmf figures. in Figure 5.1, with greater detail of the lower reaches shown in Figure 5.2. Each year, several From early May to mid-August 1992, flows were on-foot searches were made seawards of St. generally well below average, with some minor James Weir at low tide to locate fish laid up in the spates during july. From about August 10 tidal reaches. through to early October, flows were well above average, with major spates. Mid-October was 5.2 Hydrometric background dry, but from then on flows were again above average. The dmf hydrographs for Thorverton Gauging Station between May and December in two Apart from a spate at the end of May, flows contrasting years of the study are shown in through the summer of 1994 were consistently Figures 5.3 (1992) and 5.4 (1994). Also shown, below average until about September 10. A as horizontal lines, are the long term (1956-94) series of spates then kept flows well above

R&D Publication 4 39 average until the end of the month. Much of October was rather dry, but high flows prevailed River Creedy from November onwards. Pynes Weir 7.2 km The four years studied gave a wide range of flow pattern through the period May to October. There was a predominantly dry summer (1994) and a predominantly wet one (1993), long dry spells (1991, 1994) and dry spells punctuated with isolated minor spates (1992). This range of Exwick conditions provided an excellent picture of the Weir x Duryard 4.9 km impact of flows on fish movement to be built up.

1 km Flow characteristics were gathered from a variety of sources, listed in Table 5.2. X = Automatic Listening Station TL = Tidal Limit Gauged flows were available as dmf values and St Davids 15-minute instantaneous readings from the NRA Bridge Gauging Stations at Stoodleigh and Thorverton. Calculated dmf values for the tidal limit were provided by SWW.

5.3 General patterns of movement

X Head Weir 2.4 km Of the 318 fish tagged in the estuary, 1 71 (53.8%) were recorded passing the tidal limit of the Exe, and a further 47 (14.8%) were last recorded in the tidal reaches. The fate of the remaining fish is discussed in Section 8.2.1.

The daily numbers of fish tagged and recorded passing selected recording stations between the Trews Weir X tidal limit (0 km) and Brushford and Pixton 0.9 km Gauging Stations on the Barle and Little Exe Belle Isle (49 km) in 1992 and 1994 are shown in Figures Nursery 0.5 km 5.3 and 5.4. Also shown are the dmf values for Thorverton GS, 15.5 km from the tidal limit.

The pattern of arrival at the tidal limit closely follows the pattern of fish being tagged, as most fish arrive there within a few days. At times of low flow, however, fewer fish complete the journey to the tidal limit promptly. Some of the delayed fish then migrate when flows subsequently increase again e.g. in September 1994, but many fail to enter the river at all; this phenomenon is discussed in Section 8.2.

Figure 5.2 The lower reaches of the River Exe, showing location of automatic listening stations. Migration past the tidal limit, by ascending St james Weir, again shows a similar pattern to arrival downstream, but with further delays at times of low flow. Considering each recording station upstream, a greater and greater •

R&D Publication 4 40 B rushford C .5 . (Barle) 49.3 km ÜLJL. , EUQ- nnlln.

Pixton G.S. 49.1 km J-JIL

Exbridge intake A ? ,9 km ÙUL

Oakfordbridge 43.9 km M -J MJL

Stoodleigh G.S. 35.2 km MbJUflJI . J nn

'Olham intake 32.3 km. No recorder insialled

1-May l-(un l-Jul l-Aug 1-Sep 1-Oct 1-Nov 1-Dec

Figure 5.3 Daily records of fish tagged and passage of fish past recording stations in 1992. Also shown is the dmf for Thorverton Gauging Station and the long-term mean monthly flow (horizontal line). The distances shown are from the tidal limit at St. James Weir.

R&D Publication 4 41 Brushiord C.S. (Bade) 49.3 km

Pixton C.S. 49.1 km JUL

Exbridge intake 47.9 km am MSL

Oakfordbridge 43,9 km. Mo recorder installed

Stoodleigh C.S. 35.2 km IltDfnifl , flU . L

1-May 1-Aug 1 - Sep 1-Oct 1-Nov 1-Dec

Pynes intake 9.4 km Jt n m n n n n n

Passed tidal lim it 0.0 km nun nn Urn Ih n m n n n n I nn

Arrive tid a I limit 0.0 km Jill Ja n tn ill Ih n ^ tm JU tL~. 15 -

10 -

5 - Fish tagged in estuary

1-Mav 1-lun 1 -lui 1-Aua l-Seo 1-Oct 1-Nov 1-Dec

Figure 5.4 Dally records of fish tagged and passage of fish past recording stations In 1994. Also shown is the dmf for Thorverton Gauging Station and the long-term mean monthly flow (horizontal line). The distances shown are from the tidal limit at St fames Weir.

R&D Publication 4 42 Table 5.1 Monthly distribution of tagged fish and ALS records.

Net catch Fish tagged Arrive TL Pass TL North Bickleigh Stoodleigh Oakford Exebridge Bridge Bridge Distance km 0.0 0.0 9.4 23.1 35.2 43.9 47.8 Years 1991-94 1991-94 1991-94 1991-94 1992-94 1991-94 1991-94 1991-93 1991-93

no. tagged 318 318 318 238 318 318 239 318 March 6 April 38

May 129 7 4 2 1 1

June 816 91 57 37 3 0 0 4

July 1,895 153 101 85 34 37 16 3 1

August 660 56 21 27 26 25 23 18 15

September 4 13 20 24 22 10 19

October 2 3 2 26 29 33 35 November 1 1 2 1 7

December 1 Total 3,544 318 193 170 87 115 93 68 77

Table 5.2 Naturalised flow parameters used in this report. Data supplied by SWW.

Site ADF Q95 Basis of calculation Years of records m'/sec m Vsec used

Wimbleball Dam site 0.748 0.079 Theoretical calculated mean Pixton CS 4.562 0.500 Theoretical calculated mean Brushford CS 4.996 0.612 Theoretical calculated mean Exeb ridge 9.706 1.160 Theoretical calculated mean Stoodleigh GS 12.627 1.385 Naturalised, derived from gauged flow 1961-1995

Thorverton CS 16.095 1.794 Naturalised, derived from gauged flow 1957-1995

Tidal limit 24.608 3.450 Naturalised, derived from gauged flow

proportion of the fish pass later and on higher 5.4 Influence of flow on fish flows. This is particularly marked in years with a migration dry summer such as 1994; in that year (Figure 5.4) only two tagged fish reached Bickleigh (23.1 5.4.1 Introduction km) before September. In a year with a wetter summer (1992), almost 95% of fish had passed For analysis of the results of the Exe study, the here before the end of August (Figure 5.3). The flow frequency approach is used, as described in dependence of fish movement on elevated flows Section 2.6. is clear, with even minor flow events such as that occurring in July 1992 (Figure 5.3) being effective Ideally each fish event is allocated a flow value at stimulating migration into the middle reaches. that is the instantaneous flow measured at that The relationship between flow and migration is time at that location. This can almost be analysed in detail in Section 5.4. achieved where the recording station is situated at a continuous gauging station, e.g. at Stoodleigh, though in practice the most recent

R&D Publication 4 43 15-minute recorded value is used. For other seasons indicated above, for the years of stations upstream of the Culm confluence, the operation of the fish recording station. This is all flows used are the Stoodleigh or Thorverton four years in most cases, but only two or three instantaneous flows corrected for time shift by years in some. the adoption of a standard correction factor for rate of travel of flow events down the river of 5 5.4.2 Presentation of analyses km/hr, based upon the observed passage time of a minor spate. While representing an The migration indices for each station are shown approximation, it is suggested that the minor in Figure 5.5. These are plotted with the flow errors in flow that this is likely to cause are of little axis as a percentage of the naturalised Q95 as consequence for this purpose. listed in Table 5.2 as three different flow records are involved.

The situation in the lower reaches is complicated There is a pattern in the apparent chaos in Figure by the inflow of the Culm and Creedy, and no 5.5. It is clear that fish migration is under- effective mechanism for deriving instantaneous represented at low flows at all sites, and that this flows is available. Daily mean flows have been phenomenon generally becomes more marked at calculated by SWW for the tidal limit, and these sites further upstream. The flow index departs are used for allocation of "fish flows" for the from zero (indicating the lowest flow at which a stations from North Bridge Intake (= Pynes intake, tagged fish was recorded migrating) at a little 9.4 km from the tidal limit) downstream. below Q95 at the lowest tracking station (St

The records for all four years are combined, but James Weir arrival), and the relative flow (in terms the months included vary between stations as of Q95) for the first fish increases steadily as one indicated in Table 5.3. proceeds upstream; at Brushford it is in excess of five times the Q95 (Table 5.4). A similar pattern

These seasons were allocated on the basis that is apparent for the flows at which the lines reach most fish migration at these points occurred a value of 1.0, i.e. fish migration occurs at an within these months, and it is important that the average rate. In general, the higher the flow "available flows" allocated really were available; value for first fish and index = 1.0, the higher the thus inclusion of a month when few or no fish index eventually goes, because there is more were available to migrate could bias results, as "catching up" to be done. The Pynes Weir station discussed in Section 2.6.2. is apparently "out of step" with the North Bridge one upstream; this arises because arrival at the The available flows (or the "all-flows") are the Pynes station is affected by the flow dmf values for the appropriate gauging station arrangements downstream at Cowley (this is (Stoodleigh, Thorverton or tidal limit) for the discussed in Section 5.5).

Table 5.3 Time periods and flow gauging records used for fish movement analysis at each station.

Station km Period covered by analysis Flows used for fish-flows dmf record used for all-flows

St. James Weir May-Sep, 1991-94 Tidal limit dmf Tidal limit

Duryard 4.9 June-Oct, 1992-94 Tidal limit dmf Tidal limit

Pynes Weir 7.2 June-Oct, 1992-94 Tidal limit dmf Tidal limit

North Bridge intake 9.4 June-Oct, 1992-94 Tidal limit dmf Tidal limit

Bickleigh Bridge 23.1 June-Oct, 1991-94 Thorverton inst. Thorverton

Bolham 32.3 June-Nov, 1993-94 Sloodleigh inst. Stoodleigh

Stoodleigh CS 35.3 June-Nov, 1991-94 Stoodleigh inst. Stoodleigh

Oakfordbridge 43.9 June-Nov, 1991-93 Stoodleigh inst. Stoodleigh

Brushford 49.3 lune-Nov, 1991-94 Stoodleigh inst. Stoodleigh

R&D Publication 4 44 Table 5.4 Lowest flows at which fish are predicted to migrate, and flows at which the migration index reaches 10

Distance Flow as % of Q95 Flow mVsec Station (km) First fish Index exceeds 1.0 First fish Index exceeds 1.0

Arrive at St. |ames 0.0 91 160 3.14 5.52

Ascend St. James 0.0 105 176 3.62 6.07

Duryard 4.9 110 191 3.79 6.58

Pynes Weir 7.2 167 206 5.75 7.12

North Bridge Intake 9.4 126 246 4.34 8.47

Bickleigh 23.1 173 304 3.10 5.46

Bolham 32.3 256 323 3.55 4.48

Stoodleigh 35,3 240 387 3.32 5.36

Arrive Oakfordbridge 43.9 314 314 4.35 4.35

Ascend Oakford Weir 43.9 475 599 6.58 8.29

Brushford 49.3 516 531 7.15 7.35

3.0 Arrive St |ames Weir 0.0 km —Ascend St james 2.5 Weir 0.0 km

Duryard 4.9 km

2.0 Pynes Weir 7.2 km

North bridge Intake I 1-5 9.4 km 2 — Bickleigh 23.1 km

1.0 Bolham 32.3 km

Stoodleigh 35.3 km Oakfordbridge 43.9 OS km Ascend Oakford Weir 43.9 km 0.0 Brushford 49.3 km 100 200 300 400 500 600 Flow as Percentage of Q95

Figure 5.5 Migration indices for ALS on the River Exe.

S .4.3 Arrival at the tidal limit having an adverse effect upon fish passage. Flows below 5.52 mVsec occurred for 35.8% of the As observed in Section 5.3, fish tend to arrive at period covered by this analysis, and thus 35.8% the tidal limit (St James Weir) within a few days of of fish migration would be expected during that being tagged in the estuary, but low flows may time. The fact that only 26.0% of fish arrived at delay this migration until flows increase again. such flows suggests that typically 9.8% of the From Table 5.4 it can be seen that the migration total run of tagged fish for the year was delayed index reaches 1.0 at a flow of 5.52 m Vsec; below in arriving at the tidal limit, by flows falling below that flow, fish are arriving at a lower than average about 5.52 mVsec. rate. Above that flow, fish migration takes place at or above the "expected" level and thus flows above this level cannot be considered to be

R&D Publication 4 45 The effects of state of tide and time of day on by fish arriving at and passing the weir are shown arrival at the tidal limit are discussed in Sections together in the upper part of Figure 5.6, and the 8.6 and 8.7. difference between them in the lower part. The peak difference is about 13% at about 4.8 5.4.4 Passing the tidal limit m'/sec. Thus it can be stated that 13% of the total run of tagged fish was delayed by St James Most fish travel up the main river to St james Weir, due to flows falling below about 4.8 Weir, and upstream by ascending the weir which m3/sec; this represents the great majority of fish represents the tidal limit. A few tagged fish, arriving at the weir at such flows. This is reflected particularly in 1994, are known to have used the in the different migration index lines for arrival at alternative mill-stream channel (see Figure 5.2). and ascending past St James Weir in Figure 5.5. This analysis is limited to fish ascending via the It is suggested that these figures may under- weir. represent the obstruction caused by the weir, as it is likely that a number offish passing the tidal As is apparent from Table 5.4, flows associated limit at lower flows did so via the mill stream. with fish passing the tidal limit are a little higher than those reaching the tidal limit, leading to a In 1994 the millstream represented a greater build-up of fish at St James Weir (Salmon Pool). proportion of the flow past the tidal limit than in It is therefore clear that the weir itself is an earlier years, partly because of low flow impediment to the movement of fish at certain conditions generally and partly, it appeared, flows, a situation exacerbated by the leat because flow control structures were adjusted to abstraction bypassing the weir. The flows used take more flow. An ALS sited at the mill between mid July and October showed that three of the four fish passing the tidal limit between July 7 and early September did so via the mill stream. Retrospective consideration of the records for 1992 suggested that some fish may have used Hows used by fish if riving at the weir this route at times of low flow in that year also.

llowi used by fiih This situation poses an interesting problem. ax ending the weir While the extra flow in the mill stream allowed The !0i(id 90 percentilei ol flows passage of some fish past the tidal limit that used by f iih arriving. IQ would otherwise be unlikely under the prevailing 0 I- flow conditions via the main river, it is taking water from the main channel. This means that passage over the weir itself stops sooner on a falling hydrograph, and depletion of lowest flows may make the weir pool unattractive to fish, such that they return seawards. Overall, the best option would appear to be improvement of fish passage facilities under low to medium flows at St E i 6 L-TI James Weir, including ensuring that minimum flow is taken into the mill stream during the summer.

5.4.5 Fish passage through Exeter

At times of low flow many fish remain in the deep ponded water area through Exeter City Figure 5.6 Flows used by salmon arriving at and ascending St. James Weir. Upper: Cumulative frequency of dmf values (all-flows) and dmf values used by fish arriving at between Trews Weir (0.9 km) and Head Weir (2.4 and ascending the weir. Lower: Difference in cumulative frequency of flows used by fish arriving at and ascending the weir. km), with lesser numbers remaining in the 0.9 km between Trews and St James Weir, and upstream of Head Weir (see Figure 5.2).

R&D Publication 4 46 The flows used by fish leaving the ponded area represented by this series of weirs are indicated by the fish-flow arrival line for the Duryard ALS (Figure 5.7), 4.9 km upstream of the tidal limit. # BO From this figure and from Table 5.4 it is immediately clear that there is a considerable - 60 effect of low flows, with the first fish being S recorded at 3.79 m3/sec (110% of Q 95) and the riih-liows arrival ai Duryard i II -Hows migration index reaching 1.0 at 6.58 mVsec fiih-tiows arrival at Pyn« Weir (191% of Q95). These values are higher than those for fish ascending St James Weir 4.9 km downstream, and from these observations it could be suggested that there is some structure causing a hold-up between the two. However, from the records from the recording station located at Head Weir (2.4 km) in 1991, it would appear that the majority of salmon taking up •In residence in the ponded reach through Exeter do so without first having approached the next obstruction upstream, i.e. they choose to remain in the deep water area. This is a similar situation u to that which prevails on the Hampshire Avon and Tamar, where large numbers of fish take up residence in deep, slow reaches in the upper tidal and lowermost non-tidal reaches, in the absence flow m'/we of any apparent physical barrier to free Figure 5.7 Flows used by salmon arriving at and leaving Cowley Bridge area, River movement. Exe. Upper: Cumulative frequency of dmf values (ali-flows) and dmf values used by fish arriving at the Duryard and Pynes Weir ALS sites. Lower: Difference in cumulative frequency of flows used by fish arriving at Duryard and Pynes Weir. The origin of the difference line is below zero because a single fish reached Pynes Weir 5.4.6 Fish passage upstream at a lower flow than any reached the Duryard ALS.

Similar flow analyses have been undertaken for arrival at recording stations upstream, Pynes Weir 5.5 Obstructions to passage (7.2 km). North Bridge Intake (9.4 km), Bickleigh upstream Bridge (23.1 km), Bolham Weir (32.3 km), It is evident from the discussion of analyses so far Stoodleigh Gauging Station (35.5 km), that the relative flows used by fish increase as the Oakfordbridge (43.9 km) and ßrushford (49.3 fish move up river (Figure 5.5). It is also apparent km). The migration indices derived from these that much of this increase occurs in steps data are shown in Figure 5.5, and the flows associated with particular weirs. The situation at associated with the first fish of the migration St James Weir at the tidal limit has already been index reaching 1.0 are given in Table 5.4. considered in Section 5.4.4 above. A similar analysis of the flows used by fish arriving at and The phenomenon of the impact of low flows ascending a number of weirs upstream is now increasing as one moves upstream has already described; of the twenty or more weirs on the been discussed in Section 5.3 and is examined for Exe only those observed to have a significant all these rivers studied in Section 8.3. The impact on fish migration are described in detail. restricting effect of various weirs is discussed below in Section 5.5. The effect of time of day First, the situation in the Cowley Bridge area is on fish movement is discussed in Section 8.7. considered. This is complicated by the diversion of part of the Exe flow via a flood relief channel to the lower reaches of the River Creedy, which then rejoins the Exe about 1 km downstream. Flows prevailing when fish enter this area are

R&D Publication 4 47 Examining the two fish-flow lines in the upper part of Figure 5.7 indicates that the Pynes line is displaced to the right of the Duryard line by a rather variable distance that is typically of the order of 1 - 2.5 mVsec. The level of obstruction Msivüows arrive represented in this zone can thus be equated in all-llowî its impact to a non-consumptive abstraction of fish-(lows depart the order of 1.5 mVsec.

10 and 90 percentile fishHpwi (arrive) The problem at Cowley appears to be due largely to the diversion of much of the Exe flow into the lower reaches of the Creedy via the flood relief channel, leaving the flow in the Exe channel depleted at low to medium flows. The fish pass at Cowley Bridge Weir (River Exe) appears to be of excellent design and construction and to pass fish with minimal delay once they are inclined to approach it (an ALS was located here in 1992 and 1993). However, at flows when fish are held up in this area, they do not ascend the channel as far as the weir, but remain downstream of the Creedy confluence or ascend the Creedy for a short distance, as far as Exwick Weir.

Next, the situation at Bolham Weir is examined, by comparing the flows at time of first record at Figure 5.8 Flows used by salmon arriving at and ascending Oakford Weir. Upper: Cumulative frequency of dmf values (all-flows) and dmf values used by fish arriving the station situated close to the weir (time of and ascending the weir. Lower: Difference In cumulative frequency of flows used by fish arriving at and ascending the weir. arrival) and the last record (a close approximation to the time of ascent). Comparison of these two available from the Duryard ALS (4.9 km from the fish-flow lines indicates that 1 2% of the total run tidal limit). Those for fish leaving the area are of tagged fish was delayed here, at flows below obtained from the arrival time at the ALS at Pynes about 5.5 mVsec. Further details are presented Weir (7.2 km) Although this latter station was in Table 5.5. some distance upstream from the two Cowley weirs (Cowley Bridge Weir on the Exe and Exwick Finally, we consider the situation at the weir at Weir on the Creedy), few fish were found to lie Oakfordbridge. Comparison for the fish-flow up between Pynes Weir and the Cowley weirs, so lines for arrival and departure here (Figure 5.8) that its records are in fact a good indicator of indicates a delay of about 24% of the total run of conditions when the fish ascended the latter tagged fish, at flows below 8.9 m Vsec. About weirs. The migration indices for these two 15% of the run arrived at flows below 6.8 stations can be compared and contrasted in mVsec, but no fish ascended the weir at such Figure 5.5. Comparison of the flows used by fish flows. Of all fish arriving at flows below entering and leaving this zone (Figure 5.7) 1 OmVsec, over half were then delayed until flows indicates that a total of 17% of all tagged fish exceeded that value. were delayed in this area, at flows below 13.2 Of the four obstructions considered (St James m Vsec. tn the range up to 8 m Vsec, almost haff Weir, Cowley Weirs, Bolham Weir and Oakford the fish arriving in this area are delayed; this flow Weir), Oakford Weir is clearly the most severe in range represents half the time in the period its immediate impact on fish movement (Table under consideration (June - October, 1992- 5.5). The severity is somewhat reduced because 1994), during which almost a quarter of stock this lies well upstream, so that the spatial arrived in the area. truncation of dispersion of fish is limited. The

R&D Publication 4 48 Table 5.5 Proportion of fish delayed at obstructions at low flows. The figure in brackets after each flow value is the % of Q95 which that flow represents (see Table 5.2).

St James Cowtey Bolham Oakford

Flow below which 10% of fish have arrived •4.01 (116) 6.14 (178) 4.39 (317) 5.46(394)

Percentage of that 10% which is delayed. 71.5 86.0 62.0 100

Flow below which 20% of fish have arrived 4.97 (144) 7.42 (215) 5.24 (378) 7.29 (526) Percentage of that 20% which is delayed. 45.6 54.0 32.5 91.5

Flow below which 30% of fish have arrived 5.89 (171) 8.62 (250) 6.12 (442) 9.67(698)

Percentage of that 30% which is delayed. 35.1 38.1 22.0 58.0

second most severe is the situation at Cowley sub-stock was under-represented among the Bridge, which is serious because it is in the lower tagged fish. reaches and affects much of the catchment. Although the situation at St James Weir is It is likely that the numbers of fish last recorded in numerically less severe, it is critical because the each reach between North Bridge Intake and the delayed fish are gathered in a small area and Barle confluence reflects the level of spawning appear to be vulnerable to both legal and illegal within these reaches and the tributaries entering exploitation - many radio-tagged fish them. The relatively large numbers of fish last "disappeared" from this area. It also has the recorded between the tidal limit and North potential to affect dispersion throughout the Bridge intake appears anomalous, however. catchment. The impact of Bolham Weir is less While it is likely that some fish spawn in the than the other three, but could still cause a delay Creedy and Culm, and even in the lower main to migration under certain flow conditions. river, it is probable that many of these fish did not spawn here. It is likely that some were 5.6 Distribution of spawning caught (legally or illegally) and not reported, salmon some may have returned seawards, and some may have migrated upstream late in the year An indication of the distribution of spawning can after their transmitting tag batteries were be obtained from examining the most upstream exhausted. record of each fish. The figures for the four years of tracking are combined in Table 5.6. Table 5.6 Distribution of most-upstream freshwater records of tagged fish, 1991-4.

Clearly the Barle is the most important spawning Numbers of fish area. The relatively small number of tagged fish River reach ascending the Little Exe is surprising, as parr River Barle 68 surveys indicate that this, too, is a major River Little Exe 12 spawning area. It was noted that the Little Exe Stoodleigh to Barle confluence 14 (+ 9 caught) fish passed Exebridge on average a week or two Bickleigh to Stoodleigh 15 (+ 4 caught) later than the Barle fish. It is possible that the N. Bridge intake to Bickleigh 6 (+ 6 caught) two sub-catchments have separate sub-stocks 38 with different migratory habits (e.g. seasonal Tidal limit to N. Bridge intake timing of estuary entry), and that the Little Exe

R&D Publication 4 49 6 THE TAW AND TORRIDGE STUDY

6.1 Introduction The fish were tracked using a series of ALS sites on the two rivers, from the tidal limit upstream. The This investigation was undertaken by NRA staff data were archived without being processed during as part of the overall study of the impact of the the project. In 1995 Dr D Solomon was contracted Roadford Water Resource Scheme, of which by SWW to analyse and report on those data abstractions from the Taw and Torridge form a relevant to the movement of fish in the lower part. It was undertaken over a period three reaches of the Taw, in relation to development of years (1 992 - 1994) and involved a common the Newbridge PWS abstraction. The data for the source of fish in the joint estuary (Figure 6.1) Torridge was analysed specifically for this study, but and an array of ALSs in each river. Although it was not feasible to extend the Taw analysis into there has historically been a commercial net the upper river. fishery in the estuary, at this time it was temporarily closed as a conservation measure, Of the 255 fish tagged and released, 122 were and the licensed netsmen were being paid recorded by ALS in the tidal and non-tidal Taw, and compensation for not fishing. One or more 42 in the Torridge. Three others were found dead crews were therefore contracted to undertake in the estuary and three were recorded caught in netting on each occasion when fish were other rivers (see Section 8.2). The other 85 fish required for tagging. In total 255 fish were (33.3% of those tagged) were not recorded again at tagged between May and September over the any time. three years. The results for the two rivers are now considered separately.

Brounton(( chivenor Pill

Barnstaple

Appeldore) Bishops Tawton

Torridge Estuary

The lower reaches and estuary of the RIVER TAW

t = netting sites used to obtain fish for tagging s = automatic listening stations

Figure 6.1 Map of the estuary and lower reaches of the ftiver Taw, indicating features referred to in the text.

R&D Publication 4 50 6.2 River Taw interest in movements in the lower Taw with respect to water resource management, the situation was 6.2.1 Hydrometric background examined particularly closely there by placing several stations in the 8 km zone between the tidal Flows in the lower reaches are complicated by limit and Umberleigh Gauging Station, particularly a PWS abstraction which is taken from the in 1993 and 1994. The locations of the tagging and river immediately downstream of the recording station sites are shown relative to the footbridge at Chapelton, about 2 km upstream various elements of the abstraction scheme in Figure of the tidal limit. At the time of the study, 6.1. some of the abstracted water was returned to the river about 1.2 km downstream of the The numbers recorded within the Taw are intake. This meant that three related flow considerably lower than the numbers tagged, as regimes had to be considered in the many of the tagged fish were returning to the lowermost few kilometres of the river (see Torridge or even other more distant rivers, and did Section 11.5). not ascend the Taw as far as the lowermost recording station. For analysis of fish movement data, computer files were prepared of flows appropriate for 6.2.3 Flows used for migration each ALS site, using abstraction data supplied by SWW applied to the J 5-minute readings The results for the five ALSs detailed in Table 6.1 fron-r the gauging station at Umberleigh. No were analysed using the cumulative flow frequency allowance was made for elapsed time between method described in Section 2.6. The analyses are flow events at the gauging station and various limited to the period May to September inclusive points downstream, or for any flow gain from each year, as discussed in Section 2.6.2. run-off within this 3 - 4 km reach. This is not considered to introduce any significant bias The patterns observed on the Taw are different in into the interpretation of the results. detail from results obtained on most other rivers; in particular, the curves showing the difference 6.2.2 Description of the results between the cumulative frequency of available flows and of flows used by fish, do not have the smooth Fish tagged in the joint estuary of the Rivers dome-shape characteristic of analyses involving Taw and Torridge were detected as they large numbers of fish. Rather they have a number migrated up the Taw by a series of recording of peaks or steps caused by coincidental "clumping" stations sited at strategic locations throughout of the occurrence of the catchment. Because of thç particular

Table 6.1 Monthly distribution of tagged fish and ALS records.

Station Fish tagged d/s Newbridge d/s Chap Yeotown u/s Chap Umberleigh

Years 93-94 92-94 93-94 92-94 93-94

Distance km -10.0 to-18.0 0.0 2.7 4.8 6.0 8.4

May 17 6 9 4 5 2 june 50 12 8 14 16 11

July 108 30 29 25 21 19

August 48 8 12 13 21 12

September 38 8 13 17 20 6

October 0 1 4 0 8 0

November 0 3 5 3 0 0

December 0 0 0 1 0 0

Total 255 68 80 77 91 50

R&D Publication 4 51 1.0 •d/s Newbridge d/s Chapelton

Yeotown

•Umberleigh

8 10 12 14 16 18 20 Flow m3/sec

Figure 6.2 Migration indices for ALS on the Taw.

fish passage, a near-inevitable phenomenon of a relatively small sample size of fish spread over a wide range of flows. For example, for the lowermost stations there is an initial peak caused fish-Hows by a disproportionate number of fish running at — all-flows flows between 2 and 4 m3/sec. Examination of the data indicates that this step is due to the occurrence of two distinct groups of fish running within short periods, viz. July 3 1992 and July 7-8 1993. The step is more pronounced on the d/s Chapelton site as this covers both groups of fish, whereas the other sites were not operating in 1992 and thus were not influenced by the earlier group. These bursts of activity were due to a combination of a good number of recently tagged fish being available to migrate, and conditions that were clearly conducive to migration. In the case of the earlier period (July 3 1992) the fish moved on the ascending limb of a minor spate in an otherwise low-flow period. In the latter case, the fish moved on the descending limb of a major period of high flows.

In any event the multiple peaks or steps appear to be artefacts of the limited data set, and do not represent a true statement of the relationship between fish movement and flow. The

Figure 6.3 Flows (gauged dmf) when fish were caught by anglers in Lower River Taw, smoothing process inherent in the fitting of the May to September 1992-94. Upper: cumulative frequency of dmf values (all-flows) polynomial lines overcomes most of the problem, and of flows used by fish (fish-flows). Lower: Difference in cumulative frequency of all-flows and fish-flows. but it is likely that a larger, more randomly

R&D Publication 4 52 distributed data set would give a somewhat The location of these sites in relation to the different picture. The migration indices derived tagging sites is shown in Figure 6.4. The for each station are shown in Figure 6.2. monthly distribution of fish tagged, and records of passage at the four recording stations, are The Taw is unique among the six rivers being shown in Table 6.2. considered here in having no weirs at the tidal limit or in the lowermost reaches. This may partly explain the similarity between the migration indices for the sites analysed. Chivenor

6.2.4 Flow and angling success

Daily rod catch records for the years covered by Estuary FiWngton the tracking are available for the Lower Taw (Mole confluence to the sea) from a concurrent statistics gathering exercise. Catches for May to September, 1992 to 1994, are analysed using the cumulative frequency method in Figure 6.3. The threshold flow (fish catch index = 1.0, peak in lower graph in Figure 6.3) is about 6.0 m3/sec, a little lower than the threshold flows for the migration index. The relationship between migration flows and flows associated with angling success is discussed further in Section 9.5. 5 km

X = recording stations 6.3 River Torridge 1" = netting sites where fish released

6.3.1 Description of the results

The entry of tagged fish to the river, and their progress upstream, were recorded by a series of automatic listening stations sited at strategic locations throughout the lower reaches of the catchment. Because of the small number of fish entering the Torridge and the steadily Weare Giffard - tidal limit diminishing numbers recorded by the further upstream stations, quantitative analysis is only possible for the lowermost four sites on the Beam Weir Torridge. These were: TR7

• TR6. 1.4 km upstream of the tidal limit at Weare Giffard. Torrington

• TR7. 3.0 km upstream of the tidal limit.

• TR8. Situated at Taddiport, 5.1 km upstream of the tidal limit TR39*

• TR9. Situated at Torrîngton CS, 6.6 km upstream of the tidal ümit. In 1994 this station was replaced with one at Darkham

Wood (TR39), 7.3 km upstream of the tidal Figure 6.4 Map of lower Torridge and its estuary. Indicating features of Interest. limit. The data from these two stations are treated together for this exercise.

R&D Publication 4 53 Table 6.2. Monthly distribution of tagged fish and ALS records.

Fish tagged TR6 d/s Beam TR7 u/s Beam TR8 Taddiport TR9/39 Weir Weir Torrington

Distance km -18.0 to -26.0 + 1.4 +3.0 +5.1 +6.6/7.3

Years 1992-94 1992-94 1992-94 1992-94 1992-94

May 17

June 50 2

july 108 4

August 42 7 4 2

September 38 10 20 12

October 0 2 3 2

November 0

December 0

Total 255 36 28 36 25

6.3.2 Flows used for migration low flows, and the flows associated with the first fish and the migration index reaching a value of The data were analysed using the flow frequency 1.0 are lowest for the lowermost station and method described in Section 2.6. Only data for increase steadily upstream. In fact this upstream the months June to September were used, as phenomenon is stronger in the few kilometres of discussed in Section 2.6.2. The 15-minute records the Torridge than on any other river in the study; for flows recorded at Torrington GS were used for this is discussed further in Section 8.3. However, all flow analyses. the close similarity between the index lines for the station upstream and downstream of Beam The migration index lines derived for each of the Weir (TR6 and TR7) indicate that this structure is ALS sites are shown in Figure 6.5. As for the not a major obstruction to migration. other rivers fish migration is under-represented at

Figure 6.5 Migration indices at tracking stations on the lower Torridge.

R&D Publication 4 S 4 7 THE RIVER TAVY STUDY

7.1 Introduction and Weir Quay (c. 10 km) were recorded entering the Tavy; their numbers are included in This study was undertaken by NRA staff between the ALS totals in Table 7.1. In turn, 24 of the fish 1991 and 1995 as part of the Roadford Water tagged in the Tavy estuary were recorded Resource Scheme investigations in 1991 and ascending the Tamar. 1992, and subsequently as part of the low-flow investigations on the river. The number and distribution of recording stations varied between years. Those located in Each year between 1991 and 1995, live salmon the upper reaches and tributaries generally were purchased from the commercial netsmen in recorded few fish, and quantitative analysis of the the Tavy estuary, fitted with radio transmitting flows favouring migration there is not possible. tags, and released. Their progress into and up However, six of the lowermost stations which the river was monitored by a series of automatic recorded the largest numbers of fish had a similar listening stations at strategic points throughout distribution each year. The location of these ALS the Tavy catchment. The location of the netting sites is shown in Figure 4.1. The monthly sites and the ALS sites is shown in Figure 4.1 in distribution of records of tagged fish passing each the section describing the Tamar study. station is shown in Table 7.1.

The numbers of fish tagged and released are The considerable fall In numbers between TV4 shown by month in Table 7.1. Also shown is the (45 fish) and TVS (19 fish) is largely explained by total declared net catch by month; comparison fish ascending the River Walkham, a major with the numbers tagged indicates that the spawning tributary, which joins the Tavy close to tagged fish were a fair representation of the the TV4 site. commercial catch. The netting area is situated at Maristow, about 1.5 km seawards of the tidal As discussed in Section 2.6.2, detailed analysis of limit at Lopwell Dam (Figure 4.1 ). In addition, migration and flows Is limited to the period June 25 salmon tagged in the estuary of the Tamar in to September. the vicinity of Cotehele (c. 15 km from Lopwell)

Table 7.1 Monthly distribution of tagged fish and ALS records.

Total Net Fish TVId/s TV2 u/s TV 3 "UM Double TVS d/s TV6u/s catch tagged Lopwell Lopwell Ludbrook Waters Abbey Weir Abbey Weir

Distance km •0.5 0.5 3.25 7.75 14.5 15.3 Years 1991-95 1991-95 1991-95 1991-95 1991-95 1991-95 1991-95 1991-95

May 55

une 258 19 My 854 95 17 27 13 11

August 957 60 13 11

September 11 12 October

November

Dec-fan

Total 2,124 174(1) 34 62 50 45 19 18

(1) In addition, 25 fish tagged in the Tamar estuary were also recorded entering the Tavy.

R&D Publication 4 55 This does of course limit the number of fish This loss rate was highest at times of low flow records still further (Table 7.1) so that the whole and this Is discussed in detail in Section 8.2. data set becomes marginal for valid analysis. The results for tracking stations TV! - TV4 were analysed using the cumulative frequency method 7.2 Hydrometrie background (Section 2.6). It did not prove possible to The five years of study included a good range of undertake further analysis for the other stations flow patterns, including basically dry summers because the number of observations on fish with occasional spates (1992, 1994 and 1995) migration are too few, and continuous estimates and wet summers with frequent major flow of instantaneous flows are not available for these events and only short periods of low flow (1991 sites. Even for the lower ALS sites the numbers of and 1993). Flows in the lower river are affected fish are marginal for valid analysis. by HEP abstraction at Abbey Weir in Tavistock, and in addition residual flows to the estuary are It is important that reliable instantaneous flow affected by the water supply abstraction at the figures are available; attempts to use Ludbrook tidal limit at Lopwell (Section 11.6). Ludbrook GS values for Abbey Weir highlighted the rapidity CS flows (15-minute values) were used for the with which flow can change in a small spate river analysis of fish movement at TV3, TV4 and TV5. like the Tavy, particularly with added Analysis for TV1 and TV2 used values for residual complication of pulsed releases from Mary Tavy flow to the estuary, after the Lopwell abstraction HEP station (Section 7.2). Large errors can occur is taken in to account. Hourly values were if time-of-travel estimate for flows are only calculated by the Hydrometrics Section of the slightly wrong. If a valid model for Abbey Weir Agency. flows does become available, then it may be possible in the future to examine the tracking Another factor affecting flows throughout the results there, but with only nine fish observations length of river involved in these analyses is the for TVS and TV6 in the June - September period, pulsing caused by impoundment and release of such an exercise would yield only extremely water by the HEP operation at Mary Tavy, tentative results. upstream of Tavistock. Water is continuously abstracted from the river at Tavy Cleave and Hill The relationship between river flow and salmon Bridge, and is stored in two small reservoirs. migration index at each station analysed is shown Thus while no HEP generation is taking place, in Figure 7.1. Although two different flow generally at night, the river flow is depleted. records were used for these sites (Lopwell residual During generation, usually during the day, flow is and Ludbrook gauged), the naturalised mean enhanced downstream of the power station. The flows at these two points vary by only about 4% timing of flow changes.will clearly.be differentat from each other, so a direct comparison is valid. different distances downstream, but the use of 15-minute or hourly values for flow will generally A number of features are apparent. First, the ensure that appropriate figures are used. The lines for TV! and TV2 are very similar, indicating pulsing does have some implications for the a similar response to flows by fish passing these analysis offish migration however; this is two points. This suggests that the dam and fish discussed further in Section 7.3. pass arrangement at Lopwell represents a minimal obstruction to migration, and that fish arriving at the seawards side of the dam generally 7.3 Influence of flow on migration continue directly up into non-tidal water to be Of the 1 74 fish tagged in the estuary, only 45 recorded at TV2. Thus any impact of abstraction (25.9%) were recorded ascending beyond the operates on fish arriving at Lopwell Dam and tidal limit of the Tavy and five more (2.9%) were appears to have no influence upon actually briefly recorded at the tidal limit. Sixteen (9.2%) passing the dam once having arrived there. From were reported caught in the estuary nets (Tavy, the line forTV2 in Figure 7.1, it is apparent that Tamar, Lynher and one in the Dart), and 24 there is a flow below which no fish ascend (13.8%) were recorded in the Tamar. However, beyond the dam (about 0.80 mVsec), 83 (47.7%) were not seen again after tagging.

R&D Publication 4 56 4 5 6 Natural flow m Vsec

Figure 7.1 Migration indices at recording stations on the lower Tavy. and that at flows below 1.91 mVsec fish, arrive The curve for TV4 is rather similar to that for TV3, here at a rate that is lower than expected; such but the flows at which the first fish appear (1.57 flows obtained for about 59% of the time in June mVsec) and where the index reaches 1.0 (2.50 - Sept 1991-95. Second, the peculiar mVsec) are a little lower. This apparent decrease fluctuations in the two lines are suggestive of an in the impact of low flows as one moves artefact, due to either "clumping" of fish or flow upstream is unexpected, given the situation in events or of the curve-fitting process. It is an other rivers. The raw data certainly show some inevitable factor that, where the slopes of the fish moving at lower flows at TV4 than at TV3, lines are either very shallow or very steep and/or and this is likely to reflect the fact that most fish the numbers of fish are few, this methodology is migrate on a falling limb of a spate hydrograph, sensitive to the way in which the lines are fitted; a and fish "keeping going" between the two tiny change in the line can halve or double the stations will pass the upstream one on slightly slope value. The lines are very much less sensitive lower flows than prevailed when it passed the to such vagaries when the slope is around 45%. downstream one. The numbers of fish are also at the lower limit at which this analytical approach is feasible. There The phenomenon of pulsing of flows by release is no doubt, however, that fish movement is of stored water from the operation of Mary Tavy under-represented at low flows, and from the Power Station (see Section 7.2) may be data and analyses available, the "threshold flow" implicated here in either of two ways. First, the appears to be in the region of 1.9 mVsec. From rapid changes in flow as releases stop and start the raw data, it is apparent that 69.3% of fish may mean that the flow values used for the "fish were tagged at or below such flows, whereas flow" analysis may be inaccurate; the only 40.0% of fish arriving at TV2 did so under instantaneous flow value for Ludbrook CS is used such conditions. for both TV3 (at the gauging station) and TV4 (4.5 km upstream of the CS). Thus the TV4 flow The curve for TV3 is rather different to those for value is likely to be of the order of an hour out, the more downstream stations. The first fish depending on the prevailing discharge and thus appear at higher flows (2.07 mVsec) and the rate of travel. Around the time of migration index attains 1.0 at a higher flow (2.70 commencement of a release pulse, the value used m Vsec). for TV4 fish flows is likely to err low, and around

R&D Publication 4 57 the time of cessation of a release it is likely to err receding hydrograph when a release ceases, it high. may continue to migrate for a short while flows fall. It may thus pass TV4 on a lower flow than The second way in which pulsing may affect the would normally be the case on a natural receding situation h through the behavioural responses of flow that would be falling more slowly, the fish. If a fish is migrating on a naturally

R&D Publication 4 58 8 EFFECTS OF RIVER FLOW AND OTHER ENVIRONMENTAL VARIABLES ON MIGRATION AND SURVIVAL

• Elevated flows in the autumn are associated 8.1 Introduction with large-scale movements of fish In this section the observations on factors throughout the catchment into the upper influencing migration in the six rivers are reaches - the secondary phase of migration. compared and contrasted to assess commonality A very dry autumn and early winter (e.g. of influences which control migratory behaviour 1990, 1994) may be associated with a in different situations. The rivers involved significantly truncated spawning distribution. represent a wide range of types and sizes of water-course, even though they do not These and other features of migration in the six encompass the whole spectrum of salmonid river rivers are explored further in the remainder of types in, for example, the British Isles as a whole. this section.

It is immediately apparent that the pattern of 8.2 Entry to the river migration in the six riversi s broadly similar in the following ways: 8.2.1 Fate of the tagged fish

• At very low flows, fish migration through the The last known locations of the tagged fish in upper tidal reaches and into the river is each study are shown in Table 8.1. Entry to own under-represented. Fish delayed passing the river is taken to be passage past the tidal limit, or tidal limit by low flows are less likely to in the case of the Avon, passage past the ALS contribute to the in-river stock than those situated 1.2 km seawards of the tidal limit. A achieving prompt passage. small number of fish, having entered the rivers, left them again and were recorded elsewhere • Above a certain level of flow, entry to the before the spawning season; such fish are river is positively stimulated, i.e. most fish excluded from the "own river" category and are arriving in the estuary at such times will pass included in the appropriate category. the tidal limit with minimal delay, and some fish delayed by earlier low flows will be Of the 1,514 fish tagged in river estuaries, only stimulated to migrate. 851 (56.2%) were recorded as entering and remaining in that river or another sharing a • Progress upstream is dependent upon common estuary. Some of the other fish were progressively greater relative flows. Thus in recorded as being captured by net in the same or periods of low to medium flow, most fish another estuary, or as entering other rivers entering the river tend to remain in the indicated by rod catches, ALS sites or being lower reaches, while in high flows more will found dead. However, 161 (10.6%) were migrate to the upper reaches. recorded only in the tidal reaches of the river or other nearby rivers, and 395 (26.1%) were not • Summer spates are of considerable seen again at all after tagging. importance for upstream migration, especially at times of otherwise low flow. The two categories (not seen again, and recorded only in tidal reaches) should be considered • Fish tend to move upstream, subject to together, as it is likely that the variation in the adequate flows, for a period of a few weeks relative magnitude of the two groups between after entering the river. This is termed the rivers and between years is partly a function of primary phase of migration. They then tend the thoroughness of the surveying of the tidal to remain laid-up until later in the year. reaches for tagged fish.

R&D Publication 4 59 Table 8.1. Fate of the radio-tagged fish - location of last known records

Notes Avon/ Exe Tamar Taw/ Tavy Totals Stour Torridge Years 1986-90 1991-94 1986-94 1992-94 1991-95 1986-95 255 174 1514 Fish tagged 437 318 330 (100%) (100%) (100%) (100%) (100%) (100%) Own river(s) 308 171 132 155 46 811 (non-tidal) (70.5%) (53.8%) (40.0%) (60.8%) (26.4%) (53.6%) Own river(s) 43 47 45 12 5 152 (tidal only) (9.8%) (14.8%) (13.6%) (4.7%) (2.9%) (10.0%) 12 83 Local net catch 28 23 20 n/a (6.4%) (7.2%) (6.1%) (6.9%) (5.5%) 15 40 Nearby river, n/a n/a 25 n/a non tidal (7.6%) (8.6%) (2.6%) Nearby river, 0 9 9 n/a n/a n/a tidal only (0%) (5.2%) (0.6%) Distant river (rod, 2 1 2 3 0 8 found dead) (0.5%) (0.3%) (0.6%) (1.2%) (0.0%) (0.5%) 6 Distant river(ALS) 6 0 (1.4%) (0.4%) 1 2 0 4 10 Distant net 3 (0.2%) (0.9%) (0.6%) (0.0%) (2.3%) (0.7%) 84 Not seen again 49 73 104 85 395 (11.2%) (23.0%) (31.5%) (33.3%) (48.3%) (26.1%)

Notes

a) Only includes fish captured, tagged and released in the estuary. This is all fish in most studies, but excludes the trap-caught fish on the Tamar. b) Last known records only - some fish entered the river but are known to have subsequently migrated elsewhere and are recorded in other categories. c) Last known records only. Excludes net catch (recorded below) but includes fish found dead. d) Includes estuary of river where tagged; Tavy includes Tamar and vice versa. e) Tamar - 25 in Tavy. Tavy - 15 in Tamar. f) Tavy - 9 in Tamar. g) Avon; one by rod on Piddle and one by rod on Test. Exe - one by rod on Teign. Tamar - one by rod on Lynher, one found dead on Lynher. Taw/Torridge - one by rod on Slaney (Eire), one by rod on Towy (Wales) and one by rod on Camel (Cornwall). h) Avon; six by ALS on Frome and Piddle. i) Avon - one on Exe. Exe - two on Teign, one on Tamar. Tamar - two on Lynher. Tavy - three on Lynher, one on Dart. j) No record whatsoever after tagging, n/a Not applicable

On the Avon and Exe there were regular, reaches without being recorded, though the extensive searches of the estuary area at low tide, topography of the Tavy estuary means that few so that most fish remaining there for longer than fish are likely to have done so there. a few days are likely to have been recorded. On the other rivers fewer such surveys were The likely explanations for fish failing to be conducted, and most of the "tidal reaches only" recorded passing the tidal limit are records were obtained from ALS sites in tidal water, and reports of fish found dead. The array • transmitting tag failure of ALS sites on the tidal Tamar was quite extensive and numerous tagged fish were • tags regurgitated in the estuary or tidal recorded there. However, many of the "not seen reaches again" fish on the Taw and Torridge may have resided for considerable periods in the upper tidal

R&D Publication 4 60 o fish caught by legal or illegal netting in the capture, handling and tagging, and unreported estuary or tidal reaches but not reported recapture with a very minor contribution from tag failure and regurgitation. In the Avon study it • the fish died without entering the river was concluded that this represented a loss of the order of 20% of fish tagged. However, the fact • the fish return seawards and entered another that the tidal losses are much higher at lower river undetected. flows (more than double in some cases) indicates that one or more of the causes of loss increases or Tag failure is not likely to explain the major that an additional source of loss operates at such fraction of fish not detected again. If it were so, times. This is discussed below. about a quarter of tagged fish caught by anglers would contain failed tags. In fact all but one of 8.2.2 Influence of flow on survival to the 52 tags returned by anglers were working. enter the river Similarly, if regurgitation of tags was a major feature then many of the externally tagged fish From the descriptions of the individual studies, it that were reported by anglers would have is clear that relatively fewer tagged fish enter the contained no radio tag. While occasional river promptly when flows to the estuary are low, externat tags were returned without radio tags, in though some may subsequently enter when flows every case where the stomach was opened, a increase. However, in all cases the proportion of radio tag was recovered. Thus tag failure and fish entering the river at any time falls as flow at regurgitation can be ruled out as dominant the time of tagging falls. This phenomenon contributors to the apparent tidal loss rate. warrants further examination. It must be borne in mind that a wide range of environmental There is undoubtedly a level of illegal fishing variables may be linked with low flows including activity in many of the estuaries, and it is likely water temperature, water quality, lack of spates, that some legally netted recaptures of tagged fish and characteristic weather patterns. It is not were not reported. However, the overall tidal possible always to identify which of these loss rate of 36.1% exceeds the reported local net associated factors is responsible for behavioural recapture rate of 5.5% by a factor of 6.6. responses and survival of the fish. Unreported catches of anything approaching the level required to explain a significant part of the In Section 3.3.2 it is concluded that virtually ail tidal loss rate are virtually inconceivable. Avon salmon entering Christchurch Harbour at residual flows in excess of 9 m Vsec survive to A significant factor is believed to be fish enter the river, the great majority doing so within migrating from, and returning to, other rivers. ten days. Of Avon fish entering Christchurch The reporting of seven tagged fish caught by Harbour at residual flows of less than 9 m'/sec, rods and ten by nets in other rivers, where overall only 63.9% enter the river, about two-thirds exploitation rates are likely to be of the order of within ten days and one-third in the autumn. At 20-30% and tag reporting rates unknown, residual flows below 5 mVsec, only about 30% of support this explanation. Further evidence is fish survive to enter the river, virtually all when provided by the recording of six Avon tagged fish flows increase in the autumn, many weeks or in the Frome and Piddle by ALS deployed there in even months after tagging. 1988-90. The situation on other rivers is more difficult to It is likely that a few fish die as a direct or indirect quantify, because in contrast to the Avon, result of the trauma or injury of capture, handling residual flows fluctuate widely on a day-by-day and tagging, and that this is a minor contributor basis. Thus a fish tagged at a time of very low to the tidal loss rate. flow may enter the river a few days later on a high flow. However, it is clear that prolonged At times of good migration conditions (i.e. periods of low flow are associated with an medium to high flows) it is concluded that the increased tendency for fish to remain seawards of tidal losses are a combination of fish migrating to the tidal limit, and that this behaviour is other rivers, fish dying as a result of the trauma of associated with a decreased likelihood of

R&D Publication 4 61 subsequent entry to the river. The summers of late in the year, downstream within the year of 1991-93 were unremarkable in flow terms on the tagging, i.e. they were repeat spawners when Exe, with fairly frequent spates interspersed with recaptured. periods of lower flow. Of the 239 fish tagged on the Exe in these years, 143 (59.8%) were It is therefore concluded that many of the fish recorded upstream of the tidal limit. This delayed in entering the river by low flows do not contrasts with 36.7% in the dry summer of 1994, enter fresh water at any time. From the evidence even though delays and losses did occur in the of the tracking programmes it appears that half earlier years associated with periods of low flow. or more of a year's run of fish can be lost in this way in a drought year. This shocking conclusion On the Tavy, in the years 1991 to 1993, 63.6% clearly has major significance for management of of the fish tagged in the estuary were water resources and for salmon fisheries subsequently recorded entering the Tavy or management in general. Tamar. In drought years of 1994 and 1995 the proportions fell to 42.9% and 30.9% The actual cause of death is not obvious. There respectively. are few predators of adult salmon in the estuaries of SW England, though grey seals are It is therefore apparent that the delay in entering occasionally observed within the netting zones. the river at times of protracted low flow is While significant fish kills are apparent in the associated with a considerable reduction in the Tamar estuary in drought years and occasional numbers of fish that succeed in entering the river dead salmon are found elsewhere fairly compared to average and wet years. There are frequently, there is little evidence of mass three possible exptanations:- mortality of thousands of salmon in most estuaries. Examination of catch statistics and • At times of low flow, the mix of fish in the observations made during these investigations estuary is different, with a greater proportion show that legal net catches are not elevated at offish belonging to, and subsequently times of low flows; this is consistent with the returning to, other rivers. conclusions of the Salmon Advisory Committee (1990). Nor is there evidence of increased illegal • Fish delayed in entering the river return to netting at such times. At times of prolonged sea and return to the river in later years. drought the nets sometimes start to catch coloured salmon which the netsmen call • Fish delayed in entering the river die without "droppers", and observations of tagged fish on contributing to the spawning stock in any the Exe in 1994 confirm that these may indeed river. be fish returning seawards from the upper estuary. These fish appear unhealthy, with very Small numbers of tagged fish are indeed reported soft flesh. It would appear that, having missed from other rivers, but there is no evidence that the "physiological window" for passing into fresh this is a greater phenomenon at times of low water soon after entering to the estuary many flow. However, the numbers of such reported fish lose the ability to make the transition at all. recaptures are small and it is not possible to state Presumably they return to sea and die. categorically that increased numbers of fish returning to other rivers are not responsible for 8.2.3 Migration indices at tidal limit - part of the apparent loss of fish. comparison of rivers

There is no evidence offish delayed in entering The migration index lines for arrival at the tidal the river returning in subsequent years - the limit on each river are shown in Figure 8.1. The occurrence of external tags would indicate this. Tamar is not included because of the difficulties While three tagged fish have been reported experienced with line-fitting to that particular recaptured in subsequent years (two on the Avon data set - see Section 4.2.4. The essential and one from the Taw; see Section 9.4), all were statistics are provided in Table 8.2. recorded passing upstream and subsequently,

R&D Publication 4 62 2.0

1.5

1.0

0.5

0.0 M 100 150 200 250 300 350 400 Flow M % of fMturalljed Q95

Figure 8.1 Migration indices at the tidal limit of five rivers (Tamar excluded).

Table 8.2 Characteristics of the migration index for arrival at the tidal limit of the six rivers.

River Location Q95 ADF Actual lowest fish-flow Predicted first fish Index = 1

Flow % Q95 Flow %Q95 Flow % Q95

Avon d/s Knapp Mill 6.97 19.85 4.43 63.6 6.19 88.9 7.07 101.4

Exe St james Weir 3.45 23.48 2.97 87.4 3.14 91.0 5.52 160.0

Tamar Cunnislake Weir 1.77 21.01 0.86 48.6 n/a n/a 2.30 129.9

Taw d/s Newbridge 1.21 18.64 1.20 99.2 1.20 99.2 11.50 950.4 Torridge d/s Beam Weir 0.98 15.34 0.96 98.0 0.78 79.6 2.78 283.7

Tavy Lopwell Dam 1.21 7.22 0.74 61.2 0.80 66.1 1.91 157.9

There are a number of features of these indices though this is in fact the highest threshold flow in that warrant comment. First, as is apparent from terms of mVsec, reflecting of course the very Table 8.2, the computed index line is an high Q95 value. The Tamar is close behind at imperfect predictor of the flow at which the first 129.9%. The Torridge is considerably higher, at fish actually migrates. This arises because the 283.7%, probably reflecting the very low Q95 line-fitting exercise, as described in Section 2.6, is relative to the ADF. The Taw at 950.4% stands not very precise at the extremes when just one or out as being quite different to the others. The a few fish are involved. In most cases the actual slope of the index line is very flat, so that the lowest flow used by fish arriving at the tidal limit value is in fact higher than some of the other was in fact close to the lowest flow recorded rivers at low flows, but it approaches a value of during the investigations, though of course 1.0 only very slowly (see Figure 6.2). There is salmon migration was greatly under-represented some uncertainty however regarding the at such flows. Thus there was little evidence reliability of the River Taw line - see overall that the lowest flows recorded in these Section 6.2.3. studies represented a complete bar to fish migrating up to the tidal limit. The relationship between the size of the river at the tidal limit (represented by the naturalised The Avon migration index reaches a value of 1.0 at the lowest multiple of Q95 flow (101.4%)

R&D Publication 4 63 both parts of the graph the threshold flow for the Exe appears high. As discussed in Section 5.4.4, this is likely to be due to the flows over St James Weir being depleted by the mill stream take, so Avon that the flows experienced by the fish approaching Salmon Pool are in fact significantly lower than the total flow values used for the Torridge calculation. Unfortunately there are no reliable measurements or estimates of the flow down the mill stream, during the study but at low flows it was a significant part of the total residual flow at the tidal limit.

The nature of relationship between river size and threshold flow at the tidal limit discussed above Torridge can only be considered as tentative, being based on just five river catchments. Further, there is likely to be considerable variation between rivers f S Exe of similar size (Q95 flow) due to, inter alia, the I ISO 3 following variables:-

£ 100 Tamar • the length and topography of the tidal reach

• channel dimensions and availability of "safe" and "comfortable" areas for fish to remain at

Nituntwd QfS (m'/wt) various states of tide and river flows

• the salinity profile Figure 8.2 Relationship between the size of river (represented by the naturalised Q95) and the migration threshold flow expressed as a volume (upper) and as a percentage of the Q9S (lower). The Exe is Ignored for line-fitting (see text). • water quality.

Q95 flow) and the threshold flow for migration Nevertheless, If the results from tracking studies are shown in Figure 8.2. In the upper part of the and other investigations of fish migration are to figure the threshold flow is represented in flow be extrapolated to other rivers, development of a terms (m3/sec) and in the lower part as a multiple predictive mode! is necessary, it is strongly of the Q95. In both graphs the value for the Taw recommended that all available results of has been excluded because of uncertainties over appropriate studies in the British Isles are its reliability (see Section 6.2.3). analysed using the approach developed in this investigation, so that they can be incorporated Considering first the upper part of Figure 8.2, if into a developing model. Appropriate studies the threshold flow was a constant proportion of will include radio tracking investigations, and the Q95 the graph would be a straight line with a trapping and automatic counter studies slope of 1.0. If it were a constant absolute flow conducted at or close to tidal limits. Each river value (e.g. 2.0 mVsec) in all rivers, the line would analysis should take account of local factors that be horizontal. The line appears to lie between may influence the relationship between river flow these two scenarios. This is perhaps more clearly and migration, as listed above. seen In the lower half of the figure; the line approximates to horizontal for the larger rivers 8.3 Changes in migration indes (i.e. the threshold flow is a constant proportion of upstream the Q95), but becomes much steeper for the smaller rivers,indicatin g that there may be a As noted in the description of the individual lower limit to the absolute value of flow on which studies, migration past more upstream ALS sites salmon will enter any river, e.g. 1.0 mVsec. In is associated with increasing relative flows. In

R&D Publication 4 64 order to compare different rivers and different movements (typically May - September). While sites, the flows as a % of the appropriate Q95 it is immediately clear that the relative flow tends when the migration index first reaches a value of to increase as one move upstream, it is also clear 1.0 (an indicator of positively favourable that the rivers exhibit different patterns. The Taw migration conditions) for each site on each river is not included here because of doubts about the are shown in Figure 8.3. The data and analyses reliability of the threshold flow values that were used here relate specifically to summer derived (see Section 6.2.3).

800

700

600

.500

*400

300

200 •

100

o J- -5 10 15 20 25 30 35 40 45 50 55 60 65 Distance from tidal limit (km)

140

120

100

80

s 60

40 •

20

0 -»- -5 10 15 20 25 30 35 40 45 50 55 60 65 Distance from tidal limit (km)

Figure 8.3 Upper: The relationship between the threshold flow as a percent of Q 95 and distance upstream. Lower: The slope profiles of the rivers.

RfirD Publication 4 65 The Exe is considered first, as it lies in the middle terms of dimension (the mean flow near the top of the range of slopes and is the result of an of this reach is about 80% of that at the tidal extensive study with many ALS sites covering 50 limit, compared to 34% for the 49.3 km reach on km of river. The general trend of the line is the Exe), and gradient. Most of the Avon from upwards, indicating that increasing relative flows Salisbury seawards is of gentle gradient and are required for good migration conditions. This moderate current, and apart from a few mill is likely to be because the river becomes smaller pools (see Section 3.5.1) salmon would have little and the gradient steeper upstream, such that low difficulty in ascending at most levels of flow. The flow conditions are increasingly difficult for safe shape of the migration index curve at the ALS site upstream passage. At the tidal limit and at 43.9 at Knapp Mill Gauging Station, falling from a km there are vertical segments of the line. This value of 1.0 at 112% of Q95 to zero at 101 % of reflects the difference in flows associated with Q95, means that at times of low flow (around arrival at, and passage past, obstructions to Q95 and below) fish accumulate in large migration in the form of weirs, and is discussed in numbers in the lowermost non-tidal and tidal detail in Section 5.5. Two other segments of the reaches. However, at flows greater than 112% of line have a negative slope. This may reflect easier the Q95 fish distribute themselves well migration conditions upstream of a particular throughout the 60km of river between the tidal weir, such as that at 43.9 km. It may also partly limit and Salisbury. This situation is discussed reflect the tendency of fish to migrate on falling further in Section 3.4.1. flows following a spate, largely because the falling limb of a hydrograph is generally of The situation on the Torridge represents the considerably greater duration than the rising other extreme from the Avon. The value for the limb. Thus a fish passing point A will tend to do threshold flow, in terms of multiple of Q95, is so on a slightly higher relative flow than when it much higher, and the slope of the line is much passes part B some distance upstream if it "keeps greater than in equivalent lower reaches of the going" between the two. This effect will be other rivers. This is a reflection of the very low greatest where the channel size and slope do not Q95 flow (7% of the ADF) compared to the Exe change very much between the two points, so (15%) and Avon (35%), and the more "upland" that the "upstream effect" does not over-ride the nature of the lower reaches of the river. "falling flow" effect. The Tamar line is basically in three sections. The The phenomenon of migration with distance stations around the tidal limit and in the ponded upstream being associated with increasing section upstream of Gunnislake Weir (0.0 - 1.5 relative flows leads to a differential pattern of km) have similar low values. There is then a dispersion of fish through the catchment in wet sharp increase between 1.5 and 4,9 km, and dry years. This is clear from Figures 5.3 and reflecting the change in character to a steady- 5.4. In 1994, a dry year (Figure 5.4) most fish flowing large river. Similar values obtain up to spent most of the season up to mid-September in 21.9 km, reflecting the rather similar character of the lower reaches. In 1992 (Figure 5.3), high the river throughout this zone. The last point for flows from mid-August distributed most of the 26.7 km is for Lifton Park on the River Lyd, the fish beyond the upper-most ALS on the river main spawning tributary. The sharp increase in (49.3 km). The distribution of angling catch also threshold flow reflects the transition to a smaller, clearly reflects this pattern of behaviour. fast-flowing upland stream. It must also be borne in mind that most of the movement in the Lyd The Avon clearly represents an extreme among falls into the secondary migration phase. this group of rivers, as the slope of the line in Figure 8.3 is very shallow, indicating a minimal Only a short stretch of the lower Tavy is included "upstream effect". This is partially a reflection of in this analysis, but there too it is clear that the the high Q95 flow for the river, being of the threshold flows are considerably higher order of 35% of the AOF, in contrast to the Exe upstream. where the proportion is about 15%. It also reflects the similarity in channel characteristics Also shown in the lower part of the Figure 8.3 are throughout the 60.4 km study length both in the slope profiles of the reaches of the rivers

R&D Publication 4 66 covered in the upper part of the figure. These of months may occur in a few days during and are plotted as the distance upstream for each shortly after one or two minor spates. increment in elevation of 25 feet (7.63 m), as derived from 1:25,000 O.S. maps of the rivers. It In the Avon study, a number of tagged fish is immediately apparent that the pattern is rather ascended beyond Knapp Mill in the five days similar to that of the change in flows at which the following a thunderstorm and discharge increase migration index = 1.0 in the upper part of the in july 1989. These represented 44% of the figure. The Avon has the most gradual slope tagged fish (and it is assumed an equivalent part throughout the 60+ km of interest, and the of the stock that the tagged fish represented) Torridge has the greatest slope in its lowest few that passed this site between April and the end of kilometres. The Exe and the other rivers lie the angling season that year (see Section 3.4.2). between these two extremes. On the Exe, only 14 tagged fish passed the North Bridge abstraction point in 1992 before heavy The clear conclusion that the relationship rain triggered large scale movements from mid- between river flow and migration changes August onwards. Of the 14 salmon, 11 (78.6%) markedly along the length of the river, and is migrated on two days on a minor spate in mid different between rivers, has major implications july. On the Tamar, the only tagged fish passing for examining the impact of water resource upstream out of the ponded lowermost reach of management and for fisheries management in the river between mid-July and earfy October general. Clearly the potential impact of any 1990 did so in a two day period associated with a abstraction with particular operating rules could minor spate in mid-August. The largest single- be quite different depending upon its location. day's trap catch in the investigation at Gunnislake (115 fish) occurred on a minor spate in August in As is suggested in Section 8.2 for migration the drought year of 1989. An even more around the tidal limit, careful analysis and extreme example comes from a trapping synthesis of all available data sets of observations investigation on the Girnock Burn, an important on salmon movement upstream is strongly spawning tributary of the Aberdeen Dee (Buck recommended. Of greatest value will be and Hay 1984). Here the whole run of fish for situations where observations are available for a the year entered the trap in a single 6 hour number of different locations within a river period in early November 1989, having been system, such as two or more fish counter sites held up by a protracted drought. and radio tracking studies, which would allow development of models for patterns of migration Examples of various options for protecting certain throughout different river types. Once validated, critical spates from any impact are discussed for such models could be used to predict the individual rivers in Section 11, and as a general relationship between flow and salmon migration principle in Section 12.2.4. on other rivers, to derive threshold flows and to examine the impact of existing or proposed 8.5 Effect of obstructions water resource schemes. They could also be used to focus further study onto areas where As described in Sections 5.5, ALS records for sites clarification is needed. This investigation should at or close to weirs and other potential be integrated with that on patterns of migration obstructions can be used to describe obstructions at the tidal limit. in terms of the time delay caused, and the flows at which fish approaching from downstream can 8.4 Effect of spates and cannot pass without delay.

In all the rivers studied, spates are of critical Such observations were made during four of the importance to migration of salmon during the studies; on the Avon, Exe, Tamar and Tavy. summer. This is particularly so in dry years, when such events may represent the only occasions On the Avon (Section 3.5.1 ) it was noted that during a period of several months that the flow few of the weirs, mills and hatches represented a exceeds the threshold level for migration. Thus major obstacle during the primary phase of the majority of salmon movement over a period

67 R&D Publication 4 Table 8.3 Range of flows at which weirs on the Exe represented total and partial barriers to migration.

Site Distance from Complete barrier Causes delay to % of run tidal limit to movement migration experiencing delay

St James Weir 0.0 below 4.7 m Vsec 13%

Cowley diversion/weirs c. 5.0 4.0 - 13.2 m Vsec 17% Bolham Weir 32.2 3.4 - 4.4 mVsec 4.4 - 5.5 mVsec 12% Oakford Weir 43.9 4.4 - 6.8 mVsec 6.8 - 8.9 m'/sec 23%

migration. This is illustrated by the smooth, design criteria. In the case of the situation at gradual slope of the line in Figure 8.3 (upper Cowley, the problem appears to be caused by part). However, it was noted that two mill weirs the split of the river flow between two channels, (Bickton and Breamore) did represent major and one solution would be to concentrate most obstructions in dry autumn and early winter of the flow in one channel at times of low to seasons. This increased level of difficulty in medium flow, achieving passage is likely to have been due to the lower flows on which passage was being On the Tamar, observations were possible on the attempted, and the increased level of maturity or extent to which Gunnislake Weir at the tidal limit proximity to spawning of the fish involved. In represented an obstruction. This was done by the dry spawning seasons of 1989/90 and comparing the migration index from fish arriving 1990/91 it appeared that these obstructions at the weir with that for fish arriving at the ALS at contributed to a major truncation of spawning Hatch Wood, 0.3 km upstream. The migration dispersion (see Section 8.8). index reached a value of 1.0 at the same flow (2.3 mVsec) at each site indicating minimal flow- On the Exe, several weirs and water diversions induced delay due to the weir. Similar were identified as significant obstructions to observations were also made at Lopwell Dam on migration, while others that were previously the Tavy; here too the results suggested minimal thought to be possible problems proved not to flow-related delay actually at the weir. be at the flows at which fish were attempting passage. In Section 5.5 the situation is examined 8.6 State of tide in detail to establish the range of flows when the individual obstructions are having an impact Although the radio tagged fish cannot be tracked upon movement: Clearly at low flows when fish in salt or brackish water, they can be detected in are not approaching the area there is no the upper tidal reaches beyond the limit of saline problem; equally, when flows are high enough to influence. In most studies an ALS at or allow passage without delay there is no impact. downstream of the tidal limit allows examination No structures were identified that became a of the state of tide when fish arrive in the area. passage problem at high flows. In the Avon study, ALS were deployed both at The flow ranges when four problem sites the tidal limit and at a site 1.2 km seawards. The represented some obstruction to passage, and state of tide (hours after HW) of first records at the proportion of the total run of fish these stations is shown in Figure 8.4. At the experiencing delay, are shown in Table 8.3 lower site (Island ALS) there is a tendency for fish to arrive on the flooding tide, though all states of This approach can be used to identify the most tide are represented. At the tidal limit, only 1.2 severe passage problems and as an input to km upstream, the tidal "signal" has been lost, design of facilities to ease access. Many fish pass with fish arriving apparently randomly with designs work optimally only over a limited range respect to tide. The diurnal pattern is of head water heights (and thus river flows) and strengthened, however (Section 8.7). the above results would represent important

R&D Publication 4 68 Island AIS (3.4 tun) Aprif - fuly with similar numbers arriving on the flood tide and ebb tide. There does appear to be a

— Spring !ide tendency for fish to arrive late on the flood or — Nrjp Ilde ebb rather than early, however. £ f The numbers of fish involved in the other studies are too small to allow analysis of state of tide at arrival at the tidal limit. M tlO 1*11 11 II Uli

Thus while there appear to be patterns in the Tidal limit AIS {4.6 km) April - (uly state of tide, they vary on different rivers and indeed may do so in different parts of the tidal zone of individual rivers. It is likely that salmon z utilise the flood tide for much of the landwards } migration in estuaries and the pattern of timing of arrival at any point will reflect the "starting point" of the fish and the time taken to reach the 'S It-ll 11.11 particular ALS site.

It would be of interest to examine the pattern of Figure 8.4 State of tide at first record of fish arriving at ALS movement with respect to the neap-spring tidal on the Avon, April to July. cycle, as there is a common perception that greater movement through the estuary takes place on spring tides. Such an analysis is not valid with the results of these radio tracking projects because the date of arrival at the tidal limit is strongly correlated with the date of tagging, and the tagging dates were generally not randomly distributed around the neap-spring cycle. This arises because Spring high tides, for example, occur at a fixed time of day at any site. Obtaining fish from the nets proved most reliable Figure 8.5 State of tide at first record of fish arriving at St when the appropriate state of tide occurred in James Weir (tidal limit). River Exe, May to August. the early evening, and most of the licensed netsmen were operating. Thus there is a tendency for tagged fish to be concentrated on particular days of the neap-spring cycle. A more reliable indication can be obtained from fish counter results, which do not suffer from this potential bias. However, an analysis of daily salmon counts at the Gunnislake counter on the Tamar and high tide height indicated no relationship.

Figure 8.6 State of tide at first record of fish arriving at 8.7 Time of day Cunnislake Weir (tidal iimit), River Tamar, May to September. In the studies for which we have adequate data for analysis it is clear that there is a strong diurnal The situation at the tidal limit on the Exe is signal in the timing of arrival in the upper tidal shown in Figure 8.5. Here the tendency is for fish reaches and at the tidal limit; (Avon, Figure 8.7; to arrive shortly after high water, though again all Exe, Figure 8.8 and Tamar, Figure 8.9). states of tide are represented. The pattern at the Movement is greatest at night, with peaks soon tidal limit on the Tamar is not clear (Figure 8.6),

R&D Publication 4 69 Island ALS -1.2 km Tidal limit 0.0 km

id 0 3 * » S 10 lî M H IS ÎO 7! 0 1 I t 4 5 r, 7 6 ft 10 ïi Ï; Il mi* ts i t U i" M H ?i Tan«

Duryard 4.9 km Tidal limit ALS 0.0 km

h-rm m I 1-Ü * & i ic i: u IÛ la JO :J 0 I 1 i • 5 6 1 t * Iff 1) II )» 11 13 It U II » 30 » II It Time

North Bridge Intake f Knapp Mil) CS. ALS 1.4 km 9.4 km

r

a ; * A 8 10 12 M 16 18 11) 22 0 I 2 I 1 i rt ! « f ID 11 \; M M IS 16 1/ I» 1« JO II IJ .'i Tunc

Stoodleigh C.S. All ALS upslream of Knapp Mill C.S. 35.3 km

t a 10 U M lb IS 20 22 0 I 7)1 S » ! t » 10 11 II II 11 li It II U H » Il II 21 Time TV"»

Figure 8.7 Time of day of first record of fish arriving at ALS sites on the Avon, April to |uly. Bnjshford 49.3 km

after dark and around dawn. Few fish move during the middle of the day. Analysis of the much greater data set from the Gunnislake counter allows the inter-relationship among time of day, flow and fish movement to be examined « i i ) i i B i • * io ii ïi ti ïi is H î' ÏI i* is a il n (Figure 8.10). It is clear that the diurnal pattern is greatest at low flows, and weakest or

altogether absent at high flows. Figure 8.8 Time of day of first record of fish arriving at ALS sites on the Exe.

R&D Publication 4 70 obstructions to migration that are more readily overcome during daylight. Stuart (1962) Tidal limit 0.0 km concluded that sight was an important sense for salmon ascending falls and weirs, and stated "the writer has never seen a fish attempt to leap up a fall in relative darkness and has been present on numerous occasions when leaping ceased

O i I J • 1 ( I 1 > IS 11 IJ 11 |l 11 14 If II It ID It II 11 abruptly, after a period of great activity, with a marked diminution in daylight". The Knapp Mill GS ALS on the Avon (Figure 8.7) is situated about 200 m upstream of a large weir. It is likely that Gauging Station 3.0 km fish arrive at the foot of the weir and numbers build up during the night, and that the peak of activity at the ALS around 07:00 hours is due to these fish ascending the weir after dawn.

There are three important conclusions to be

o > i i • i i i i i m ii u il i« it it il u i» jo JI a u drawn from these observations in terms of management of water resources. First, it may to be possible to reduce the impact of abstractions close to the tidal limit by diurnal modulation of Leigh Wood 13.8 km take during peak migration periods. For example it may be possible at times of low flows to cease abstraction for a few hours around dawn and dusk. This is considered further in Section 12.4.

Second, such an approach is much less likely to oi ( i < i « i g « ID n u il M il is i; m i> io Ii li il Tim. work for abstractions situated significantly upstream of the tidal limit as the relationship between time of day and movement is likely to —i Greystone Bridge 21.9 km be different. Third, observations made at a particular fixed location such as a counter, fish pass or trap should not be taken as a reliable indicator of the situation at other points on the river for, as we have seen, the pattern can vary radically within a distance of a few kilometres

; j I 10 n 17 11 Il It 17 11 I« 70 II II II (see Section 13.9). rtn>

8.8 Spawning distribution Figure 8.9 Time of day of first record of fish arriving at ALS sites on the Tamar, May to September. Observations on spawning distribution on the Avon, Exe and Tamar are made in the However, within a few kilometres upstream of the appropriate earlier sections of this report. What tidal limit the strong diurnal pattern has been is considered here is the extent to which flow either weakened (Tamar) or lost (Exe), or is may influence spawning distribution. replaced with a distinctly different one (Avon), often with any apparent peaks occurring during On the Avon (Section 3.5.2) it was concluded the hours of daylight. The loss of the diurnal that in the very dry autumns of 1989 and 1990, signal is likely to occur because migration takes the geographical distribution of spawning activity place at higher relative flows further upstream - was severely truncated due to low flows. Fish cf. the situation at Gunnislake counter discussed appeared to be particularly held-up by specific above. The occurrence of distinct peaks during mills and weirs (Sections 3.5.1 and 8.4). The daylight may occur as a result of major proportion of tagged fish located at

R61D Publication A 71 spawning time upstream of Breamore Mill (41.6 km from the tidal limit) averaged almost 70% in 0 - S m'/sec the "normal" autumns of 1986-88, but less than 1 9% in the two very low-flow autumns of 1989 and 1990. The impact upon salmon stocks of this limited spawning dispersion is unknown, but it is likely to have been a major factor in the poor runs and catches of the resulting year-classes in the Avon from 1992 onwards.

On the Exe and Tamar there was no evidence of 0 i 2 i s « 7 e 9 io n u u u is 16 u ia is m « JJ major differences between spawning distribution Tim* between years. High flows occurred each autumn of the study period allowing effective dispersion. A very dry autumn could cause problems for spawning dispersion however; an examination of the flow records for the period 1966-94 indicated four years (1972, 1975, 1978 and 1990) when part of the spawning migration past the Exe-Barle confluence was likely to have been delayed beyond the end of October; only in 1978 was November dry enough to raise some doubts about eventual spawning dispersion.

10 11 lî 11 11 15 16 17 IB 19 20 2\ 11 23 As no tracking was undertaken upstream of Tlm* Brushford on the Barle and Pixton on the Exe, or within the spawning sub-catchment on the Tamar, these results give no information on possible flow-mediated variation in spawning distribution in the upper catchment.

However, although there was no direct evidence i» of spawning distribution on the Exe being affected by low flows, the proportion of the stock surviving to spawn is influenced by river flow, in Section 8.2.2 it is concluded that the spawning stock may be halved in dry years relative to wet years. 3 * S Û ? S 9 10 11 U t] 14 IS 1& 17 IS 19 ÎO 71 21 21 Ilm«

Figure 8.10 Time of day of records of fish at the Cunnislake counter, May - October 1994 at different rivers flows.

R&D Publication 4 72 9 OTHER FISHERY MANAGEMENT ISSUES

9.1 Introduction (Eire), Towy (Wales) and Camel (Cornwall). In addition, a fish tracked back to the sea having In this section a number of aspects of the study spawned in the Mole (Taw tributary) in 1993 was are examined which are not directly related to recaptured in a net at Dingle (Eire) the following the relationship between migration and summer. environmental factors, but are nevertheless of fisheries interest. Of 36 fish radio tagged in the Fowey estuary between 1985 and 1987, four were recorded outside the area (Solomon and Potter 1988, 9.2 Fish going elsewhere Potter 1988). One was caught in a coastal In most studies involving tagging fish in estuaries, trammel net near Weymouth, 160 km to the a proportion of fish are reported subsequently east. Three were reported from the Tamar; two entering other river systems. In most cases this in the estuary nets, and one recorded by a diver involves the capture and reporting of the external in a tributary. tag by an angler or netsman - such fish will of course only be a small proportion of the fish Thus fish entering temporarily the estuary of a which really are returning to other rivers. In the river which is not their final destination is a case of the Avon study, however, ALS sites on widespread and frequent occurrence. It is adjacent rivers recorded some fish radio tagged presumably a normal part of homing behaviour. at Mudeford. Most of these "deviations" are fairly local, to adjacent or nearby salmon rivers. Even in the In the Avon study, a proportion of the fish tagged case of some of the apparently longer excursions (about 6.5%) entered the River Stour. As this has (e.g. Avon to Exe, Taw/Torridge to Towy and a joint estuary with the Avon, these fish were "on Camel) the other river involved was almost the course" for their home river. Nine of the 437 fish "next" major salmon river in that direction. Most tagged were recorded elsewhere; single fish of the radio tagged fish which went elsewhere caught in a net on the Exe, by rod on the Test did so without being detected in the river of and rod on the Piddle. In addition, three fish tagging, i.e. they did not penetrate beyond the were recorded on the Frome and three on the tidal limit. However, two of the fish tagged on Piddle by ALS equipment deployed there in the Avon that subsequently entered the River connection with a pilot-scale tracking project. It Piddle spent a considerable period in the was estimated that about 6.5% of the fish tagged freshwater reaches of the Avon near the tidal limit at Mudeford subsequently returned to rivers before returning seawards. other than the Avon and Stour. 9.3 Fish caught by anglers, and In the Exe study, four of the 318 fish tagged were population estimates reported outside the Exe system; two by nets on the Teign, one by rod on the Teign, and one by All the radio-tagged fish in these investigations net on the Tamar. were also fitted with an external anchor tag at the base of the dorsal fin. This was done so that There was considerable interchange between the any catches of tagged fish by nets or rods could Tavy and Tamar fish tagged in these estuaries, be reported. While not all captures of such fish with smaller numbers being recorded on the were reported (the field workers would Lynher which joins the lower part of the Tamar occasionally hear of a tagged fish having been estuary. One fish was recaptured in a net in the caught which was not fully reported) the patterns estuary of the Dart (Table 8.1 ). of catches are of interest and can provide a minimum estimate of exploitation rate. In the Taw/Torridge study, four of the tagged fish were recorded outside those river systems. Single In the Avon study, the number of tagged fish fish were reported caught on rods in the Slaney entering the river within the angling season, the

R&D Publication 4 73 Table 9.1. Estimation of yearly runs and exploitation rates of salmon in the Avon.

1986 1987 1988 1989 1990

Number tagged 76 86 99 101 75

Tagged fish enter river in season 57 65 68 52 32

Reported Rod catch 1,025 600 708 441 295

Reported Net catch 685 568 667 406 312

Reported Tag recaptures by rod 10 10 11

Estimated run into river in season 5,317 3,551 4,017 2,873 1,579

95% CL + or - 1,953 1,015 1,105 949 591

Each tagged fish represents 93.3 54.6 59.1 55.2 49.3

Total run at Mudeford to 30 Sept. 7,091 4,696 5,851 5,580 3,698

10 Total run into river 5,317 3,877 4,078 3,481 2,219 n Exploitation rate by nets 9.7% 12.1% 11.4% 7.3% 8.4% 12 Exploit, of available fish by rod 19.3% 16.9% 17.6% 15.6% 18.7%

13 Exploit, of whole river run (rod) 19.3% 15.5% 17.4% 12.7% 13.3%

numbers of tagged fish caught by anglers, and fish were caught by angling in the tidal reach of the total reported angling catch each year were the river. Because many of the fish were trapped used as inputs to a mark/recapture population and tagged at the tidal limit, and were thus not estimate. This calculates the total run entering subject to the tidal reach fishery, calculating an the river before the end of the angling season. overall rod exploitation rate is complicated, but From a consideration of how many fish each clearly it appears to be lower on the Tamar than marked fish represents, one can estimate the total on the Avon, Exe and Tavy. river run and the total number of fish entering Mudeford Run. In turn, estimates of exploitation On the Tamar, the mean elapsed time between rates can be derived. The inputs and results are entering the river (taken as first record at or shown in Table 9.1. approaching the tidal limit) and capture by rod was 32.6 days. However, many of the fish were Twenty (46.5%) of the 43 same-year recaptures caught while migrating or soon after stopping; of were made within 10 days of tagging, and 67% the 23 captures reported upstream of the tidal within 20 days of tagging. Thus most recaptures limit for which adequate details are available, 15 were made-within-the three-week-period during (62.2%) were taken within three days of passing wiiich the initial migration phase is contained the nearest ALS site. (Section 3.4.1). It is generally considered that fish are most vulnerable to capture while resting On the Exe, 19 (11.8%) of the 161 fish that during migration and for a few days after arrival ascended beyond the tidal limit within the in a lie, though fish actually travelling are less angling season were reported caught by anglers, prone to capture. These recapture results all in the same year as tagging. In addition, one support this view. The 43 fish reported caught fish was reported caught in tidal water. Although by rod represent 15.7% of the tagged fish some of the fish were caught some time after entering the river during the angling season. entry to the river, most catches were of fish that were migrating or had recently migrated a Two others were caught two years following significant distance. Of 16 fish for which tagging, i.e. as a repeat spawners (Section 9.4). sufficient details are available, 9 (56%) had been recorded passing the ALS situated downstream A total of 463 tagged fish ascended beyond the within the previous three days. tidal limit on the Tamar, of which 24 (5.18%) were reported caught. In addition, five tagged

R&D Publication 4 74 Finally, on the non-tidal Tavy 10 tagged fish were investigation allows a direct comparison to be reported caught by anglers, represented 14.08% made. In Section 4.4 and 6.2.4 the relationship of the 71 fish recorded passing the tidal limit. Of between flow and migration, and flow and the 8 fish from which adequate details are angling success, are compared for the Tamar and available, all but one was caught within three Taw. In both cases it is concluded that the weeks of entering the river. In addition, one relationships are basically similar, but the further fish was caught by rod downstream of the threshold flows when the angling index reaches a tidal limit. value of 1.0 may be lower than those when the migration index does so, especially in the 9.4 Fish recaptured in subsequent summer months. This is considered to be years because salmon are most catchable both while resting briefly during migration but also for a few Three radio-tagged fish were reported recaptured days after stopping migrating. Thus good in the years after they were tagged. All three had angling persists for longer on a falling been recorded extensively in the river in the year hydrograph than does large-scale migration. In of tagging, and are presumed to have spawned general, enhanced catchability appears to extend there before returning seawards. for one or two weeks after the end of the primary migration phase. Analysis of autumn catches Two fish were reported caught by anglers on the appears to give a threshold flow closer to that for Avon, each two years after having been tagged migration (Section 4.4), but the number of sites and tracked in the river. As the fish had grown analysed is too few for firm conclusions to be considerably in this period, it is assumed that drawn. Use of angling records alone, at face they had not returned to the river in the value, could thus give a biased assessment of the intervening year. They were thus exhibiting a flows used for migration. On the other hand, long-absence return habit rather than returning examination of the pattern of angling catches in one year later. space and time following small summer spates could be very effective in identifying the One fish tagged and tracked on the Taw was importance of such events both for migration recaptured in a coastal net near Dingle, Ireland in and angling success. July of the following year. On the assumption that this fish was returning to the river when Generally, the potential for use of angling success caught it was showing a short-absence habit. as a surrogate for migration is most important as it represents a low-cost and short time-scale 9.5 Angling success and river flow option, assuming that catch records are already available in adequate detail. Serious In the absence of direct migration information, consideration should be given to an investigation salmon angling success has on occasion been examining all situations in England and Wales used as a substitute for analysis with river flow. where catch success can be compared to direct In many rivers it is clear that at times of low flow measurements of migration made by trapping angling is poor in the middle and upper reaches counters and tracking studies to fully evaluate largely due to a shortage of fish in the area, so this approach. there is clearly some relationship. This

R&D Publication 4 75 10 MIGRATION MODELS

10.1 Introduction is limited, the allocation of these fish is not too critical and the following is adopted: 20 are Any smoothed, extrapolated or interpolated assumed to have gone to rivers other than the relationship between fish movement and flow Avon, 5 were caught but unreported by the legal derived from the results of the tracking studies net fishery, and 10 are explained by tag failure. does of course represent a simple model in its own right. In this section, development of two It is assumed that the fish recaptured by the nets more complex models is described, which and the tag failures would otherwise have shown incorporate a daily or weekly availability of fish to a distribution similar to the remainder. Thus for allow simulation of numbers of fish migrating for each 1,000 fish evading capture by the nets at any period for which appropriate flow figures are Mudeford Run at residual flows in excess of available. These models are of course specific not 9mVsec, 846 are Avon fish, and enter the river only to the particular river but also to a particular within 10 days, 24 are Avon fish, and enter the point on the river, as it is clear that the river in the Autumn, and 1 30 are fish from relationship between flow and fish movement elsewhere, which will return elsewhere. changes with location on the river (Section 8.3). The models are for salmon migrations in the At lower flows, the behaviour and fate of fish are lower reaches of the Avon (Section 10.2) and at increasingly influenced by residual freshwater the tidal limit on the Tamar (Section 10.3). Once flow (Figure 3.6). As flows fall, increasing constructed and validated, such models can be numbers of fish are either not seen again or are used to examine the impact of any existing or last seen in the harbour, and increasing numbers proposed future abstraction on fish movement. of fish delay entry to the river until the autumn. Of fish entering the river, increasing numbers remain for the summer in the tidal reach or 10.2 Avon migration model downstream of the Knapp Mill abstraction until the autumn (Figure 3.7). 10.2.1 Inputs to the model

In Section 3.3, it is noted that at residual flows As discussed in Section 3.3.2, it is suggested that (flows to the estuary) in excess of about 9mVsec, the steady "disappearance" rate for fish tagged at the pattern of entry to the river appears to be flows in excess of 9m3/sec represents the rate at independent of discharge. The pattern of which tags fail, fish die as result of capture and "behaviour" of the 281 fish tagged at such flows handling, and fish go to rivers other than the is described in Section 3.3.2. Avon. The same allowance is therefore made in the proportion of "disappearing" fish at lower A number of assumptions and decisions are now flows, and they are re-apportioned as above. The necessary. Of the 20 fish not "seen" again after excess "disappearance" rate is assumed to be tagging, some are explained by tag failure, some caused by death in the harbour, or return by tagging or handling mortality, some by seawards without subsequent entry to the river. natural mortality, some by unreported capture in the harbour (legal or illegal), and some will have 10.2.2 Relationship between flow and fish gone to other rivers. It should be noted that, movement although a number of fish were reported in other rivers, most fish going elsewhere will be The inputs discussed above are used to develop a undetected unless captured or entering a river single model of fish migration on a flow band which is also instrumented for a tracking basis in Table 10.1. programme (e.g. Stour, Frome and Piddle 1988- While the figures follow closely the actual results 1990). Similarly, the failure to enter the river by of tracking, some arbitrary "smoothing" of the 15 fish only recorded in the upper harbour area trends has been applied, particularly where single will be explained by the same range of factors. fish in a small batch have resulted in points lying As the total numbers involved (35 fish = 12.5%) off the general trend.

R&D Publication 4 76 Table 10.1 Allocation (%) of migratory pattern of Avon Hsh evading capture in the Mudeford nets.

Residual Fail to Enter Avon Enter Avon Lie-up tidal Avon Lie-up d/s Knapp Ascend beyond flow m3{sec enter <10days | Autumn (-1.2-0.0 km) (0.0 - 1.2 km) i Knapp in season

20+ 97.2 2.8 0 97.2

16-20 97.2 2.8 12 85.2

14-16 97.2 2.8 12 85.2

13-14 97.2 2.8 5.4 12 85.2

12-13 97.2 2.8 8.8 17.7 70.7

11-12 97.2 2.8 10.5 18.4 68.3

10-11 97.2 2.8 16.2 24.3 56.7

9-10 97.2 2.8 14.5 29.0 53.8

8-9 8.5 70.2 21.3 19.1 14.9 36.2

6-8 17.5 49.5 33.0 38.5 2.1

5-6 45.5 37.7 16.8 29.3 6.3 2.1

<5 60 20.0 20.0 13.3 6.7 0

10.2.3 The numbers of fish entering the river during the angling season, and the number of tagged fish reported caught by The final input of the model is the numbers of anglers, indicated that the exploitation rate by fish approaching the Avon. In Table 10.2, the nets in 1986-89 averaged 9.8% (Section 9.3). By mean net catches by week for 1986 to 1990 are applying the estimated exploitation rate, an shown. Calculations based on the reported estimate of the mean run by week is given. angling catch, the number of radio tagged fish

Table 10.2 Reported net catches of salmon at Mudeford during the main migration season. Week 1 starts about April 10, and the netting season ends in week 16 (July 31).

Week Net catches in Year Mean Mean Avon Escape Adjusted No '86 '87 I '88 I '89 •90 catch run total nets escape 3.4 35 30 27 22 10 4.2 43 37 34 27 2.4 24 21 19 15 16 7.2 73 64 58 46 14 19 10 9.8 100 87 78 63 6 26 19 Ï2.4 127 110 99 79 21 28 14 15.0 153 133 120 96 52 24 22 22 25.0 255 222 200 160 58 25 34 13 25 31.0 316 275 248 199 10 65 25 34 33 19 35.2 359 312 282 225 11 74 57 55 55 33 54.8 559 486 i 439 351 12 84 90 76 56 33 67.8 692 602 543 434 13 87 92 124 59 44 81.2 829 721 650 520 14 77 I 103 106 57 59 80.4 820 714 644 515 15 50 56 ÎÎ7 68 32 64.6 659 573 517 414 16 23 46 28 48 21 33.2 339 295 266 213

R&D Publication 4 77 A further calculation adjusts for the mean proposed abstraction patterns. This is explored estimated proportion of the Mudeford net catch further in Section 11. that comprises Avon fish (see above, 88.1%), and to allow for the fact that part of the run enters 10.3 Tamax migration model the harbour outside the netting season (see below). 10.3.1 Introduction

The adjustment for the proportion of the run The Tamar migration model is a spreadsheet occurring outside the netting season is necessary model of the relationship between flow on any because the overall estimated exploitation rate by day of the year and the number of fish expected the nets is 9.8% of fish entering the harbour to pass the tidal limit. It is based upon the between the start of the netting season on analysis of the radio tracking results and on February 1 and the end of the angling season on availability curve derived from several years of net September 30 (Section 9.3). Thus the weekly run catch data. It represents a considerable level of sizes in the table are over-estimates by a factor trial iteration and adjustment, comparing the equivalent to the proportion of the run which output results with the fish counter results. The enters the harbour outside weeks 1-16 i.e. results generally follow the overall pattern of before about April 14 and after july 31. It is fluctuations in numbers of fish recorded by the believed that only a limited proportion of the run counter, but the model is still relatively poor at occurs at such times, and a 20% adjustment is reliably predicting individual daily counter made to account for this. figures. Further refinement of the model would improve its performance, but much of the Using the "whole season" estimate of exploitation variability appears due to stochastic day-by-day rate for individual weeks is of doubtful validity, as variation in the numbers offish entering the it is certain that effective fishing effort is much estuary that the availability curve in the model lower in the early part of the season when fish are cannot meaningfully generate. few. Thus the exploitation rate is likely to be lower in the early weeks than at the peak of the Nevertheless, the model is potentially useful in fishery (possibly as little as a quarter of the peak) predicting the overall effects of changes in flow and, higher than the average in the peak weeks. due to abstraction and so Is presented here. It is This would lead to the numbers of fish running in expected that it could be improved significantly the early weeks being a major underestimate. with more feedback from fish counter results. For example, if the exploitation rate were only one-quarter for the first five weeks what it was for 10.3.2 Inputs to the model the remaining 11 in the table above, the runs in the first five weeks would be underestimates by The basic flow diagram for the model is shown in 75% and the remaining weeks would be over­ Figure 10.1. Details of the values of each input estimates by only 20%. As the weeks of greatest and multiplier are given in Appendix I. impact of abstraction are indeed the later ones, and in the absence of reliable effort figures, the The fundamental input into the model is the values derived in the table are used. stock availability curve, which determines what proportion of the total years run of salmon Using these inputs, we can now calculate the becomes available to enter the river each day. expected numbers and behaviour patterns for This determined from the net catches between any week, and indicate the different results 1990 and 1994, and is dome-shaped, peaking in obtained using the natural flow record, the actual ]uly. The actual run entering the river on any day flow record, or any predicted residual flow for a will comprise a proportion (often all) of that day's new scenario of abstraction. We can thus availability, plus under certain conditions indicate the impact on salmon movement, and to contribution from earlier days "unused" some extent angling success, of existing and availability and subsequent days availability, less the fish taken by the nets.

R&D Publication 4 78 Today's availability (rnay be reduced by draw-i ( over previous 2 days)

Backlog 1A 10 days / Next two days after 10 days I availability \ (draw-in) Backlog 1B 10 days flow 3+

after 10 days

J Backlog 2 "I (entered alter 20 \ days in backlog 1) flow 6'i only after Sept. 16

Net catch

(catch on any day spread equally over next 8 days)

Delay

Run at Gunnislake Weir

Figure 10.1 Schematic representation of the Tamar salmon migration model. "Today's availability" fs derived from a stock availability curve. Straight lines indicate the movement of all fish remaining in the category. Curved lines indicate that part of the stock is involved, generally according to formulae dependent upon river flow. The italicised Mow values are in m Vsec are thresholds for Invoking the pathway. Details of all steps are given in Appendix 1.

The proportion of each of the day's available which may migrate on subsequent days in stock that actually runs is flow-dependent. Above response to elevated flows. After ten days, fish in the threshold flow (2.5 m3/sec) and below 20 Backlog 1A are transferred to Backlog IB where mVsec, the whole availability runs; below a lower they become less susceptible to migration cues, threshold (1.0 m Vsec), none does; and in and after 20 days they are transferred to Backlog between a part does. That part not migrating 2, from which they are not then available to that day enters a backlog (Backlog 1A), part of migrate until the secondary phase of migration in

R&D Publication 4 79 All values and threshold flows used for running the model are detailed in Appendix II.

10.3.3 Model output

A sample output for the model, for the year 1995, is shown in Figure 10.2 along with the counter record. While the general form of the output of the model is similar to that from the counter, the day-by-day correspondence is relatively poor. However, the model correctly predicts peaks in numbers associated with spates and low numbers associated with low flows in 01'fin Ol-M Ot.Mjf (M-Ap( 05-Mjy OJ.|un Ol M 0«^ 06-Sfp 07-0(1 07 Nov QBDti August and September and it is likely that further adjustment of the model parameters would improve its performance further. A limiting factor in this respect is the reliability of the counter output itself. As discussed in Section 4.3, some fish do not use the pass to achieve upstream migration and they will not be counted. Further, the species-split between salmon and sea trout becomes critical when one outnumbers the other several fold. However, model parameters that could be amenable to adjustment include:-

• the number of days over which each day's Ol-Jan Ol-Fab (M-Mat Ot-Apr 05-Mjiy OS^Juti OfKlul M-Aug 06-Sep 07-Oa 07-Nov 06'Oec net catch is spread

Figure 10.2 Model output for salmon 1995. Upper: dally runs at tidal limit predicted by the number of days availability that is the model. Lower: daily runs recorded by the counter. influenced by "draw in", and between which flows draw-in operates the autumn (after September 15). At flows over 20 mVsec, some of the run is delayed until flows the proportion of each backlog that is fall below that level again. This is based upon the affected by elevated flows observation that radio tagged fish appear to be under-represented at such-high flows. the flow threshold and the relationship between flow and movement below the The tracking data suggests that fish may take threshold (as more counter data become several days to pass the tidal limit after being available, particularly for periods with flows caught in the nets. Each day's net catch is falling well below 2.5 m Vsec) therefore spread equally as a debit over the next eight days run though this is thought to be too the number for multiplying up to generate high and can readily be adjusted. Elevated flows the numbers of fish. At present the total run between pre-set levels may draw-in a proportion for each year is assumed to be 7,500, of the stock that theoretically become available in regardless of the actual count. future days, depleting those days available stock.

R&D Publication 4 80 11 IMPACT AND MANAGEMENT OF ABSTRACTION

11.1 Introduction time, subject to the take plus that at Knapp Mill not exceeding the Knapp Mill limit. A further The primary justification for ail the investigations 0.35 m Vsec (30 Ml/d) may be taken, subject to a covered by this report has been the assessment of prescribed flow at East Mills GS of 23 m3/sec. the impact of existing and potential future water resource schemes. This aspect has of course been There are also PWS groundwater abstractions a major feature of the detailed internal reports of from the upper part of the catchment with a total the investigations. The assessments discussed licensed take of about 140 Ml/d, but these are here focus mainly on the impact of existing considered no further here. licensed abstractions, and a general examination of the options available for reducing such impacts 11.2.2 The impact of abstraction on and for minimising the effects of future migration developments. Discussion of detailed future The impact of the Ibsley abstraction is not developments is not appropriate here because considered further here as one part of the licence any such proposals are highly tentative and liable acts simply as a transfer of part of the Knapp Mill to modification, and may be confidential to the take, and the other part is subject to a pf that is Agency and the water undertakers. The possible well above the flow that is likely to have an options for reducing the impacts are considered adverse impact upon salmon movement (Section as concepts only from the fisheries viewpoint. 10.2). Most of the options would have significant cost and resource implications together with broader The combined effects of the abstraction at environmental effects that would have be taken Matchams and Knapp Mill on movement in the into account if serious consideration were to be lowermost reaches are now examined using the given to them. migration model described in Section 10.2. A period of sixteen weeks is considered running 11.2 Avon from early April to the end of july - this covers the main migration season and, in most years, 11.2.1 Description of the abstractions and the period when residual flows fall through the their impact on flow critical value of 9 mVsec.

The three main surface-water abstractions are at The model was run for the three years 1986, Matchams (14.3 km upstream of the tidal limit 1987 and 1989; the first two were unremarkable for Longham WTW), Knapp Mill (1.4 km years, the third was a drought year with low upstream of the tidal limit for Knapp Mill WTW flows (Figure 3.5). Two scenarios were modelled, and Fawtey refinery), and Ibsley (28.4 km the naturalised flows and the actual residual upstream of the tidal limit for refilling Blashford flows. The results are indicated in Table 11.1. Lakes for PWS). The first two are operated by Bournemouth and West Hants Water, and the It is apparent that while the abstraction had an third by Wessex Water. impact upon fish migration and survival in all three years, it was very much greater in the year The Knapp Mill and Matchams abstractions are of naturally low flows, 1989. In very wet years Licences of Right and are not subject to a with high flows the impact is likely to be prescribed flow rule, and may thus abstract at negligible. The impact is greatest when any time and under any conditions of river flow. abstraction causes the residual flow to be below 9 The maximum take at Knapp Mill is 1.32 mVsec mVsec, when it otherwise would have been (113.7 Ml/d) and at Matchams 0.74 mVsec (63.3 above, during the peak of migration period in Ml/d). The licence at Ibsley is more complex. Up late June and early July. to 0.23 mVsec (20 Ml/d) may be taken at any

R&D Publication 4 Table 11,1 Impact of historic abstraction indicated by the Avon salmon migration model In three years.

1986 1987 1989

Natural Q Historic Q Natural Q Historic Q Natural Q Historic Q

Fish enter estuary 3,380 3,380 3,380 3,380 3,380 3,380 Fail to enter river at all 53 43 134 169 632 Enter river within 10 days 3,174 3,117 3,147 2,900 2,962 2,010 Enter river in autumn 94 211 190 346 249 138 Lie-up tidal reach 239 386 322 476 516 910 Lie-up 0-1.4 km 556 599 559 528 702 342 Lie up beyond 1.4 km 2,533 2,155 2,298 1,906 1,758 784

11.2.3 Possible approaches to reducing the 11.3 Tamar impact of abstraction The only abstraction considered here is the PWS Moving the Knapp Mill abstraction point 1.4 km take at Cunnislake operated by SWW. seawards, to the tidal limit, would reduce the

impact by reducing the numbers of fish that The situation on the Tamar is unique among the laidup in this reach and increasing the number six rivers in this study, in that the results of the that ascended beyond Knapp Mill in their initial fish migration investigation have already been phase of migration. used to adjust operating rules for a large water resource scheme. In 1996 the results, largely as Reducing or ceasing abstraction when it would presented here, were the basis for an agreement otherwise cause residual flows below 9 mVsec, between the Water Company and the Tamar and would considerably reduce the overall impact. In Tributaries Fisheries Association (TTFA) for some years the flow falls through this zone fairly adoption of operating rules for the period up to fast, within about two weeks; in 1989, it took at least the year 2015. about four weeks. Finding an alternative source

for this period would represent a major water The abstraction point is about 1.4 km upstream resource development. A more modest "close of the tidal limit at Cunnislake Weir. It forms part season" of one week would still represent a of the Roadford Water Supply Strategy and significant reduction in impact on fish operates as a major abstraction using both movement. supported and unsupported flows. Below the prescribed flow, any abstraction must be The third option considered is that of diurnal supported by release of stored water from modulation of abstraction. Examination of Figure Roadford Reservoir. Above the pf, 50% of the 8.7 indicates that most migration up to and past excess flow may be taken subject to the overall the tidal limit takes place between 22:00 and licensed maximum. 06:00 hours. Protection of an eight or ten hour

period, allowing an increased take for the The licence for the abstraction was originally remainder of the day to maintain overall daily granted with a pf of 2.85 mVsec (246 Ml/d), but yield, would be an interesting option. for an interim period of ten years from 1992 the Alternatively, two shorter periods of flow sparing, pf was set at 5.52 m Vsec (477 Ml/d). Both these around the peak activity times of dawn and the sets of rules allowed half the flow above the pf to early hours of darkness, could be explored. Flow be abstracted to a maximum of 1.71 mVsec (148 sparing for periods of hours per day would place Ml/d). The Company pledged to fund a large- a requirement for significant off-river storage, but scale fisheries and environmental investigation of this would be a very much more modest scheme operation to allow appropriate operating undertaking than that required to cover for a rules to be determined in the longer term. At the week of no abstraction as discussed above. end of the ten-year interim period (in the year 2002) the pf was to revert to 2.85 mVsec

R&D Publication 4 82 Table 11.2. PWS surface water abstractions from the Exe. (246 Ml/d), but the Company had an agreement with the TTFA that they would apply for a Distance from nominal change in the pf at this time. This was a Description of abstraction Operator tidal limit mechanism to allow any interested parties the Impoundment at Wimbleball Reservoir SWW + WW 55.6 km opportunity to object, thus forcing a review of The Exe-Taw transfer from Exebridge SWW 47.8 km the operating rules based upon, mtera/ia, the Wimbleball pumped storage from SWW 47.8 km findings of the fisheries and environmental Exebridge investigation, possibly through a Public Inquiry. Bolham for Allers WTW SWW 32.3 km

North Bridge for Pynes WTW SWW 9,4 km In the event, the clear picture that emerged from the studies regarding the relationship between flow and salmon migration in the lowermost Bolham and North Bridge when the discharge reaches led to an agreement between the falls below the prescribed flow at Thorverton GS, Company and the TTFA that the scheme could and for the Exe-Taw transfer. Wessex Water takes operate with a pf of 2.85 mVsec (246 Ml/d). water direct from the reservoir to its Maundown This new arrangement was introduced in 1996, WTW. In addition to the compensation flow and six years ahead of the scheduled review date. regulation releases, a volume of 900 Ml per year The Company further undertook not to seek any is allocated as a water bank for release to the river further reduction in the pf before the year 2015 for fisheries and conservation purposes. without the prior agreement of the TTFA. Thus the results of the investigation have been used to The Exe-Taw transfer intake is located at negotiate considerably increased flexibility of Exebridge. All transfer abstractions must be operation while safeguarding salmon fishery supported by a commensurate release from interests. Wimbleball. This scheme therefore has no effect on flows downstream so it is considered no As concluded in Section 4.2 and 4.3, there further in this report. appears to be no significant impact of abstraction as long as the residual flow to the estuary The licence for the Exebridge abstraction for the exceeds 2.5 mVsec. As the recently adopted pf is Wimbleball Pumped Storage Scheme includes the 2.85 m Vsec with a 50% take rule, no adverse following conditions: impact of the scheme is foreseen. Thus no potential remedial measures are required for • no take allowed from April to October consideration. inclusive

11.4 Exe • maximum take 150 Ml/d (1.74 m'/sec)

11.4.1 Description of the current • take can be 50% of flows above the pf abstractions (Exebridge) of 100 Ml/d (1.16 m Vsec)

There are currently five PWS surface water • no abstraction shall take flow at Thorverton abstractions from the Exe operated by SWW and GS below 273 Ml/d (3.16 m Vsec). Wessex Water, as listed in Table 11.2.

The PWS abstraction licence at Bolham comprises Wimbleball impounds the flow of the River two parts: a LOR element of 2.7 Ml/d, plus 29.3 Haddeo, modifying flow downstream when not Ml/d subject to a prescribed flow of 273 Ml/d at spilling by the difference between the potential Thorverton. When the Thorverton flow is below catchment discharge at the dam, and the 273 Ml/d, any abstraction beyond the LOR must combination of releases and seepage. The be supported by a release from Wimbleball and theoretical Q95 at the dam is 0.079 m Vsec and would thus have no impact on flows the compensation flow 0.11 mVsec(9.1 Ml/d). downstream. Augmentation releases are regularly made at times of low flow to support abstractions at

R&D Publication 4 83 The PWS abstraction at North Bridge operates There are of course numerous other abstractions under two separate licences: a LOR for 24.46 from the Exe that contribute to any impact upon Ml/d subject to no operating restraints, and a salmon movement. Many are of a "non- licence for 42.00 Ml/d subject to a pf of 273 Ml/d consumptive" nature e.g. fish farms and HEP at Thorverton CS. When flows at Thorverton are operations with the water being returned to the below the pf, any abstraction beyond the LOR river a short distance downstream. While this must be supported by a release from Wimbleball may contribute to or cause a significant local and would thus have no impact on flows migration problem, there is no impact downstream. downstream of the return point. Public water supply accounts for 63.3% of the total licensed Present levels of abstraction are well within these consumptive surface-water abstraction from the licence limits. At times, the Bolham take is catchment. No analysis is available for the virtually the full licensed limit; in 1994, for percentage uptake by the non-PWS abstractors. example, the average take was 25.88 Ml/d compared to the full licence of 32 Ml/d (81%) Part of the take of water for PWS is returned to and the peak daily take was 31.84 Ml/d (99.5%). the river via sewage treatment works (STW), Takes at North Bridge are generally a lower reducing the net effect on flows. In considering proportion of the licensed limit of 64.5 Ml/d: the overall impact of water resource through the study period 199 -94, the take management on flows in the river, account must averaged 34.75 Ml/d (52.3%) and the peak daily be therefore taken of the discharges from STW to value was 44.94 Ml/d (67.6%). the non-tidal catchment. The dry-weather flow from all STW in the catchment is 0.204 mVsec.

-Flow wiihoui abstiaction

-Peiidual How u/i North Bridge Botha m a bit Faction 11.4.2 Potential impact of existing PWS

-ReikJuil ttow d/s North Bridge abstractions on river flows and migration

It is stressed that the potential impact of abstraction upon flows considered here is that of the full licensed take. As discussed above, the current levels of take are rather below this level, but it is likely that they will increase in the future to be close to the licensed limit.

fJitu'il (to* mllu The relationship between the natural unsupported flow and the residual flow arising from abstraction at-Bolham on flows upstream of

-—Reiidual flow North Bridge intake, and from abstraction at

-—Flow without abstraction Bolham and North Bridge on flows downstream, supported where required by a reservoir release, is shown in Figure 11.1 (upper part). Full licensed take and dry-weather flow returns from STW are assumed. In Figure 11.1 (lower part) the impact of full licensed take at Bolham and North Bridge on residual flows at the tidal limit is illustrated; again dry-weather flow from STW is taken into account in the residual flow line. In 12 13 14 li modelling the impact of the full licensed take, Niturtl lkn> tri'liec this exercise considers the maximum future impact of the current operating rules; in practice, Figure 11.1 Residual flows in the Lower Exe assuming full licensed take at Bolham and North Bridge , and DWF from STW. Upper: Residual flows upstream and the impact to date has been less. These figures downstream of North Bridge Intake (9.3 km). Lower: Residual flow at the tidal limit. apply to the period April to October inclusive,

R&D Publication 4 84 when the abstraction from Exebridge for Wimbleball pumped storage is not operating.

The migration index for fish arriving at the tidal limit at St lames Weir, and the effect upon this of abstraction at Bolham and North Bridge, are indicated in Figure 11.2. The main impacts are an increase in the range of natural flows at which no fish arrive, from 0-3.14 m3/sec to 0 - 3.25 mJ/sec, and a minor reduction in the index (varying around 6%) between 3.25 and about 9 mVsec. About 2.4% of the run would otherwise arrive at flows between 3.14 m Vsec and 3.25 m Vsec. The difference of 0.11 m Vsec is due to the net reduction of flow caused by abstraction Figure 11.2 Impact of full licensed abstraction on migration index for fish arriving at the tidal limit. when the Thorverton flow is below the pf, and approximates to the Licence of Right abstractions for PWS less the dry-weather flow from the STW. Natural How migration Inden Rciidual How migration lnde> Overall, the impact of abstraction upon fish — Index« 1.0 arriving at the tidal limit can be considered to be Available Hows negligible, particularly as the abstraction does very little to decrease the migration index below a value of 1.0 (see Section 2.6).

As discussed in Section 5.4.4, the presence of St James Weir and the existing flow arrangements over the weir and along the mill stream represent a significant obstruction to fish migration at times of low flow. The full licensed abstraction regime exacerbates this effect to a minor extent (Figure 11.3). The range of natural flows when no fish Figure 11.3 Impact of full licensed abstraction on migration index for fish passing the tidal limit (ascending St James Weir). ascend the weir is increased from 0 - 3.62 m Vsec to 0 - 3.73 m Vsec. Only about 0.8% of the run would otherwise have ascended at such flows, Natural How migration indei

but abstraction also causes a variable impact on Residual How migration index migration index up to about 11 mVsec, typically — Index • 1.0 about 10%. The effect of abstraction is therefore Available Hows greater on fish ascending St James Weir than on fish arriving below the weir, including a greater tendency to take the migration index below the 1.0 level.

It is stressed that this impact is upon fish that have already arrived at the foot of the weir and i ro n rj tj u iä are stopped from ascending. The impact could Njurit ftow m'/Kt be removed completely, along with much of the delaying effect of natural low flows, by provision Figure 11.4 Impact of full licensed abstraction on migration index for fish passing the Duryard ALS site (4.9 km). of an effective fish pass at St James Weir. SWW is currently funding the provision of improved fish The impact of the Bolham and North Bridge passage facilities at this site. The design criteria abstractions upon the migration of fish passing for the new facilities are being recommended by through the lowermost non-tidal reaches is the Agency.

R&D Publication 4 85 The impact on fish passing through the reach Natural How migration inctei between Duryard (4.9 km) and North Bridge (9.4 Residual llow migration inde km) is complicated by the impact of the flow - -iiide* = 1,0 diversion downstream in the Cowley area. As Available (tow discussed in Section 5,5, the combination of flow diversion and weirs has an impact on that section of river equivalent to an abstraction of the order of 1.5 m Vsec at all flows. The effects of this and maximum take of 1.14 m Vsec at Bolham and North Bridge are clearly additive. It must be pointed out that the effect of the diversion thus exceeds that of the full licensed abstraction, even Natural fluw rnVw at flows when the abstraction is not being supported by a reservoir release. Figure 1 l.S Impact of full licensed abstraction on migration Index for fish passing the Pynes Weir ALS (7.2 km). The impact of abstraction on fish arriving at indicated in Figure 11.4, showing the impact on Pynes Weir (7.2 km) is shown in Figure 11.5. The the migration index at Duryard (4.9 km). impact is greater than that at Duryard (4.9 km). This arises because of the steepness of the The impact of fish arriving at Duryard is migration index line at the point when the considerably greater than at the tidal limit, unsupported abstractions first commence. This particularly when the unsupported abstractions increased steepness compared to Duryard is "cut in" between 6 and 7.5 m3/sec. Between partly due to the "upstream effect", described in flows of about 3.7 and 10 mVsec (tidal limit dmf) Section 8.3 and illustrated in Figure 5.5. there is a variable impact of abstraction typically However, the Pynes migration index line is out of of the order of 20%. At natural flows between step with the other Exe stations largely due to the about 6 and 7 m3/sec there is a considerable effect of the diversion at Cowley, and is steeper reduction in the migration index below 1.0. than expected on its ascending limb. Pynes Weir itself is not an obstacle to fish migration. The combined impact of the maximum abstractions at Bolham and North Bridge The impact on fish arriving at the North Bridge increases the tendency for fish to remain in the Intake (9.4 km) when the two unsupported non-tidal reach downstream of Duryard at natural abstractions "cut in" (Figure 11.6) is rather less flows when unsupported abstraction has the than that at Pynes Weir. This arises because the greatest impact on flow i.e. a tidal limit natural migration index line for North Bridge is back "in flow of the order of 6 - 10 mVsec. The exact step" with the other Exe stations (cf. Pynes, impact will depend upon natural flow conditions above). Again, this illustrates how the diversion prevailing at the time. In very wet summers with of flow at Cowley adds to the impact of the frequent major spates there will be little impact. abstractions. At times of very low flow also there will be little impact, as few fish would in any case penetrate The impact at North Bridge, however, as beyond the lowermost reaches under such indicated by the depression of the migration conditions. It is in unremarkable years, with index below the value of 1.0, extends to higher periods of low flows punctuated with occasional flows (c. 9.5 m Vsec) than at Pynes (c. 8.1 minor spates, that the greatest impact is likely. mVsec). This represents a true increased impact The critical value of minor summer spates at within that flow range due to the "upstream otherwise dry times is dear from Figure 5.3 effect" (Section 8.3). (around July 16 1992). Such events often put flows within the effective migration range for a The situation upstream of North Bridge is rather very short time, as fish migrating upstream soon different for two reasons: "run out of water" as the spate moves downstream.

R&D Publication 4 86 • There is, of course, no impact of the North XT '00 Bridge abstraction, any effect being limited Natural llov* migration inden to that of the Bolham abstraction. Residual Mow migration inde — lndo-1.0 • At low flows the discharge is enhanced by Available Hows the release from Wimbleball to support the

North Bridge abstraction. 2 10

Only minimal migration takes place at North Bridge and upstream at flows which require reservoir releases to support abstraction at North Bridge - for example the lowest flow at which fish are predicted to migrate at Bickleigh Bridge is 3.10 m Vsec (Table 5.4), compared to the prescribed flow of 3.16 mVsec. However, where Figure 11.6 Impact of full licensed abstraction on migration index for fish arriving at the North Bridge ALS (9.4 km). reservoir releases continue through minor spates, a useful enhancement of flow is likely to occur,

Further, it is likely that the enhanced flows Natu

The impact of the full Bolham abstraction upon the migration index at Bickleigh and Bolham is shown in Figures 11.7 and 11.8. In each case the dry-weather flow from appropriate STW is assumed as part of the scheme's operating scenario. NJturjl How m/we

At Bickleigh (Figure 11.7) the migration index is Figure 11.7 Impact of licensed abstraction on migration index for fish arriving at the Bickleigh ALS (21.1 km). in fact slightly elevated at flows around 3 m Vsec, as the STW effluents exceed the LOR take at Bolham of 0.03 m Vsec. However, once the Natural flow migration inde*

Bolham unsupported abstraction cuts in, there is Residual llow migration inde" a minor negative impact. — lr>de> = I.O

Available Hows

S l.S The impact on fish arriving at Bolham is a little c greater, mainly because there is less amelioration I from STW effluents (Bolham is upstream of the I 1,0 major STW at Tiverton), but is still considered to be minor.

11.4.3 Impact of water resource operation 1} 14 IS in winter Natural How m'h*t

The operating rules for the winter Wimbleball Figure 11.8 Impact of full licensed abstraction on migration index for fish arriving pump-storage abstraction at Exebridge are at the Bolham ALS (32.3 km). described in Section 11.4.1. This abstraction will be operated as and when required to supplement In most years the great majority of migration past the reservoir recharge and will be in addition to all the abstraction points is complete by the PWS takes during the winter. November 1, and operation of all abstractions at full take would represent minimal impact. In

R&D Publication 4 87 some years, however, low flows during the just above the Thorverton pf. The impact autumn would mean that much of the migration between North Bridge and Bolham was again through the middle reaches had not been minor. completed by the beginning of November, and scheme operation could have an impact. It is inevitable that there will be conflict between the requirements of the fish and those of water Passage past Exebridge abstraction point is resource management in this situation. The impacted, to some extent at least, by flows below current method of operation, with only minor 8.3 m Vsec occurring (Stoodleigh measurement) unsupported abstraction at low flows, represents as described in Section 5.4. It is suggested that a a responsible approach to the problem, but dmf value of 8.3 m Vsec on at least ten days nevertheless some adverse impacts do occur. between August 1 and October 31 would allow There are, however, a number of ways in which the great majority of fish to pass Exebridge the impact could be reduced. These include: during this time. Examination of the flow gauging records for a 29 year period (1966- • Raise Thorverton prescribed flow. In order 1994) indicates that only in four years (1972, for this to be totally effective, a pf of about 1975, 1978 and 1990) would a significant part of 5.6 m Vsec would be required. This the migration have been incomplete by this represents about a Q67 flow, and it is criterion. Another factor that would limit the concluded that this is an unrealistic option. impact of the abstractions on fish already delayed is the tendency for flows in November to be • Move intake for Pynes Waterworks (currently either low or high, with few days of intermediate at North Bridge) downstream to the reach flows when Wimbleball recharge pumping would between St James Weir and Trews Weir. The be permitted yet flows were low enough so that present abstraction at the existing pf would the maximum abstraction would bring residual have virtually no impact here. Perhaps more flows below 8.3 m Vsec. However, the year 1978 realistically, moving the intake downstream stands out as having both a very dry late to Duryard would remove much of the summer/autumn (highest Stoodleigh dmf impact for any future scheme development. between August 10 and October 30 being 4.4 m Vsec on August 16) and a high proportion of • Improve fish passage facilities at St James potential "pumping days" in November when Weir. Current operation has a minor impact pumping would reduce the flow below 8.3 here, but spreading the fish that gather in mVsec (8 out of 13). the weir-pool upstream through the city is likely to increase survival and would give the Overall, the impact of the Wimbleball refill fish a head start in the event of a short-lived abstraction on salmon migration will-be summer spate; this is scheduled.to be negligible, except in years when a prolonged undertaken in the near future. The range of period of low flows persisting into November flows (tidal limit dmf) for which improved coincides with reservoir storage levels that require fish passage would be particularly beneficial pumped refill during November. Even then, is 3 - 5 mVsec. If existing facilities are to be there is scope for impact to be reduced. removed or modified, rather than added to, it is of course important that the new 11.4.4 Possible ameliorative action facilities maintain fish passage at higher flows too. It is important that the improved fish In Section 11.4.2 it was concluded that pass provision at the weir itself should be abstracting the full licensed volume of water at associated with minimising the flow down Bolham and North Bridge had a negligible the mill leat (see Section 5.4.4) in order to impact upon fish movements to the tidal limit, a maximise the benefits. Further, in order that minor impact on fish ascending St James Weir, the benefits are maximised in terms of fish and a greater impact on movement between the distribution throughout the river, it is tidal limit and North Bridge. The main impact important that the situation at Cowley (see occurs when abstractions are taking place next bullet point) is also resolved. unsupported by reservoir releases, when flows are

R&D Publication 4 88 Improve fish passage facilities in the Cowley Although adverse conditions resulting from the Bridge area. One option would be to pump storage abstraction at Exebridge are likely engineer the flood relief channel so that at to occur fairly infrequently, any potential impact flows below about 10 m Vsec (at tidal limit), could be reduced in two ways. These are: virtually the whole of the Exe flow remained in the most easterly channel (see • Avoid any November pumping which would Section 5.5). reduce Stoodleigh flow to less than 8.3 m Vsec during "critical" years. Protect minor spates and critical recession flows. In current practice, however, "spate • Ease fish passage at Oakford Weir; it is likely sparing" either occurs anyway, or when it that the critical flow could thus be reduced does not, it is likely to be impractical. This by about 2 mVsec. arises because of the considerable response and lag times involved in adjusting reservoir Reservoir operational control curves for releases to support abstraction. If a release is Wimbleball have been designed to minimise taking place when a minor spate occurs, the November pumping. Otherwise, in a November release is likely to continue throughout. If in which both reservoir refill and fish migration the flow before the spate occurred was conditions were critical, it is suggested that above the pf and a release was not being consideration should be given to using part of made, the time taken for a release to reach the fisheries and conservation water bank to the lower reaches means that, in the absence protect critical flows. If the current year's water of considerable off-river storage specifically bank had been exhausted, it is possible that for this purpose, unsupported abstraction is access to part of the following year's allocation likely to continue throughout. These real could be negotiated. practical constraints do not detract from the validity of the concept of protecting critical Regarding the second option, it appears that the spates, however, and the comments made conditions under which fish arrive at Exebridge above stand. The concept of spate sparing are dictated by those required to ascend Oakford should be borne in mind in considering any Weir. If an effective fish pass were installed there future modifications to operating it is likely that the critical flow for arrival at procedures. Factors that might make spate Exebridge would be similar to those for arrival at sparing more realistic (or more critical) Oakfordbridge, about 6.4 m Vsec. In any event, include increased conjunctive use, improved correction of the poor fish passage arrangement pipeline infrastructure, changes in and at Oakford Weir is strongly recommended for greater flexibility of operating rules, and fishery management purposes, to ease fish changes in operating procedures that might migration and to prevent possible damage that make "spate mining" more realistic. If spate might occur to fish trying unsuccessfully to sparing provisions are considered at some ascend the weir. The Agency has recently made time in the future, care would be needed to modifications to the fish passage facilities at identify the critical spates and the critical Oakford Weir. features of those spates for conserving, in order that this did not become a wasteful 11.5 Taw exercise in terms of water resources. However, any operation that protected 11.5.1 Description of the abstraction and critical minor summer spates from its impact on flow abstraction that otherwise reduced flows The only abstraction considered here is at New within the 3-10 m Vsec range (measured at Bridge (Chapelton), about 2 km upstream of the the tidal limit) would be an environmentally tidal limit. sensitive move. Similarly, it might be possible to protect part of this flow range of At the time of this investigation, water was a receding hydrograph of a larger spate. pumped about 1.2 km to Newbridge Water Treatment Works. Because of the particular

R&D Publication 4 89 configuration of the pumps, at most levels of increases as flows increase. The migration index abstraction more water was taken from the river reaches a value of 1.0 at a flow of about 12 than was treated at the works, and the balance m Vsec. was returned to the river about 1.2 km downstream of the abstraction point. Because this threshold flow is so high, the slope of the migration index line at low flows is very The maximum take from the river is 0.208 mVsec shallow. This means that the impact of a small (18 Ml/d). Unsupported take is limited by a abstraction is in turn extremely minor, though prescribed flow of 2.26 m Vsec (195.3 Ml/d) some negative impact is apparent over a very measured at Umberleigh GS, but abstraction at wide range of flows. The greatest impact of the lower natural flows is permitted if it is supported present level of abstraction on the migration by a transfer from Wimbleball Reservoir via the index, at natural flows below 5.0 m Vsec, is less Exe to the Molland Yeo. Of the water transferred than 1%; the maximum impact at flows from the Exe, only 80% may be abstracted at between 5 and 10 mVsec is about 1.5%, and at Newbridge. The maximum transfer is 23.0 Ml/d, flows between 10 and 25 mVsec the maximum allowing the full licence take of 18 Ml/d at impact is about 3.4% (natural flow of Chapelton. 19.8 m Vsec).

The nature of the operation of the abstraction Similar conclusions apply to the stations at d/s during this period means that there are three Chapelton and Yeotown. Zero impact is indicated levels of flow that should be considered for fish for the stations at u/s Chapelton and Umberleigh, movement: as the flows here are unaffected by the current abstraction. However, as the fish arriving at these 1. Upstream of the abstraction point the flow is stations have had to pass through the depleted equivalent to that at Umberleigh CS. This is and residual flow reaches, some impact is likely. termed the gauged flow. At times of low It will not exceed the greatest impact for the natural flow this gauged flow may be stations situated downstream. Thus all impacts enhanced by transfers from Wimbleball of the present regime of abstraction can be Reservoir via the River Exe. considered as negligible.

2. Between the abstraction point and the return The effect of the Exe-Taw transfer, to support of the untreated overflow water from abstraction at low flows, represents a net gain in Newbridge WTW, the flow is depleted by the flow. The enhancement of flows upstream of the full abstracted volume. This is termed the intake by up to 23 Ml/d must, by the same depleted flow. criteria used to assess the depletion by a similar volume from flows above pf, be considered 3. Downstream of the untreated bypass water negligible in terms of migration conditions in the return, the flow is reduced below the gauged lower reaches. However, further upstream, flow by a volume equivalent to that treated especially in the Mole sub-catchment, the flow at Newbridge WTW. This is termed the enhancement is relatively greater, and here it residual flow. may improve overall conditions for migration, angling and salmonid production. Since 1996, SWW has undertaken engineering work such that only the water required for PWS is 11.6 Tavy abstracted, and the residual flow figure would apply downstream of the abstraction point. 11.6.1 Description of the abstractions and their impact on flow 11.5.2 The impact of abstraction on migration There are four large-scale abstractions from the river that have a major impact upon flows in If we first consider the situation at the d/s certain reaches. Newbridge station (Figure 6.2), it can be seen that salmon activity is lowest at low flows, and

R&D Publication 4 90 Two abstractions at Tavy Cleave and Hill Bridge take water from the upper reaches of the Tavy for — Heiidual (low with Abbey Weir abstraction

HEP generation at Mary Tavy Power Station, — Flow without abstraction operating under Licences of Right. The water is returned to the river via the Cholwell Brook, upstream of the reaches of river covered by this report. They are considered no further here, although the pulsing of flows caused by storing and releasing water may have some influence on the migration of salmon throughout much of the river.

ludbrook ruturJl llow m Viet The third abstraction is that for HEP generation at Morwellham Power Station. Water is taken from the Tavy at Abbey Weir in Tavistock, is conveyed Figure 11.9 Potential impact of full authorised abstraction at Abbey Weir on flows at Ludbrook CS along the Tavistock Canal and is discharged to the Tamar estuary. The operating rules have changed somewhat in recent years, with — Reiduil 1>ow with Abbey Weir abstraction implementation of a phased increase in pf — Fk>w without abstiaction

declared at the Roadford Public Inquiry in 1978. — Flow without abstraction The current pf is 26.06 mgd (1.371 m3/sec) measured at Abbey Weir, and the maximum take is an hourly volume of 4432 m3 which is equivalent to 1.23 m Vsec. The potential impact of this take on flows downstream at Ludbrook GS is shown in Figure 11.9. This assumes that all water that may legally be taken is in fact abstracted; in practice this will often not be the Lopwïfl nuturi! Jtow m'/i« case. Since 1996, the HEP scheme has been owned and operated by SWW and it is likely that the take at Abbey Weir will be controlled to allow Figure 11.10 Potential impact of full authorised abstraction (Abbey Weir and Lopwell) on flows to the estuary. the maximum potable water supply take at Lopwell when it is required. When there is no Finally, water Is abstracted immediately above the take at Abbey Weir for power generation at head of tide at Lopwell Dam for public water Morwellham, there is nevertheless a "sweetening supply. This abstraction forms an integral part of flow" taken to maintain conditions within the the Roadford Water Supply Strategy and may canal. In the past the magnitude of this flow has take 50% of the flow above the pf of 73 Ml/d varied and at times much of it has been returned (0.845 m Vsec) up to a maximum take of 91 Ml/d to the lower Tavy via the River Lumburn. Future (1.053 m Vsec). Lopwell Dam was constructed in arrangements are currently being considered. the 1950's as part of the abstraction scheme The details of operation of this abstraction during about 2 km seawards of the previous tidal limit, the period covered by this investigation are not converting the upper estuary area into a non- considered here. It should be pointed out that tidal freshwater lagoon. the flow figures used for the analyses were based upon the appropriate gauge records and de facto In order to calculate the impact of the abstraction took account of the depletion due to abstraction at Abbey Weir on flows at Ludbrook and Lopwell, at Abbey Weir. we need a ratio of flows at the three points to calculate the flow at which the effect of According to the agreed schedule, the pf for the abstraction starts and its relative impact upon the Abbey Weir abstraction will increase to larger flows downstream. The figures used for 1.529 m Vsec (29.06 mgd) in the year 2000, and this are the theoretical mean flows assessed using to 1.709 m Vsec (32.48 mgd) five years later. the Micro low-flows software. The values of

R&D Publication 4 91 The potential impact of the Abbey Weir and — Natura! llow migration Index

— Reiiüual llow migration intlc* Lopwell abstractions on residual flow to the — Cummulative all-flowi estuary is illustrated in Figure 11.10. The lines indicate residual flows if all water that may legally be taken is in fact abstracted. In practice this is often not the case; the Lopwell potable water supply source is used conjunctively with Burrator, Roadford Reservoir and Gunnislake sources. The introduction of the stepped increases in the pf at Abbey Weir will retain most of the summer flow within the river, with only a sweetening flow being taken into the canal. Nltural flow mVirc

11.6.2 The impact of abstraction upon Figure 11.11. Impact of abstraction at Abbey Weir and Lopwell on migration of saimon past Lopwell Dam (TV2) migration

The analysis of the impact of the combined Natural llow migMtion inde» Abbey Weir and Lopwell abstractions on — -Index = 1 movement past the tidal limit at Lopwell Dam is Heiidual llow migration indei shown in Figure 11.11. No fish migrate below a Cumulative all-llows flow of 0.8 mVsec, and for a narrow band of t '-s flows between this level and about 0.845 mVsec J there is no abstraction and thus no impact, 2 1.0 because of the pf rules of 73 Ml/d (0.845 m3/sec) for the Lopwell abstraction. Above that level of natural flow, the 50% take at Lopwell steadily reduces the migration index, and at natural flows above about 2.6 m3/sec the Abbey Weir Niturnl (tow m'/v abstraction starts to add impact. At natural flows above 3.24 m3/sec the residual flow is above 1.91 Figure 11.12 Impact of abstraction at Abbey Weir on migration of salmon at mVsec and the residual flow migration index is Ludbrook (TV3). above 1.0 (Figure 7.1 ), and no further significant impact is suggested. However, at natural flows

-Migtation index natura between 0.845 and 3.24 the migration index -Migration inde* reiiriual could be lowered by abstraction and remains Index = 1.0 below a value of 1.0.

The impact of the Abbey Weir abstraction on movement at Ludbrook (TV3) is considered next. The potential effect on flows is shown in Figure 11.9 and upon salmon migration in Figure 11.12. This shows an impact commencing at a natural flow of about 2.5 mVsec. Between natural flows of about 3 and 6 m3/$ec the relative impact on Njturil now mJ/w( the migration index is about 20 - 25% but it is only between flows of 2.47 and 3.01 m3/sec that

Figure 11.13 Impact of abstraction at Abbey Weir on migration of salmon past the abstraction causes a reduction in migration Double Waters (7V4). index that results in a value of less than 1.0. Adverse impact at this point is therefore theoretical mean flows used in this report are considered to be restricted to natural flows 3.597 mVsec at Abbey Weir, 6.472 mVsec at between these flow values and is minor. It is Ludbrook GS, and 6.703 mVsec at Lopwell Dam. appropriate to note that while the Abbey Weir abstraction operating rules protect flows below

R&D Publication 4 92 2.47 m Vsec at Ludbrook (Figure 11.9) which relative effect on flow, and because such flows prevail for 65.9% of the time (June - September, are used by salmon for entering and ascending 1991-95), only 5.5% of fish migration is the river. Large spate flows will be largely predicted to occur at or below at such flows. unaffected, and though the falling limb of the hydrograph will reach the various critical levels The impact of the Abbey Weir abstraction sooner, similar periods of flows for optimal appears to be less at Double Waters (TV4), as the migration will still occur to about the same impact of low flow seems to be less there as extent. It is during dry summers, with few but discussed in Section 7.3. At virtually no flow level disproportionally important small spates, that the does the abstraction cause a reduction in the resource operation has the greatest potential for migration index to a value of less than 1.0 (Figure adverse impact. The potential impact is partly 11.13). offset by the conjunctive use of sources in the Roadford Scheme, and further protection could The impact is likely to be greater further possibly be achieved by refinement of the upstream, particularly within a short distance of operation of the HEP scheme. Abbey Weir itself, where the natural flow is lower and thus the impact of loss of the volume of In order to progress further the determination of water relatively greater. Further, the effect of low the most critically important aspects of the flow natural flows on fish migration appear to be regime, more data are required for migration greater than for the more downstream stations, close to the tidal limit. Radio tracking has though as concluded in Section 7.3, the data are provided a good, semi-quantitative picture in too few for quantitative analysis. space. What is now needed is a quantitative picture in time. Probably the most cost-effective As has been referred to several times in this technique to obtain this is by deployment of a report, the number of fish records available for fish counter at Lopwell. It will of course be these analyses is very small, and marginal for a essential for any such device to differentiate reliable result to be obtained. It is likely that between salmon and sea trout by some means, as some of the details of the migration index curves there is abundant evidence that their responses are artefacts of low numbers, biased results (not to flow are different. The current development of enough fish tagged at medium to high flows) an acoustic counter installation at Lopwell is and the curve fitting process. There is little doubt therefore firmly supported. however that migration is "under-represented" at low flows and that the combined effects of the 11.6.3 Possible approaches to reducing the two major abstractions discussed here are having impact of abstraction an impact upon the patterns of migration around the tidal limit. It is ironiç that the prescribed flow Based upon the available evidence, it is rules appear to be protecting exactly those low concluded in Section 7.3 that the present regime flows that salmon do not use for migration! It is of abstraction at Abbey Weir and Lopwetl is recognised of course that protection of low flows having an impact upon salmon migration. has other roles to play, e.g. maintenance of water Although it is concluded that more data on quality and protection of conditions conducive to movements are needed to confirm or modify the healthy juvenile stocks and to migration of sea initial conclusions, a consideration of potentially trout, which were not covered by this viable options for ameliorative action is investigation. appropriate here because it may help to focus future investigations. The discussions below are The licences for the abstractions at Abbey Weir intended only as a stimulus to further (HEP) and Lopwell (PWS) combined authorise consideration of options and to help focus future approaching half the natural flow to the estuary fisheries investigations. The scenarios considered to be taken at natural flows of a few times the have not been evaluated in terms of water Q95, though in practice this rarely occurs. It is resource implications or potential knock-on abstraction at such medium flows that are likely environmental impacts if other sources are to represent the potential for greatest impact on affected. fish migration, because of the considerable

R&D Publication 4 93 In Section 7.3 it is tentatively concluded that 0.47 mVsec), 't is suggested that flows can, migration past the tidal limit is adversely affected on occasions, be taken below the present pf by reduction in flows resulting in a residual flow without obvious fisheries and environmental between 0.8 m3/sec and about 1.9 mVsec. In impact. There would therefore appear to be theory, one possible option for protecting this scope for negotiating greater flexibility in the range of flows is that of introducing a prescribed operating rules for the overall benefit of flow of 1.9 m Vsec at Lopwell, with 100% take salmon movement - with only minor or even allowed above that. SWW has estimated that no effect upon scheme yield. Any move to adoption of such an operating rule would result take water below the present pf would of in a loss of overall scheme yield of the order of course need to be evaluated in terms of 9.6 Ml/d, which would cost of the order of £15 - implication for movement of sea trout, water 20 million to replace. The possibility of allowing quality and ecology of the estuary. some abstraction at flows below 0.8 mVsec (the present pf) could replace some of that lost yield, • The observation in Section 8.7 that fish but probably not most. However, a more flexible movement is not evenly spread through the operating scheme based around these values may 24 hours suggests that there could be scope well be viable and is worthy of further for some diurnal modulation of abstraction. consideration. This would require careful evaluation as it is by no means certain that protection of flow As the Lopwell source is used conjunctively with for (say) the three hours of peak movements other sources in the Roadford scheme, there is (21.00 - 24.00) would be as effective as it likely to be scope for ceasing or reducing may appear at first sight. It is likely that the abstraction from the Tavy at the most critical "decision" to enter the river is effectively times for salmon migration. If scheme yield were made some hours earlier at some distance not to be seriously compromised, such reductions seawards from Lopwell Dam. An effective would have to be relatively infrequent and of fish counter would provide the monitoring limited duration, or made up in some other way, facility to evaluate tests of this approach. e.g. by allowing some take below the present pf. It is suggested that there may be scope for this * The observations in Section 8.6 regarding approach for the following reasons: the state of tide when fish pass the tidal limit are equivocal, probably because of the small • The Lopwell abstraction is at the tidal limit, numbers of fish involved. Most studies on so any reduction in abstraction would have other rivers have indicated fairly marked tidal immediate effect on residual flows to the influence on arrival at and passage past the estuary. tidal limit. If subsequent study on the Tavy indicated that there was a tidal pattern, • Although the results of this study are there could be scope for some modulation of inadequate to draw firm conclusions, abstraction on a tidal basis. As with the analyses of angling catch records carried out option for diurnal modulation above, this as part of previous Roadford studies indicate would need careful evaluation, as the that certain minor spates are very much decision to migrate past the tidal limit may more important than some other periods of effectively be taken some hours before similar flow - so there could be scope for actually doing so. targeting spared periods both in terms of the months when such operation would be most As already stated, the viability of all these beneficial and in terms of identifying critical concepts is dependent upon operational flow events within those months. considerations such as compromise of scheme yield and operational flexibility, and costs of any • From the observations on fish movement at additional off-river storage that might be low flows and from experience with the required. previous lower prescribed flow (41 Ml/d,

R&D Publication 4 94 11 GUIDELINES FOR PLANNING AND MANAGING ABSTRACTION

12.1 Introduction sizeable abstraction from these flows would have only minor impact on the migration index. It is From the studies on the six rivers described in this where the migration index line is steepest that report emerge certain patterns of fish behaviour even a minor reduction in flow can reduce the in relation to flow and other environmental migration index significantly and thus have an variables. In Section 11 the impact of a number impact on fish movement. Thus, based upon the of existing abstractions on several of the rivers is analyses in Section 6, a pf of the Q95 on the Taw assessed. From these analyses it is possible to would avoid any major impact, as the migration produce some general guidelines for minimising index line is very flat above this flow (Figure 8.1). the impact of abstraction on salmon migration. However, any abstraction which reduced natural In some cases the effectiveness and operational flows lying between 90% and 100% of the Q95 viability of options which would appear to be where the migration index line is steeper, would promising is not yet established; in these cases have a significant effect. recommendations for further investigations are made. This raises the possibility of operating rules designed to protect critical flow bands or flow 12.2 Migration flows and operating events. Generally, the steeper the migration rules index line, the narrower the range of flows that justify protection, and the more effective such 12.2.1 General protection becomes. There are three options:

At all sites on all rivers studied, migration of • a prescribed flow such as that on the Taw salmon is under-represented at the lowest (discussed above) and on the Tamar (which naturally occurring flows. (For a definition of lies above the threshold flow, see Section under-representation and expected levels of 4.2), which avoid critical areas of the activity see Section 2.6.) migration index curve.

Despite the fact that all the rivers studied appear • a protected range of flows which contains to experience a relationship between low flows critical areas of the migration index curve, and salmon movement that conforms to the but which allows abstraction at flows above above pattern, the "threshold flow" below which and below the protected range. migration is under-represented, and the slope of the migration index line around this point, vary • protection of certain flow events that are markedly between rivers. In Section 8.2.3 the associated with major fish movements threshold flow at the tidal limit of each river is ("spate sparing"). compared, in terms of multiples of the Q95. These range from 101.5% on the Avon to These are discussed below. A general 950.4% for the Taw for summertime movements. consideration applicable to all approaches is that While the Avon value is the lowest in terms of there should be scope for operating rules to vary Q95 it is the highest in absolute flow terms (7.07 seasonally, to maximise the benefits to fisheries mVsec) reflecting the high base-flow of the river and minimise any disruption to scheme yield. A (Section 8.2.3). Some tentative relationships are second consideration is that unnecessarily discussed but more data from more rivers are stringent operating rules, while giving the required to develop a reliable model. appearance of honouring the precautionary principle, may in fact have indirect adverse A number of pointers for planning abstraction impacts on other rivers' systems. If the yield of rules can be derived from this information, existing sources is not maximised, consistent with however. If a river has a very high threshold negligible or acceptable environmental impact, flow, and the migration index line has a very low there could be an increase in the requirement for slope over much of the range of flows below the water to be taken elsewhere and at other times threshold (e.g. River Taw in Figure 8.1), then where a significant impact could occur.

R&D Publication 4 95 12.2.2 Prescribed flow would appear to effectively remove the impact of operation of the Lopwell abstraction. While this A single prescribed flow, below which abstraction could represent major periods of "no abstraction" must cease, is the most usual operating rule, during the summer months, it must be borne in apart from the almost ubiquitous maximum take mind that Lopwell is part of a conjunctive-use regulation. As already noted, it is effective at scheme involving Burrator Reservoir on the protecting salmon migration at Chapelton on the Meavy, Roadford Reservoir and abstraction from Taw and at Cunnislake on the Tamar, but less so the Tamar at Gunnislake - thus alternative downstream of North Bridge on the Exe, Lopwell sources are potentially available. The scope for on the Tavy and Knapp Mill on the Avon (though abstraction at flows below 0.845 mVsec, the same "take" on the Exe at the tidal limit especially in very dry years, may make up for would have much less impact - see Section 12.3). some or even most of any yield lost in the On the Tavy a pf of 1.91 m Vsec would have to protected flow range, in terms of drought-reliable apply to protect the threshold flow, which would yield of the scheme as a whole. Such a radical radically impact scheme yield and is likely to have development would of course require very careful knock-on enviromental impacts if other sources evaluation, both in terms of implications for had to supply the lost yield. The present pf of 73 water resources and environmental impact. Ml/d (0.845 m Vsec) is almost irrelevant to While salmon may not use residual flows to the salmon migration as it lies at the bottom of the estuary below 0.845 mVsec, such flows may range of flows used for migration. There is no pf nevertheless be important for sea trout and for on the Avon, but again a pf to protect salmon water quality, and an ultimate lower pf would migration would be impossibly high, requiring also be required. months of supply from alternative sources in dry years and consequential knock-on impacts. While in theory the protected flow range concept may be particularly attractive for abstraction well Prescribed flows to protect salmon migration will upstream where salmon migration is associated only be relevant while fish are migrating at that with a higher flow band (Section 8.3), it is point. At many sites, such rules could be relaxed important to consider the impact of the reduced during the winter and early spring, depending residual flow all the way to the estuary, where the upon the pattern of migrations. relationship between flow and salmon movement will be different. The concept is still likely to have It is of course recognised that there may be many valid application here but again a thorough reasons other than salmon migration for impact assessment would be required. imposing a prescribed flow.

12.2.4 Protection of critical spates 12.2.3 Protected flow range This concept arises from the observation that, Where the flows justifying protection are too high particularly in dry years, certain flow events of for a pf rule (and indeed in some cases when a pf limited magnitude and duration may be already applies), there may be scope for adopting associated with a disproportionally important a protected flow window. On the Avon, ceasing level of migration (Section 8.4). abstraction for a week or two when the residual flow first falls to below 9 m Vsec on the falling This leads to the consideration of protecting such summer hydrograph (if this occurs between May flow events from abstraction. It is of course and July) would significantly reduce the impact of unlikely that abstraction could or would have abstraction. Usually, after two weeks, natural completely eradicated the effectiveness of any of flows have fallen further so that protection would these events, but any significant impact would of be much less effective in terms of additional fish course be worthwhile avoiding if doing so entering the river. This does have major represented only a minor inconvenience for the implications for off-river storage or alternative abstractor. Where abstraction may be supported sources of course. On the basis of the limited by reservoir releases from upstream at times of information available for the Tavy, protection of low flow (e.g. Exe and Tamar), it may just be a the flow range between 0.845 and 1.9 m Vsec matter of not "switching off" the reservoir release

R&D Publication 4 96 during such an event, even if the flows do go practice it may also have the effect of increasing above the control flow. Indeed, in some the lowest flows from which abstraction takes situations such protection happened de facto place, as there is usually a minimum take because the operation cannot respond quickly associated with pump capacity. How does this enough to "exploit" short-lived flow events. In 50% take rule fit in with the approach based other situations, however, switching to an upon migration indices and threshold flows? alternative source for a few days, or even a single day, may make a significant contribution to The 50% rule represents the precautionary reducing any impact of abstractions. The principle, and any effects it has upon fish selection of spates for protection, and the length migration and wellbeing will be beneficial or at of time that the receding hydrograph should be the very least neutral compared to a 700% take. protected, are important considerations. However, compared to a 100% take it is Unnecessary protection would be potentially very potentially wasteful of water resources, wasteful of water resources and careful evaluation decreasing the time when the full take can be of local data would be needed to prepare abstracted, and reducing reliable scheme yields. effective operating rules. The 50% take rule therefore necessitates other resources to be accessed to achieve satisfaction of An earlier exercise considering management of the overall demand for water supply, with the Roadford Reservoir Water Supply Strategy associated knock-on environmental impacts explored possible ways of identifying critical elsewhere. Adoption of a 50% take should spates without the benefit of hindsight, and therefore only be considered where it is perceived potential periods of "sparing". The approach to offer significant environmental advantages taken was to identify historic flow events that over a 100% take. Such situations might include: would have justified sparing, based upon analysis of angling catches, counter results, trap catches • schemes where there are insufficient data to and radio tracking results. Attempts were then evaluate the impact reliably; made to develop guidelines that would have identified those spates, in real time, and would • schemes where the abstraction is large not have resulted in sparing of other less critical relative to the pf, where a 1 00% take could flow events. Factors explored included absolute cause very long periods of "plateau" flows, flow level, relative increase in flow level, at times of environmental sensitivity; occurrence of spates in the previous days and weeks, spate thresholds and duration of spate • situations where flow variability persexs sparing (Lawson et al, 1991 ). considered important, e.g. for biota other than migratory fish. It is likely that subsequent observations and operating constraints would modify the details of However, where adequate data on fish migration this approach, but the principles are broadly exist, such as on the Avon, Exe and Tamar, the valid. Clearly, local information is required for impact of abstraction upon migration can be the targeting of spate sparing, but this is an area assessed reliably enough that operating rules where analysis of angling success might be used could be set without a requirement for a 50% as substitute for detailed migration data take rule. Far from representing an adverse (Section 9.5). environmental impact, such efficient use of existing water resources may reduce the requirement for the promotion of alternative 12.2,5 Percentage take rules sources within their own potential adverse In a number of water resource schemes in South impacts. West England, abstraction operating rules allow only 50% of the flow above the prescribed flow Further, the efficient use of such resources at to be taken. This preserves flow variability, times when no adverse impact is apparent, avoiding a "plateau" of residual flow which can particularly in conjunctive use schemes, may offer occur where an abstraction takes all the flow the potential for reduced abstraction under more above the pf within a certain flow range. In environmentally sensitive conditions.

R&D Publication 4 97 12.3 Location of the abstraction considerably increased again if it were to be sited, for example, 5 km above the head of tide at Weir The lowermost reaches of rivers are often the site Cottage (see Section 4.2.4). selected for abstraction because they are likely to provide a higher drought-reliable yield than sites This does not of course mean that all head-of-tide further upstream where the flows are lower, abstractions have no adverse impact. From the because the demand is often concentrated in the available information it appears that the lower reaches, and because of a perception that, abstraction at Lopwell on the Tavy does have an overall, less environmental damage will be caused impact on salmon movement at residual flows by abstraction near the tidal limit. The results of between the pf of 0.845 mVsec and about 1.91 the studies presented in this report vindicate this mVsec. Its impact would be greater if it were last perception but also present a vivid picture of sited several kilometres further upstream, how critical even short distances can be in however. determining the impact of any particular level of abstraction. The clear conclusion from this, is that abstraction from salmon riversdurin g the main migration On the Exe, taking the full licensed abstraction at season should be located as far downstream as the existing intake sites at Bolham (32.3 km possible, ideally close to the tidal limit. Not only upstream of the tidal limit) and North Bridge (9.4 would this ensure the least possible adverse km from the tidal limit), is concluded to have an impact, but it also makes more realistic the impact upon salmon migration through the possibility of diurnal, tidal or flow-window lower non-tidal reaches at certain unsupported modulated abstraction (see Section 12.4). Such flows. The same level of abstraction taken at the an approach could both reduce impact and same time at the tidal limit instead would have a enhance drought-reliable yield if properly negligible impact. This is partly because of a exploited. higher flow at the tidal limit, but is mainly due to the change in the relationship between flow and 12.4 Diurnal and tidal modulation fish movement over the lower part of the river of abstraction (see Section 8.3). It is noted in Sections 8.6 and 8.7 that the On the Avon, the situation is even more finely pattern of migration up to and past the tidal limit balanced. The abstraction point at Knapp Mill is in most studies is modulated to a considerable just a matter of 1.4 km upstream of the tidal extent by the time of day and state of tide. This limit, yet on falling flows in the summer raises the interesting possibility of equivalent abstraction leads to considerable numbers of fish modulation of abstraction in order to protect collecting within that short reach. Moving the critical times when migration.is at.its greatest, abstraction point downstream to the tidal limit allowing a commensurate increase of take at would significantly reduce the overall impact on other times to maintain overall yield. How fish movement. realistic is this idea?

The situation on the Tamar is rather different. Two limitations are immediately apparent. First, Although the Gunnislake abstraction point is it is likely to be effective only for abstractions at about 1.5 km upstream of the tidal limit at or near the head of tide, since the time lag Gunnislake Weir, the flow/migration relationships involved for more upstream abstractions is likely there do not differ from those downstream. Thus to be too great. Even for tidal limit abstractions it moving this abstraction seawards would produce would take time for the "spared flow" to reach no useful reduction in impact, which is in any the fish which are likely to be some distance case minimal, due to the adoption of an seawards. Second, a diurnally modulated regime appropriate prescribed flow. On the other hand, would of course impose a requirement for off- moving the abstraction point a further 1.5 km river storage of at least several hours supply, to upstream to the vicinity of the Gauging Station, buffer any resulting mis-match between demand with the same operating rules, would start to and availability. However, such capacity already have an adverse impact which would be exists in many cases.

R&D Publication 4 98 This approach would clearly not be feasible in all of any available information, however limited it situations, but it is suggested that it could make a is, should be undertaken at the earliest stage. significant contribution to reducing the impact of abstraction in some schemes. A careful For preliminary analyses, for review of existing evaluation of the potential for this approach is schemes and possibly as the sole analysis for recommended, both from the point of view of some minor schemes, use of angling catch results operational implications and also, with field trials, may be considered (Section 9.5). A breakdown to establish just how effective it can in fact be. by date and river zone or beat is essential. The This approach is of course only relevant to takes collation of rod catch returns by river section by from unsupported flows and would be justified the (then) South West Water Authority only when significant numbers of fish were commencing in 1983 is to be particularly migrating. commended in this respect, and the fact that the introduction of the national fishing licence 12.5 The way ahead scheme in 1992 precluded its continuation is to be regretted. The studies on the six rivers described in this report, and similar investigations elsewhere, were tn many situations, individual fishery records can fundamentally designed to explore the situation be made available by riparian owners; the data on the particular rivers involved. In Section 8 and used for the analysis on the Tamar (Section 4.4) Sections 12.1 to 12.4, attempts are made to and the Taw (Section 6.2.4) were obtained in this derive generic relationships between river flow way. Again, in Section 9.5 a recommendation is and salmon movement, and to derive guidelines made that appropriate data are analysed to for management of abstraction. As stressed in further validate this approach. Section 8.2, this approach is at an early stage and the guidelines cannot yet be safely transported Overall, the approach being developed between rivers. Recommendations are made in represents a major step towards more efficient Section 8 for analyses involving all relevant UK use of water resources. In some cases it is likely studies which should improve the predictive to indicate an adverse impact of existing resource ability of the approach considerably. Results of management. Reduction of this impact is likely any future studies can also be incorporated as to involve some loss of scheme yield, but the they become available. tactics discussed throughout this section are all aimed at achieving a balance between In the meantime, how can the results of these maximising scheme yield, minimising investigations be applied to other rivers? It is environmental impact, sustainability and cost- likely that major new schemes, or significant effectiveness. In other cases the operating rules changes in existing schemes, will for the may be unnecessarily cautious, and relaxation foreseeable future require a major programme of there could significantly increase yields. A good fieldwork as part of the Environmental example has been the early adoption of a Assessment. Guidelines for such programmes are proposed lower pf on the Tamar following given in Section 13. extensive fisheries and environmental studies, described in Section 11.3. By ensuring efficient On some rivers there may be some tracking or use of existing resources, consistent with fish counter data available that fall short of the negligible environmental impact, overall yield will full programme that would be required for a be maximised, and the requirement for new reliable analysis. Such data may nonetheless be sources, with associated potential environmental valuable in terms of indicating similarities with impacts, will be reduced. other riversstudie d in more detail, and in establishing the specification for gathering further information. Full evaluation and analysis

R&D Publication 4 99 13 GUIDELINES FOR SALMON TRACKING STUDIES

13.1 Introduction It was concluded that the 1 74 fish tagged on the Tavy gave only marginally valid results, while the From the results in this report and the analyses 255 fish on the Taw/Torridge did likewise on the developed from them, it is clear that a Torridge, where only 42 fish were recorded. It is thoroughly planned and executed radio tracking therefore concluded that 300 or more is an programme can be most effective in allowing the appropriate target for a tracking programme. It relationship between river flow and other is also necessary to spread the tagging of fish variables, and salmon migration, to be throughout much of the season for several years elucidated. in order to cover the full range of seasonal flow conditions. The experience of these studies, both in terms of gathering the data and analysing it, has shown With the tagging system used in these studies, up strengths and weaknesses of the individual the individual identification of tags was achieved programmes and their execution. It is important from the combination of the radio frequency and that the lessons learned are clearly recorded so the pulse rate. Only ten rf channels are available that future programmes can take full advantage in the ALS design. Tendency for pulse rates to of this knowledge and experience. These notes drift imposed a limit on the separation between add to the general guidelines offered by Solomon pulse rates, and battery capacity meant that very and Storeton West (1983). fast pulse rates were precluded. In practice, this limited the number of unique tag codes to about The first essential is a clearly stated project 120, which also represents the annual limit on proposal and plan, stating the primary and any numbers of separately identifiable fish that can be subsidiary aims and giving detail of how these used in any river in any year. Where fish are aims will be achieved. All other aspects of the being tagged on nearby rivers with movements programme will be derived from these between the two (e.g. Tamar and Tavy), the limit statements. applied to fish tagged in both rivers combined.

13.2 Numbers of fish A system using tags with unique signal codes operating on one or a few radio frequencies It is apparent that the number of fish would have several potential advantage:. observations available at each site is important for reliable interpretation. While no exact statistical • more uniquely-identifiable fish could be evaluation in terms of confidence limits has been tagged per river or per group of nearby conducted, it is clear that the "best" sites with a rivers hundred or more observations give very clear results when analysed using the flow-frequency • scope for mis-identification of tags should be analysis methodology described in Section 2.6. reduced The smallest number used in these analyses was 20 (TR9 on the Torridge), and several others • ALS design should be simplified were based on 30 or fewer. It is concluded that such data sets are marginal for reliable • more could be learned from fish migrating interpretation, and that 50 or more valid records between more distant rivers (e.g. Exe-tagged per site is desirable. It must also be borne in fish could be recognised if they entered the mind that these numbers represent valid, Tamar). complete, records within the months appropriate • by allowing the use of more tags for each analysis - in the cases mentioned above, simultaneously, it may be possible to shorten many more fish may have passed these points the overall timespan for a project to obtain but are not included, either because some detail valid sample sizes - though it is important is missing (usually the exact time) or because that a wise range of river conditions is they did so outside the months being considered. covered (see Section 13.3).

R&D Publication 4 100 13.3 Number of years and winter fish proportions. A comparison of the distribution of tagging effort monthly net catches and the monthly tally of fish tagged in the first table in Sections 3 to 7 gives It is clearly important to include a range of some indication of how well the tagged fish seasonal flow conditions within the timescale of a represents the net catch, and thus stock, in each tracking project. Specifically it is desirable to study. include very dry seasons and years as well as "average" or unremarkable years. Given that it is 13.4 Source of fish well established that it is low flows that adversely affect salmon migration rather than high flows, As the movements of fish up to and past the tidal and that the usual impact of water resource limit is of considerable interest in most studies, it schemes is to reduce flows, including a wet year is clearly desirable to tag and release the fish is less important. Most dry and average years will seawards of this point. As tracking in saline and in any case include wet interludes with elevated brackish water is not feasible using radio tag flows. technology, there is little to be gained from tagging a great distance seawards. The elapsed As it is not possible to predict rainfall and flows, it time between tagging at some more distant site is necessary to select a time period that is likely to and arrival at the tidal limit provides some useful incorporate the desired range of conditions. In information, but this is likely to be offset by the this respect the studies in this report were fact that fewer of the fish tagged at greater fortunate, in that the last ten years have included distance will enter the river. In most situations, several very dry seasons, especially 1989, 1990, any existing commercial net fishery in the estuary 1994 and 1995. Most studies have been started is the optimal source of fish. The fishermen are with a planned life of four or five years, The Exe usually skilful at catching salmon and are for the study was planned for five years but was stopped most part very interested to be involved in such after four because it was considered that an projects. Further, there is no undue pressure to adequate range of conditions had already been tag fish damaged during netting as there might covered, and that the expenditure on the fifth be if a dedicated netting programme were to be year would have added little. A run of five years deployed. is therefore recommended, with the option to stop after less time if appropriate or indeed to In some situations it may not be possible to extend if the full range of conditions of interest obtain sufficient fish in the estuary. Trapping at has not been experienced. or close to the tidal limit has been used in several studies. This clearly reduces the geographical For meaningful analyses it is important that the limits over which useful observations can be tagged fish are representative of the stock, or the made, but has few other drawbacks. part of the stock of interest. This generally means that fish must be tagged throughout most of the Displacing fish seawards from traps has been netting season, ideally longer. Both the smaller deployed in some studies including that on the studies in the series analysed here (Taw/Torridge Tamar described here. Displacement a short and Tavy) suffered from "clumping", with too distance seawards within fresh water appears to many fish being tagged on one or a few days have little effect on fish behaviour and can relative to the remainder of the study period. extend the range of observations, but significant This causes a potentially serious bias in the doubt must remain regarding the validity of flow/frequency analysis, with a narrow range of observation made on fish displaced significant flows tending to be over-re presented. The ideal distances back from fresh water to saline approach is to tag fish on one or two days a week conditions. throughout the season; small net catches, indicating low runs, will result in few fish tagged 13.5 Handling and tagging fish and large catches will result in larger numbers. Care should also be taken to ensure that a Tagging and releasing fish in good condition is representative sample of the catch is tagged, for most important, both to maximise survival and to example with respect to the grilse/multi-sea- ensure that the fish are behaving as naturally as

R&D Publication 4 101 possible. Details of handling and tagging of valuable data. The first record of each fish methods used in these studies are given in gives a very good indication of the time when the Section 2.3 and 2.4. fish arrived from downstream. The last record is a fairly reliable measure of when the fish passed A persistent dilemma concerns whether or not the point, especially if care is taken to check that the fish should be fitted with an external tag as the next record for each fish is upstream rather well as the radio tag. The external tag allows than downstream. Care must be taken in reporting of recaptures, as long as it is large and interpreting the last record, as the fish could, for obvious enough to be noticed by the captor. example, have ascended the weir some time However, this very requirement also dictates that before and then have rested upstream. Careful the tag may have an adverse effect upon fish occasional "on foot" determination of exact health and survival. On balance however, it was locations will be needed to check if there is a decided in all the studies reported here that the significant tendency for fish to behave in this advantages outweighed the disadvantages. way. An alternative approach that has been tried is to locate two ALS near the site, one just out of 13.6 Siting of automatic listening range downstream and one just out of range stations (ALS) upstream. While in theory this is a sound concept, in practice where this has been tried, A critically important site for monitoring fish variation in radio reception conditions has migration is at or near the tidal limit, especially if confounded interpretation, with any fish that it lies at a potential obstruction to migration such remains in the area being recorded on occasions as a weir. One or more stations seawards of the by both stations, or alternatively by one then the tidal limit can be valuable as long as there is other, interspersed with periods of no record. minimal saline influence; a station that records fish only around low tide, for example, is of less It is critically important to select ALS sites value as its record is likely to be incomplete and carefully from the start and to use those selected biased. Perhaps the main value of ALS throughout. In practice, problems can arise (e.g. installations in the tidal zone is that they will finding the most secure location, radio build-up an inventory of fish which enter the interference from other sources), especially in zone, adding information about the behaviour urban areas, which require that the optimal both of fish that never reach the tidal limit, or do network develops as the project proceeds. so only after a protracted period following However, experience with analysis of the projects tagging. in this report shows that by far the most meaningful results are obtained from ALS sites Locating several ALS sites in the lowermost few that are represented throughout the kilometres of river is generally sound, as it is here investigation. that many fish remain for considerable periods, and the migration index patterns are likely to be Where a specific period for analysis of changing most rapidly. Clearly locating an ALS movements in the lower river is selected (e.g. at or near existing or potential abstraction points May to September - see Section 2.6.2), then it is important too. can be fruitful to "thin out" the ALS deployment in the lower reaches after this time, and, once Upstream of the lowermost reaches, a good most fish have passed through this zone, re­ spread of sites at fairly regular intervals is ideal, deploy them in the upper reaches as fish enter with any perceived sites of obstruction being spawning tributaries. favoured. It can also be of interest to site ALS equipment at junctions of major tributaries or Operating single ALS sites at the tidal limit of other sharp topographical changes to examine nearby rivers could provide much information of behaviour there. fish migratory behaviour. Such installations on the Stour, Frome and Piddle recorded several fish A single ALS sited very close to a point of interest tagged on the Avon (Section 8.9). Reports of fish (e.g. tidal limit, an abstraction point or an tagged on the Tamar and Tavy in this study obstruction) will generate a considerable volume being recaptured in the tynher (5) and Dart (1),

R&D Publication 4 102 and of fish tagged on the Exe in the Teign (3), uppermost potential abstraction site they were suggest that deployment of ALS in nearby rivers immune from any further effect. would have recorded many more fish. Clearly though the potential value of such information 13.8 Quality control and reporting must be balanced against the costs, both capital and manpower, of deploying such equipment. Radio tracking results, from the print-out and audio tapes from ALS and from on-foot surveys, As described in Section 2.1, the attenuation of rf represent a complex data set. Some errors and signals is very high in water. This property can ambiguities are inevitable due, for example, to an be exploited to determine the location of tagged ALS station working improperly, frequency or fish within a very few metres, using an pulse rate drift of tags, and data "overload" of an underwater aerial. This approach was used in the ALS when more than one fish on a frequency Tamar study to determine the proportion of fish channel is present at the same time. This latter that was using the Cornwall fish pass under problem was particularly marked at the ALS site different flow conditions. A simple loop aerial at the tidal limit on the Exe, when up to 25 submerged in the pass recorded only those fish tagged fish could be within range at one time! using the pass; those passing upstream without being recorded in the pass were deemed to have Many if not all errors and ambiguities can be done so by another route. corrected with a stringent quality control system which operates in real time. By checking and 13.7 On-foot and aircraft tracking validating results on the day they are collected, any contradictions or unexpected results can be While most of the results described in this report re-examined while, for example, the audio tape were obtained from carefully-maintained and from the ALS is still retained. This approach was serviced networks of ALS, detailed searches for used on the Avon and Exe studies. While careful fish within or beyond the ALS deployment can be records were kept in the other studies, the same very valuable. In both the Avon and Exe studies, real-time approach to quality control was not regular on-foot checks were made at low tide for adopted. This resulted in a proportion of the fish located in the tidal reaches. This provided results having to be rejected from the analyses. important insight into the behaviour of fish at Examples of discrepancies encountered included time of low flow and indicated the presence of fish being apparently "missed" at several ALS fish that did not ever come within detection- before being recorded well upstream (probably range of the lowermost ALS. Without these mistaken identity), records of fish arriving at, but observations, these fish would have been not departing from, an ALS site or vice versa, and presumed to have returned seawards. Similarly, passage times between ALS of impossible speed checks through the lowermost reaches to locate (probably a wrongly recorded time). These types fish suspected of being between successive ALS of errors that can occur in the best-run sites provide useful information regarding the programmes are all easily spotted and often areas where fish may spend much of the summer. rectified by an effective real-time approach to quality control, but are almost impossible to In the Avon study, use was made of an aircraft in rectify a year or two later. some years to check the dispersion of fish beyond the uppermost ALS site, near spawning time. What proved particularly useful in the Avon and The fish may spread into literally hundreds of Exe studies was a constantly updated list of last- miles of tributaries and headwaters at this time, known records of each tagged fish. Reference to and aircraft tracking can be highly cost-effective if this while tracking and servicing ALS sites allowed spawning distribution is an issue. The fact that many potential conundrums to be avoided or detailed examination of the spawning solved on the river bank; for example, by a distribution was not made in the other studies double-check on a pulse rate which represented described here reflects the fact that they were an unexpected result. specifically investigating the actual or potential impact of abstraction for the lower or middle Regular reporting at an appropriate level is also a reaches. Once the fish were upstream of the good discipline that helps to ensure that results

R&D Publication 4 103 are processed promptly and effectively. Quarterly the order of £350,000 (1994 prices). This progress reports and detailed annual reports are equates to about £425 per fish tagged. Thus for strongly recommended. a four or five year programme involving 300 fish (as suggested for a reasonable-scale study in These disciplines are most readily applied if the Sections 13.2 and 13.3), the cost could be of the work is undertaken by a small, dedicated team order of £150,000. with real responsibility for and a commitment to achieving a worthwhile result. High staff How do the two methods compare in terms of turnover can be disastrous, especially if real-time usefulness of results for evaluation of water data processing and reporting are allowed to slip. resource schemes?

13.9 Radio tracking and fish Radio tracking generally provides information on counters compared the movements of only limited numbers of fish (typically 100 per year for several years), but over In many investigations there has been a direct a considerable geographical range and time- choice taken between installation of an electronic scale. The results obtained are very reliable; the or acoustic counter, and a radio tracking study. identity of each and every fish is known, and the Both approaches were deployed on the Tamar reliability of the system is not sensitive to flow. study. What observation can be made here to Counters provide observations on the movement help decision-making in the future? of potentially very many more fish past a single point, but there may be considerable uncertainty Factors such as logistic constraints or costs are of the veracity of each record, and differentiation likely to influence the decision. Costs for between salmon and sea trout can be electronic counters are highly site-dependent. problematic. Resistivity counters may miss fish at They are most cheaply and most readily installed high flows if the fish swim at some distance from on existing Crump-section weirs or in fish passes. the electrodes, and sizing (and thus species Where a structure has to be to specially built in a differentiation) becomes an even greater problem large river, costs are likely to be very high. with increasing distance from the electrodes. Conversely, when a suitable structure already Further, given the observation from all rivers exists, such a counter may be a cost-effective studied that the relationship between flow and option. The installation of the counter in the fish migration changes markedly over sometimes pass at Cunnislake cost of the order of £85K in small distances, particularly in the lower reaches, 1992. The current generation of acoustic it is essential to realise that results from a counter counters are under evaluation on several UK rivers cannot be readily extrapolated to other sites on including the Tavy; if proved to be effective, they the river. Thus a head-of-tide counter will be may offer a more economic solution than able to provide a good indication of the actual or electronic counters if a suitable structure for the potential impact of abstraction upon migration as latter is not already available. Costs of installation well as other information of fisheries interest at are likely to be of the order of £100K. To the that point, but the relationships observed cannot installation costs of any type of counter must be reliably be transported to other locations in the added running costs, mainly manpower, and the system. Similarly, a counter some kilometres costs of validation, data processing and analysis. above the head of tide is likely to be a poor indicator of the situation at the tidal limit. The cost of a radio tracking programme will depend upon many factors including the ease Radio tracking however, provides information of with which fish can be obtained for tagging, the the relationship between flow and migration at numbers to be tagged, the period of study and potentially a large number of sites throughout the number of ALS deployed. It is the catchment. This is particularly valuable straightforward to cost the Avon and Exe studies where existing or potential abstraction sites are accurately because all work was conducted by an situated at more than one point (e.g. on the Exe external contractor. Total costs of the two at 9.4 km, 32.3 km and 47.8 km upstream of the studies together, with the cost of ALS written off tidal limit), where the site of a potential over the nine years of the programmes, was of abstraction is as yet unknown, or where it is

R&D Publication 4 104 desired to evaluate the impact of an abstraction system for coastal and estuarine water involving a at various points downstream of the intake. It combined acoustic and radio tag (CART), and also allows identification of particular problem acoustic-receiving/radio transmitting buoys. obstructions (Section 8.4), the solution of which This system has been used on salmon in a might significantly reduce the impact of an number of estuaries including the Fowey abstraction as well as, of course, representing a (Solomon and Potter, 1988; Potter 1988) and the fishery management enhancement in its own Avon (Potter, Solomon and Buckley, 1992). right. Although deployment of this system has provided fascinating biological and behavioural data, it has Counters, on the other hand, by generating large in fact added little with respect to the effect of volumes of data, allow analysis on a sounder river flow on entry and ascent of the river. It statisticai basis. For example, the time of day vs. would of course be ideal to tag fish as they first flow analysis of salmon migration at Cunnislake approached the coast, so that they could be (Section 8.6) could not have been undertaken tracked entering and passing through the reliably on the radio tracking results, as the estuary, but such an approach is impractical if number of fish involved was too low. achievement of statistically meaningful results, involving large numbers of fish entering the There is little doubt that the ideal option is to target river, is the aim. combine both approaches on the same river. The counter will provide large numbers of results Turning now to the first question, how critical the at a single point and provide continuity in time, lack of information on the movements of fish in whereas the tracking study will provide limited coastal and estuarine water? numbers of results on a wide geographical area of the catchment, providing continuity in space. It is sometimes suggested that, at low freshwater Both techniques also provide considerable but flows to the estuary, the fish caught and tagged different "spin-off" benefits for general fisheries in the estuary are not representative of the stock management. For example, the counter as a whole. It is conceivable that a fish that was potentially allows assessment of total runs for willing to enter the estuary under conditions of management of spawning targets, and the low flow may also be willing to ascend the river tracking identifies migration bottlenecks and the at lower flows than fish that had not entered the distribution of spawning effort. estuary under such conditions. Tracking such fish could give a biased picture of the flow/migration 13.10 Tracking fish in coastal waters relationship of the stock as a whole. However, there are two pieces of evidence that this As discussed in Section 2.1, saline and brackish problem does not occur to any great extent, at water are effectively opaque to radio signals, so least as far as the studies described here are radio tags cannot be used to track fish in coastal concerned. First, there is evidence that low waters and in the saline part of estuaries. Radio freshwater flows do not deter salmon from tags can of course be fitted to fish which are entering the netting zones to any great extent; released into saline water, but the first records net catches are generally maintained during that can be obtained will be at the tidal limit or in periods of low flow, and the net fisheries involved the freshwater tidal zone. This reduces the data here are mostly intercepting fish passing through that can be collected by radio tracking, but how to the upper estuary, and are not exploiting a critical is this limitation with respect to stock that has built-up over a period of days or investigation of water resource impacts, and are weeks. On occasions, at times of very low flow, it alternatives available? was observed that the nets on the Tamar and Exe catch some "coloured" fish, including occasional Dealing with the second question first, acoustic radio tagged fish that had clearly been in the transmitting tags can be used in saline water, and estuary for some time. Such fish are termed a considerable number of fish have been tracked "droppers" and are presumably returning in this way, including salmon (Hawkins et al seawards in response to deteriorating 1979). Solomon and Potter (1988) describe the environmental conditions in the upper estuary. development and deployment of a tracking However, this phenomenon is readily recognised

R&D Publication 4 105 when it occurs and does not introduce any bias been observed (Section 4.3). The counter data into the results. would not be vulnerable to the potential "low water" bias that could affect tracking results. The second piece of evidence comes from This is another example of fish tracking and fish comparison of the threshold flows obtained from counters complementing each other to provide a radio tracking and those obtained from the fish more robust conclusion, counter on the Tamar; very similar values have

R&D Publication 4 106 14 REFERENCES

Aprahamian, M.W, Nicholson, S.A., Best, R.A., Sambrook, H. (1987). Homing of sea trout: Shaw, R,A. and Kaar, E.T. (1997). Design and use evidence derived from the River Fowey stock. In: of open channel resistivity fish counters. Picken, M.j. and Shearer, W.M. (Eds). The sea Environment Agency R&D Technical Report W23. trout in Scotland, 1 3-24. SMBA/DAFS.

Buck, R.J.G. and Hay, D.W. (1984). The Salmon Advisory Committee (1990). The effects relationship between stock size and progeny of of fishing at low water levels. Atlantic salmon, Sa/mo sa/arL., in a Scottish MAFF/SOAFD/WOAD. 17pp. stream. |. Fish Biol. 23,1-11. Solomon, D.J. (1982). Tracking fish with radio Hawkins, A.D., Urquuhart, G.G. and Shearer, tags. Symp. zool. Soc. Lond. No. 49, 95-105. W.M. (1979). The coastal movements of returning Atlantic Salmon, Sa/mo sa/ar{\-.). Solomon, D.J. and Storeton-West, T.|. (1983). Scottish Fisheries Research Report 14. 15pp. Radio tracking of migratory salmonids in rivers: development of an effective system. Fish. Res. Lawson, J.D., Sambrook, H.T., Solomon, D.J. and Tech. Rep., MAFF Direct. Fish. Res., Lowestoft Weilding, G. (1991). The Roadford Scheme: (75)11 pp. minimizing environmental impact on affected catchments. ]. Inst. Wat. Env. Mgmt. 5, 671 -681. Solomon, D.J. and Potter, E.C.E. (1988). First results with a new estuarine fish tracking system. Potter, E.C.E. (1988). Movements of Atlantic J. Fish Biol. 33 (Supplement A), 127-1 32. salmon, Salmo salarV., in an estuary in south­ west England. |. Fish Biol. 33 (Supplement A), Stasko, A.B. and Pincock, D.C. (1977). Review of 153-159. underwater biotelemetry with emphasis on ultrasonic techniques. J. Fish. Res. Board Can., Vol Potter, E.C.E., Solomon, D.J. and Buckley, A.A. 34, 1261-1285. (1992). Estuarine movements of adult Atlantic salmon {Sa/mo sa/ar\..) in Christchurch Harbour, Stuart, T.A. (1962). The leaping behaviour of southern England. In: Priede, I.G. and Swift, S.M. salmon and trout at falls and obstructions. (Eds) Wildlife telemetry, remote monitoring and Freshwater and Salmon Fisheries Research 28. tracking. 400-409. Department of Agriculture and Fisheries for Scotland. HMSO. 46pp.

R&D Publication 4 107 APPENDIX I GLOSSARY

ADF "Average daily flow" or mean flow for a particular site. May be based on a period of gauged flow or may be calculated "naturalised flow".

ALS Automatic listening station (described in Section 2.3).

Base Flow An index of the base flow (or low flow) with respect to the mean flow at a site. Index Highly impervious catchments may have a 6FI as low as 0.15, while a chalkstream BFI may exceed 0.9.

DMF "Daily mean flow" or the mean flow for a specified day, usually 09:00 hours to 09:00 hours the following day.

CS Flow gauging station

HEP Hydro-electric power

HW High water

LOR Licence of Right. An abstraction licence issued under the terms of the 1963 Act, whereby existing abstraction were licensed without additional restrictions.

Naturalised (Q95, ADF). A flow statistic that has been calculated to represent the natural state, i.e. without abstraction, regulation and effluent discharges. May be calculated from a period of gauged flow or may be a theoretical figure based on catchment area, rainfall, geology etc.

pf Prescribed flow. A condition attached to any abstraction licences which precludes abstraction that would cause river level to fall below a certain level, the "prescribed flow".

PWS Public Water Supply

Q95 A flow characteristic of a site being the discharge that is exceeded for 95% of the time. It is generally accepted as a typical dry-weather flow. May be based on a period of gauged record or "naturalised".

STW Sewage treatment works

WTW Water treatment works

R&D Publication 4 108 APPENDIX II INPUTS TO THE TAMAR SALMON MIGRATION MODEL

Introduction d = multiplier for Dp to derive D.. This is flow dependent. These notes apply to the Tamar migration model described in Section 10.3 and illustrated in Figure f = multiplier for proportion of run that is 10.1. delayed by high flows. The numbers of salmon passing the tidal limit at Availability Cunnislake Weir on day n =R (Figure 10.1). n An availability curve was developed from the According to the analyses undertaken, this can be catch curves from the Tamar nets for 1985-1994 considered to comprise: (Fig Al ) and the counter curves for 1 992, 1 993 1. The potentially available number of salmon and 1994. The derived curve (in fact a series of entering the estuary on that day = A, straight lines), indicating the proportion of the year's run that becomes available to enter the 2. A as modified by D in previous days (point 6 f river each day is based upon: below) and by net catch = Am. • from March 1 to March 31 an assumed level 3. The number of those fish that enter the river of availability of 1% of peak level = A^, which is flow dependent. • a minor increase through April 4. The availability of a "backlog" of fish = B

(equivalent to the cumulative sum of Am - A^ • the net catch curve between May 1 and for the year so far). August 31 5. That part of the backlog that is stimulated to • a two-stage decline between August 31 and run = B,, which is flow dependent. November 1 6. Any tendency for elevated flows to draw in • from November 1 to November 30 an fish from the theoretical stock of future days assumed level of availability of 1 % of the = 0,. peak 7. That part of the run delayed by very high • zero availability in December, January and flows (Del). February. 8. The availability of fish delayed by very flow Availability multiplier = a on earlier days that are released by falling flows (Rel). Flow influences the proportion of newly "available" fish that will run on that day. At R A + B + D + R •'• n = ™ , , el - Del residual flows above 2.5 m3/sec all will run. At A spreadsheet programme was developed to flows below 1 m Vsec none will. The simplest predict Rn on this basis and to allow adjustment model form is a straight line between these two of input constants to develop the best "fit" with points. Thus: observed phenomena. for flows below 1 mVsec, a = 0 All reference to flows is to the residual daily mean for flows (Q) between 1 mVsec and flow at Cunnislake Weir. 2.5 m Vsec, a =(Q-l)x0.75 Development of input constants for flows above 2.5 mVsec, a = 1. The following input constants are required for the Those fish that do not run that day enter the above model: backlog. A = availability Backlog multiplier = b a = multiplier for availability, which Flow influences the proportion of the backlog of determines the proportion of A that will fish that will run that day. From the analysis of actually run that day (i.e. \_); this is flow- the tracking and counter data, elevation of runs A above that "expected" begins at around 3 dependent. m Vsec. Although the data are sparse, this b = multiplier for backlog, which determines tendency appears to increase to a maximum at the proportion of the backlog that will run about 6 mVsec. As there is a strong tendency for that day (i.e. Bj; this is flow-dependent and some fish in the backlog to never become B available, and as no spate appears to clear the varies with how long the fish have been in the backlog completely, a maximum level of 0.75 is backlog. assumed. The simplest "straight line" model is D^ = the potentially available numbers of fish thus: that can be drawn from future A.

R&D Publication 4 109 for flows below 3 mVsec, b = 0 for flows above 7 mVsec, d = 1. for flows (Q) between 3 mJ/sec and 6 mVsec, Delay multiplier (f) b = (Q-3)x0.25 Very high flows cause a delay in running by fish for flows above 6 m Vsec, b = 0.7S. stimulated to migrate. At flows of less than 20 In the first ten days in the backlog, a multiplier of mVsec, the multiplier used is 1 i.e. there is 0.5 of b applies. For the second period of ten deemed to be no delay due to high flows. At days, a value of 0.2 of b applies. It is necessary to flows above 50 mVsec, f = 0.1 i.e. 90% of the keep individual track of each day's contribution potential run does not migrate on that day. At to the backlog so that any remaining after ten intermediate flows between 20 and 50 mVsec and twenty days can be appropriately transferred. intermediate values apply. Thus After twenty days, any fish still remaining the for flows below 20 mVsec, f = 1 backlog are transferred to "backlog 2" which does not become available to migrate until the for 20 - 50 m Vsec, f = 1 - ((Q-20) x 0.03) secondary phase of migration, nominally after for flows over 50 m Vsec, f = 0.1 September 15. After that date, backlog 2 is available to migrate dependent upon high flows. The multiplier f applies to both that day's run Consideration of tracking and counter results plus any remaining fish delayed from earlier days. indicates that sizeable runs of all fish in the autumn are associated with significant increases Time-specific inputs in flow of considerable magnitude. Therefore a To run the model for a specific year, the value of b(2) of 0 applies to flows below 8 m Vsec following inputs are required: and for higher flows that are less than two times the level of two days earlier, and 0.5 for flows • Residual daily mean flows at Gunnislake above 8 mVsec which are more than two times Weir; these are used to derive values for a, b, the flow two days earlier. b(2), d and f (above). Potential drawn-in stock • Daily net catches. These are used to modify the availability of fish to run i.e. they are It is assumed that a maximum of a quarter of the taken into account in deriving Am from A. stock which would approach the river on each of The tracking studies have indicated that it the next two days will be drawn-in by high flows. typically takes 1 - 8 days for fish tagged at D^, on day „ is therefore: the estuary nets to enter the river. Thus any days net catch may effect the next eight (0.25 xA^)* (0.25 xA^J days runs. The reported net catch on each Draw-in multiplier = d day is therefore deducted equally from the availability of the next 8 days. A similar model is used here as for backlog multiplier, as it is not possible to differentiate • A multiplier for the year (M). Availability is a between fish derived from the backlog and from derived value which cumulates to 153 for the draw-in among a run associated with increased year. A multiplier is required to turn this into flow. However, the draw-in is allowed to reach numbers of fish. In the model runs so far 100% of D^, by increasing the range of influence undertaken the multiplier used is 50, giving to 3-7 mVsec. Thus: a notional total run of 7650 fish. This gives a fairly good overall fit with counter results, J for flows (Q) below 3 m /sec, d = 0 but clearly there is scope for the year for flows (Q) between 3 mVsec and 7 m Vsec, multiplier (M) to be varied to reflect high or d = (Q-3)x0.25 low runs of fish.

R&D Publication 4 110 This publication explores the relationship between salmon migration and river flow in a range of rivers in South West England. Over 1800 adult salmon were tracked from the estuaries and throughout six rivers using radio telemetry between 1985 and 1994. New analytical techniques are developed to predict the influence of flow changes caused by abstraction and river regulation. Although the catchment characteristics of the six rivers are varied, broadly consistent patterns of behaviour have emerged. General relationships and models are developed which are of relevance throughout the British Isles and beyond. This publication will be invaluable to water resource managers and fishery managers with responsibilities for balancing the needs of salmon and the best use of water. It will also prove of great interest to fishery scientists, behavioural biologists and salmon anglers throughout the distribution range of the Atlantic salmon.

Additional copies of this report may be obtained from Dr D J Solomon, Foundf) I aim. Kiln I ane, Redlynch, Salisbury, Wiltshire. SP5 2HT. UK. Tel: 01725 512523. Fax: 01725 512964. Email: Publications(«;Solomon.softnet.co.uk

ENVIRONMENT AGENCY SOUTH WEST Wi^TER