' ,- ì ' .. ¿l i : =," ¿ o '¡"i i,-.:: ' CÐ ;ti t- '- {j r-- . zo No. 130 Trout stocks in the Pukaki and Ohau hydro canals ISSN 01I3-25M New Zealand, Freshwater Fisheries Report No. 130 Trout stocks in the Pukaki and Ohau hydro canals by S. Bloomberg E. Graynoth Report to: Electricorp Freshrvater Fisheries Centre MAF Fisheries CHRISTCIIT]RCH S ervicing freshtva t er fi she ri e s and aquaculture September 799t NEW ZEALAND FRESHWATER FISHERIES REPORTS This report is one of a series issued by the Freshwater Fisheries Centre, MAF Fisheries. The series is issued under the following criteria: (t) Copies are issued free only to organisations which have commissionedthe investigation reported on. They will be issued to other organisations on request. A schedule of reports and their costs is available from the librarian. (2) Organisations may apply to the librarian to be put on the mailing list to receive all reports as they are published. An invoice will be sent for each new publication. ISBN 0-477-08445-r Edited by: S.F. Davis The studies documented in this report hâve been funded by the Electricity Corporation of New Zealand Limited. ill INISiRY OF AGRICIJLTURE ANO FISHEB'ES T: ivlÀNATU AH Ur¿VH EN U ÀHlJM0A¡J MAF Fisheries is the fìsherìes business group of the New Zealand Ministry of Agriculture and Fishe¡ies. The name ìr{AF Fisheries was tbrmalised on I November 1989 and replaces MAFFish, which was established on I April 1987. It combines the tunctions of the tbrmer Fisheries Research and Fisheries Management Divisions, and the tisheries functions of the former Economics Division. T1le New Zealand Freshwater FÌsheries Report series continues the Nevv Zealand Minisnl of Agriculture and Fisheries, Fßheries Environmental Report series. Enquiries to: The Librarian Freshwater Fisheries Centre lvfAF Fisheries PO Box 8324 Riccarton, Christchurch New Ze¿land CONTENTS Page SUMMARY 1. INTRODUCTION 2. STUDY AREA 3. METHODS 7 4. BROWN AND RAINBO\M TROUT 8 4.t Species Composition 8 4.2 Abundanee aud Distribution 8 4.3 Recruitment and Movements 9 4.4 I-ength, Weight, and Condition Factor 9 4.5 Age and Growth 10 4.6 Diet 12 5. NATIVE FISH T4 6. DISCUSSION l4 7. ACKNOWLEDGEMENTS t7 8. LITERATURE CITED l7 TABLES 1. Physical tèatures of the Pukaki and Ohau canals 't 2. Fyke net catches in the Pukâki and Ohau canals, June 1983 to June 1984 8 J. Seine net catches in the Pukaki and Ohau canals, June 1983 to May 1984 9 4. Numbers and size of brown and rainbow trout observed by divers in three sections of tbe Ohau Canal 9 on 2l February 1984 5. Details of tagged trout which were recaptured in nets or by anglers T2 Freshwater Fisheries Centre Page 6. Mean length, weight, and condition factor of brown (BT) and rainbow (RT) trout caught in fyke and 12 seine nets in the Pukaki and Ohau canals 7. Size composition of brown aud rainbow trout in the Ohau Canal based on fyke and seine net catches and 14 drift-dive counts 8. Relationship between condition factor and length of trout in the Pukaki and Ohau canals, 1983-84 14 9. Mean back-calculated length at age for trout netted in the Pukaki and Ohau canals 14 10. Diet of trout in the Pukaki and Ohau canals 15 11. Comparison of trout stocks in the Pukaki and Ohau canals with other waters in the upper Waitaki 16 catchment FIGURES 1. The Pukaki and Ohau canals showing fyke and seine netting sites 6 2. Cross section of the Pukaki and Ohau canals 10 3. Correlation between fyke net catch rate and f'low volume in the Pukaki and Ohau canals 1l 4. Length tiequency distribution of brown and rainbow trout caught by fyke and seine nets in the Pukaki 13 and Ohau canals Freshrvater Fisheries Centre SIJMMARY power stations and into I-ake Benmore. The first stage of the canal system was completeà in 19'77 when water was diverted into the Tekapo Canal. The Pukaki and The Pukaki and Ohau canals were netted and drift dived Ohau canals were constructed soon afte¡ and now carry from June 1983 to June 1984 to assess their fish stocks water from I¡kes Pukaki aud Ohau to Ohau A power aud value as a fishery. They coutained brown and station at the head of I¿ke Ruataniwha. The canal rainbow trout and four species of native fish: common system was finally completed in 1985 when the outflow bully, upland bully, koaro, and longfinned eel. Brown from l¿ke Ruataniwha was diverted through Ohau B trout comprisedS2To of the total catch and outnumbered and Ohau C canals into the Haldon arm of Lake rainbow trout in both canals. Benmore. Seine net catch rates were extrapolated to give estimates The canals have become naturally stocked with brown of 13 trout/hectare in the Ohau Canal and 15 trout/ and rainbow trout from upstream lakes and have hectare in the Pukaki Canal. Drift dive counts were developed into popular recreational fisheries. They are thought to unde¡estimate numbers of trout in the Ohau easily accessible by road and open to fishing all year Canal because there were not enough divers to cover round. Angler catch rates are reasonably high the whole width of the canal. Trout appeared to be (Bloomberg et aI. 1983) and it has been suggested that evenly distributed along the length of both canals. the canals compensate for the loss of the Ohau River which was effectively dewatered when water from I-ake The canal bed is unsuitable for trout spawning, which Ohau was diverted into the canal system (Graynoth suggests that canal stocks are maintained by fish moving le84). downstream from I-akes Pukaki and Ohau. The fish stocks and fisheries of canals have been well Brown trout (mean length : 320 mm, mean weight : studied overseas (Hynes L97O), however very little is 430 g) were slightly larger than rainbow trout (mean known about the fisheries aspects of canals in New length : 296 mm, mean weight : 375 g). The mean Zealand. The aim of this study was to collect condition factor of brown and rainbow trout was very intbrmation on brown and rainbow trout stocks in the low (102 and 108, respectively). Pukaki and Ohau canals to gain a better understanding of the fish stocks and fisheries of canals in this country. Sixty-two percent of trout netted in the canals were The presence of native fish species iu the Pukaki and estimated to be aged 2 or 3 years, although ages ranged Ohau canals was documented and the value of the canal from 1 to 10 years. The percentage of trout aged 1 fishery was assessed. year was relatively low (7%), possibly because many juveniles reared in upstream lakes and rivers. This study was part of a five-year research programme, designed to investigate the impact of hydro-electric The slow growth rate of trout in the canals is thought to power development on the fish stocks and fisheries of be the result of a lack of food and instream cover. the upper Waitaki catchment. Benthic invertebrates were scarce in the canals and winged terrestrial insects were abundant only in the spring and summer. Small fish did not appear to be an 2. STUDY AREA important prey item. Compared to many upper Waitaki lakes and rivers, the The Pukaki and Ohau canals are situated north-west of canals supported moderate numbers of mostly medium- Twizel township in the MacKenzie basin of the South sized trout in poor condition. The canals are unlikely Island (Fig. l). They are constructed of glacial outwash to be rated highly by anglers and do not compensate for gravel embankments, lined with compacted, clayey the loss of the Ohau River due to hydro-electric power gravels and au outer layer ofcompacted coarse gravels. development. The canals' dimensions are given in Table 1. Flow in the canals is regulated by control gates at the 1.' INITRODUCTION outlet of Lakes Pukaki and Ohau. Flow volume depends on the demand for electricity and on lake levels, and is usually greatest at 0700 hours and The upper Waitaki catchment has been modihed between 1700 and 1900 hours on week days. Flows are extensively by hydro-electric power development generally higher in spring in the Ohau Canal and in (Fig. 1) (James 1985). A t'eature of this development autumn and winter in the Pukaki Canal (Electricorp has been the construction of alarge canal system which flow records). carries water from upstream Iakes through a series of Freshwater Fisheries Centre Ohou ¡\ Powerhouse Pukoki Dom/ Loke Ruotoniwho Ohou B Powerhouse Ohou C Po*erhouse- Loke Benmore IIIGURE 1. The Pukaki and ohau canals showing fyke and seine neuing sites. TABLE 1. Physical featr¡res of the Pulcaki and Ohau Fyke and seine netted trout r¡/ere identified, measured to canals. the nearest mm (fork lengtå), and weighed to within l0 g if they were longer thau 200 mm or tåe nearest gram if they were less than, or equal to, 200 mm fork Pukaki Ohau length. Trout greater than 300 mm fork length were Canal Canal tagged just below the dorsal fin with a serially numbered Floy anchor tag. Tags had a Fisheries l,ength ftn) l3 8.25 Research Division address printed on them so that Water surface width (m) 7t 45.5 recapture details could be sent by anglers to MAF. Water depth (m) 10 5.8 Native fishes were identified and released. Maximum flow volume (m3ls) 4Æ 170 Gradient l:12 000 l:5880 The condition factor (CÐ of trout was calculated from Secchi depth (meau) (m) - 0.5 5.8 the expression: cF : Y.to' L The Pukaki and Ohau canals contain brown and rainbow where V/ was weight in grams and L was fork length in trout as well a few laud-locked quiunat salmon millimetres.