Fish Populations in a Tidal Estuary in Marlborough Sounds, New Zealand, from 1971 to 2004.
J. Roger Bray and Gwendolyn J. Struik
P. O. Box 494, Nelson 7040, New Zealand
http://www.oceansatlas.org
© J. Roger Bray and Gwendolyn J. Struik, 2006
Published in 2006 and obtainable from http://www.oceansatlas.org
Copyright © J. Roger Bray and Gwendolyn J. Struik 2006 P. O. Box 494, Nelson 7040, New Zealand [email protected]
All rights reserved. This book is copyright. With the exception of fair dealing for purposes of research or private study, or criticism or review, no part of this publication may be reproduced, stored or transmitted in any form or by any means, without prior permission in writing from the copyright holders. Copying for general distribution, for advertising or promotional purposes, for creating new collective works or for resale is prohibited. Special requests should be addressed to the copyright holders.
ISBN 0-473-11195-0
Acknowledgements We thank Anne and Vic Marchant, Greg and June Harney, Claire and Ernie Twose; and Paul Creswell and Scott Williamson of Ministry of Fisheries; N.Z. Lottery Science Research, and the libraries of Canterbury University, Greta Point fisheries research and Nelson.
About the Authors J. Roger Bray and Gwendolyn Struik have doctorates in plant and animal ecology from the University of Wisconsin. After teaching and research in the U.S.A and Canada, they moved to New Zealand in 1963 and have lived in Nelson and Duncan Bay where, since 1971, they have worked on forest regeneration, possum populations and estuarine fish. Dr. Bray has published over 70 papers in scientific journals and books in North America, Europe and Australasia on ecology and climatology, and has worked at the Universities of Minnesota and Toronto and at D.S.I.R. in New Zealand. Dr. Struik has scientific publications on forest ecology in North America, grasslands ecology and fisheries in New Zealand and has worked at Wheaton College, Nelson Polytech and D.S.I.R.
Fish populations in a tidal estuary in Marlborough Sounds, New Zealand from 1971 to 2004. J. R. Bray and G. J. Struik (2006). http://www.oceansatlas.org
ABSTRACT
From 1971 to 2004, we sampled the fish populations of Te Mako tidal estuary in Marlborough Sounds, New Zealand by setting a monofilament net, at low tide, in the same location for a total of 2832 tides.
Fifteen hundred and eighteen fish of 23 species were caught and measured. Results showed percent tidal occurrence was 26, number per tide 0.54, mean weight per fish 783g, weight per tide 420g, mean length per fish 355 mm and length per tide 190 mm.
The eleven major species, followed by family name and weight per tide, were divided into Early Dominants- Rig (Inshore shark) 73g and Yellow-eyed mullet (Mullet) 12g, Mid Dominants- Snapper (Seabream) 68g and Barracouta (Snake mackerel) 14g, Later Dominants- Kahawai (Kahawai) 119g, Warehou (Raft) 40g and Spotty (Wrass) 3g, and Invaders- Flounder (Right-eyed flounder) 29g, Grey mullet (Mullet) 14g, Blue mackerel (Tuna) 9g and Dab (Right-eyed flounder) 3g.
Predominant species were the Early Dominants in the early and late 1970s, the Mid Dominants in the mid 1970s, followed by the Invaders in the early to mid 1980s and the Later Dominants thereafter.
A schooling index was inversely related (Rs -0.98, p<.01) to percent of single individuals per tide. Of 55 species pairs, 18 had significant positive associations and none were significantly negative. Positive interspecific association was related to intraspecific association, to an environmental variable and to food preference, which indicated fish- eating species had the highest percentage of significantly associated species pairs, while invertebrate and bottom feeders were less associated with other species.
All species showed distinctive seasonal, temperature, rainfall and day/night distributions, and nearly every species peaked on a new or waning moon. A significant relationship was shown between spring precipitation and a consequent increase in fish weight and length followed in the next year by increases in weight, length and number per tide, and percent tidal occupancy, which may reflect a flushing of nutrients from land to sea during the crucial vernal warming period.
The population variables for the sum of all species declined between 1971-86 and 1987- 2002 and, especially, from 1971-74 to 2001-04 when there were declines in weight per fish (71%), length per fish (48%), number per tide (70%), weight per tide (91%), length per tide from (85%) and occupied tides (46%). These massive declines may reflect the large increase in unregulated foreign fishing vessels in New Zealand from 1967 to 1977, subsequent increase in domestic fishing, deterioration of nearby benthic communities by trawling and dredging, and an increase in nutrient loss to farmed mussels.
Fish populations in a tidal estuary in Marlborough Sounds, New Zealand from 1971 to 2004. J. R. Bray and G. J. Struik (2006). http://www.oceansatlas.org
SHORT ABSTRACT
Fish netted in a tidal New Zealand estuary from 1971 to 2004 declined between 1971-74 and 2001-04 by 46% in percent occupied tides, 70% in number per tide, 71% in weight per fish, 91% in weight per tide, 48% in length per fish and 85% in length per tide. There was a shift from Early to Mid Dominant species followed by fluctuation between Later Dominants and, briefly, Invaders. Degree of interspecific association was significantly dependent on food preference, a seasonal variable and level of intraspecific association. Spring precipitation was positively related to increases in fish weight and length, followed in the next year by increases in number, weight and length per tide.
Table of Contents
Chapter page
Summary...... 1
1. Introduction...... 5
2. Site description and sampling methods ...... 5
3. Te Mako fish species and their New Zealand distribution ...... 8
4. Fish species data summary ...... 12
5. Species numbers, weights and lengths...... 15
6. Species Importance Index...... 17
7. Intraspecific and interspecific association...... 20
8. Stomach content and fullness, food preference and interspecific association...... 29
9. Species number, diversity and presence ...... 34
10. Diversity and global abundance-weight profiles ...... 36
11. Biyearly fish population variables...... 40
12. Percent changes in the six biyearly population variables ...... 50
13. Biyearly running means for weight per tide and length per tide ...... 51
14. Early, Mid and Later Dominants and Invaders...... 54
15. Environmental variables and fish populations...... 56
16. Yearly fish population variables and precipitation...... 59
17. Te Mako fish population trends...... 64
References...... 71
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 1
SUMMARY
Chapter 1. Introduction. A monofilament net was set at monthly intervals on 493 occasions and 2832 tides from 1971 to 2004, at low tide, in the same location in Te Mako estuary, Duncan Bay, Marlborough Sounds, New Zealand (41o 7’ 35” S, 173o 45’75” E).
Chapter 2. Site description and sampling methods. Te Mako estuary has a catchment of warm temperate rain forest, sheep pasture and coastal shrubs, sedges and rushes. The estuary is tidal with patches of sand, silt and Eel grass. The set net was 27.4 m with stretched mesh size of 124 mm. Fish caught were weighed and measured.
Chapter 3. Te Mako fish species and their New Zealand distribution. Of the twenty- three species netted, there was a mean overlap with eleven New Zealand estuarine and coastal sites of 64%. For the eleven major species, the mean overlap was 75% with 91 to 100% overlap with the three closest sites.
Chapter 4. Fish species summary. Eleven major species occurred in twelve or more years and twelve minor species occurred in three or less years. Seven original major species were first caught between 1971 to 1974, and four later arrivals first caught from 1977 to 1981. Number per tide x 1000 and mean weight per tide were Kahawai 130 and 119g, Warehou 110 and 40g, Yellow-eyed mullet 80 and 12g, Yellowbelly Flounder 68 and 29g, Snapper 53 and 68g, Blue mackerel 19 and 9g, Rig 19 and 73g, Barracouta 16 and 40g, Spotty 16 and 3g, Dab 9 and 3g, Grey mullet 7 and 14g.
Chapter 5. Species weights, lengths and numbers. Mean weight per species was 1147g, mean length per species 418 mm, weight per tide 18g, length per tide 8 mm and number per tide 0.023. Total number of individuals per tide was 0.54, of weight per tide 420g and of length per tide 190 mm.
Chapter 6. Species Importance Index. A percent Importance Index based on an equal weighting of three relative frequency, two relative dominance and one relative abundance value showed Kahawai, with an Index of 22.7%, was double the amount for each of the next four highest species, Warehou, Yellow-eyed mullet, Flounder and Snapper.
Chapter 7. Intraspecific and interspecific association. The percent of single individuals in an occupied tide varied from 31 for Rig to 96 for Barracouta and Spotty. Schooling indexes were a near mirror image of percent single individuals, varying from 1.0 for Barracouta and Spotty to 1.9 for Blue mackerel, 2.2 for Rig and 3.3 for Warehou. Number of species per tide varied from zero to four and an index of occupied tides for the eleven major species was significantly correlated with the number of their significantly associated species pairs. Of 55 species pairs, 18 had positive significant interspecific associations. When the 55 species pairs were divided into three levels of declining interspecific association, then in seven out of eight instances (X2 4.5, p<.05) there was a decline in the rank correlations between the species pairs and four environmental variables, but only the seasonal variable was significantly related to the species pairs when they were considered separately. http://www.oceansatlas.org 2
Chapter 8. Stomach content and interspecific association. Kahawai and Barracouta eat mainly fish; Warehou, Snapper and Blue mackerel eat invertebrates and fish, Rig and Spotty eat mainly invertebrates, and Yellow-eyed mullet, Flounder, Dab and Grey mullet eat mainly invertebrates and benthic plants, detritus and sediments. The more a species depends on other fish for food, the higher its percentage of significant positive interspecific associations. Species which eat mainly invertebrates, or are bottom and detritus feeders, have the lowest percentage of significantly associated species pairs. These connections between food preference and interspecific association, and the correlations between intra and interspecific association and between an environmental variable and interspecific association illustrate the complexity of the relationships which underlie fish interspecific associations.
Chapter 9. Species number, diversity and presence. When the influence of search effort on the number of species found was reduced by dividing species numbers by the square root of the number of tides, then the transfomed number of new species per year declined from an initial maximum in 1971-75 to 1981-85 and then increased to the present. This increase occurred during a time when the species population variables were declining which raises doubts of the usefulness of species diversity as an indicator of ecosystem stability or productivity.
Chapter 10. Diversity and global abundance-weight profiles. If diversity is judged by degree of approach to a multitypic model in which each species has the same abundance or weight per tide then, by two different tests, abundance-weight profiles for Te Mako are more diverse than the mean of 52 global studies.
Chapter 11. Biyearly fish population variables. The eleven major species were analysed for changes in six population variables during 17 biyearly periods from 1971 to 2004 and during two halves from 1971-86 and 1987-2002.
Between the first and second halves: Ten of the eleven species declined in percent of occupied tides with declines of 89, 80, 77 and 74% for Rig, Spotty, Snapper and Dab, and a mean decline of 53%.
Ten species declined in number per tide with a mean decline of 54%. Rig, Blue mackerel, Spotty, Snapper and Dab each declined by over 70%.
Eight species declined in mean weight per fish, with increases for Rig and Grey mullet that were not statistically significant. The largest declines were for Warehou 31%, Kahawai 30%, Barracouta 29%, Dab 25% and Yellow-eyed mullet 25%.
All the eleven species declined in weight per tide by a mean of 64%. Major decreasers were Rig 94%, Spotty 83%, Snapper 82%, Dab 81% and Blue mackerel 79%.
Eight species decreased in length per fish, with a mean decline of 5%. Species declines were less than for weight per fish, but the rank correlation between the two variables was significant (Rs 0.93, p<.01).
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 3
All major species declined in length per tide by from 9% to 96% with a mean of 60%. Largest declines were by Rig 96%, Blue mackerel 87%, Spotty 84%, Snapper 81% and Dab 79%. Weight per tide and length per tide had a significant rank correlation (Rs 0.81, p<.01).
Population variables calculated for the sum of all twenty-three netted species showed declines between 1971-86 and 1987-2002 of 27% for percent of occupied tides,18% for number per tide, 52% for weight per fish, 30% for length per fish, 64% for weight per tide and 50% for length per tide.
Chapter 12. Percent changes in the six biyearly population variables. Between the first and second halves there were 58 declines and 8 increases (X2 37.9 p<.001) in the 11 major fish species for the 6 population variables.
Chapter 13. Biyearly running means for weight and length per tide. Non-weighted running means for both weight and length per tide showed six patterns: 1) steep declines for Rig and Yellow-eyed mullet, 2) sustained declines interrupted by lesser recoveries for Barracouta and Snapper, 3) persistent declines for Kahawai with two moderate recent recoveries, 4) declines to nil for Warehou and Spotty, then increase to secondary peaks and further decline, 5) increase from nil to peaks in mid 1980s for the major early invaders Blue mackerel, Dab, Flounder and Grey mullet followed by steep to gentle declines, 6) very slow increase for the eleven later invaders.
Chapter 14. Early, Mid, and Later Dominants, and Invaders. Patterns of change in the number per tide and weight per tide were used to designate periods of dominance for four dynamic groups: 1) Early Dominants, Rig and Yellow-eyed mullet, 1971-74, 1979- 80; 2) Mid Dominants, Snapper and Barracouta, 1975-78, 1981-88, 1993-94; 3) Later Dominants, Kahawai, Warehou and Spotty, 1979-80, 1987-2005; 4) Invaders 1981-86, 1997-98. The three groups of Dominants declined from 1971-72 to 1977-78 and partially recovered to 1983-86 accompanied by the appearance and rapid increase of Invaders from 1977-78 to 1981-86. Invaders then declined and the Later Dominants increased to the present with minor increases for the Early and Mid Dominants.
Chapter 15. Environmental variables and fish populations. Every major species had a strong seasonal occurrence and every bimonthly period had at least one dominant species except March-April. Warehou peaked in July-August, Blue mackerel and Barracouta in September-October, Yellow-eyed mullet and Snapper in November- December, Rig, Spotty and Grey mullet in January-February and Kahawai and Flounder in May-June. Warehou and Barracouta declined from colder to warmer intervals, Kahawai, Blue mackerel, Flounder and Dab peaked in the second coldest interval, Snapper in the second warmest and Rig, Spotty, Yellow-eyed mullet and Grey mullet in the warmest. Yellow-eyed mullet, Rig, Spotty and Grey mullet peaked in the lowest rainfall period, Warehou and Dab in the intermediate period and Blue mackerel, Kahawai, Barracouta, Snapper and Flounder peaked strongly in the highest interval. Warehou, Blue mackerel, Yellow-eyed mullet, Spotty, Grey mullet and Dab peaked on a waning moon in the Last Quarter. Kahawai, Snapper and Rig peaked on the New Moon, Flounder on a waxing moon in the First Quarter and Barracouta on the Full Moon. http://www.oceansatlas.org 4
Warehou, Blue mackerel, Barracouta, Yellow-eyed mullet and Snapper appeared from 79% to 90% on night tides, the other species occurred 63% to 68% on night tides.
Chapter 16. Yearly fish population variables and precipitation. The population variables were not related to Te Mako yearly precipitation , but there were highly significant relationships between spring precipitation and weight and length per fish and between summer precipitation and both weight and length per tide and the other three variables. There were discrete sequences from spring precipitation to the population variables. The basis for these sequences may be increased spring and summer rainfall, especially during the crucial vernal warming period, which resulted in increased flushing of nutrients from the land to ocean with a consequent increase in plankton at the bottom of the food chain, an increase in weight and length per fish and a subsequent increase in weight and length per tide and the other population variables.
Chapter 17. Te Mako fish population trends. The six population variables for all species declined 40% from 1971-86 to 1987-2002 and declined 69% from 1971-74 to 2001-04 which had losses of 46% for occupied tides, 48% length per tide, 70% number per tide, 71% weight per fish, 85% length per tide and 91% weight per tide. Dominant species shifted from Early to Mid to Later Dominants and, briefly, Invaders, but all major species have persisted.
The decline in the six fish population variables at Te Mako may be the result of: 1) a large increase in unregulated foreign fishing vessels in New Zealand which removed a catch of from 20,000 t in 1968 to a peak 500,000 t in 1977, 2) rapid increase in domestic commercial and recreational fishing and the use of new technology, 3) destruction of nearby benthic communities by trawling and dredging, and, 4) increase in farmed mussels in Pelorus Sound which feed on the plankton base of the food chain.
If the pressures which resulted in the fish declines in Te Mako estuary are reduced, then recovery scenarios can be predicted based on short term recovery dynamics observed over the past 34 years. Following a 45% decline, populations may recover to 60% for Dab, Flounder and Grey mullet in 2 to 3 years, for Kahawai, Yellow-eyed mullet, Barracouta, Snapper and Spotty in 5 to >31 years and for Blue mackerel, Warehou and Rig in >19 to >32 years. Mean recovery to 100% was 7 years for the fast species, and >27 years for the medium to slow recovery species. The three fast recovery species plus Spotty were the four least commercially valuable fish. Fast recovery species have a mean age to maturity of 2.9 years and a life span of 8.2 years compared with 3.6 and 23 years for the medium recovery species and 4.7 and 17 years for the slow recovery species. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 5
1. Introduction
For 34 years, from 1971 to 2004, we set a monofilament net on 493 occasions and 2832 tides in the same location in Te Mako estuary, Duncan Bay, Tennyson Inlet, Marlborough Sounds, South Island, New Zealand (41° 7’ 35” S, 173° 45’ 75” E – Figure 1). An initial publication (Struik & Bray, 1979) summarised results after the first eight years during which time the number of fish per tide and the mean weight per fish declined significantly for the five species with an adequate sample size for statistical analysis. Since 1994, we have been granted Ministry of Fisheries research permits and supplied them with annual information, for their data bank, on number and size of fish collected, species, number of tides fished, and dates of fishing.
2. Site description and sampling methods
2.1 ESTUARY LOCATION AND SIZE
Te Mako tidal estuary is adjacent to the small settlement of Duncan Bay and is a part of a complex 600 km coastline of steep sided drowned valleys jutting north eastward into Cook Strait. The estuary is approximately seven ha in size with a maximum intertidal zone nearly 800 m long and 300 m wide at its widest diameter. The vertical tidal range at the net at high tide varies from 1 to 2.8 m. http://www.oceansatlas.org 6
2.2 CATCHMENT DESCRIPTION.
The estuary’s catchment is warm temperate angiosperm-podocarp rain forest. Some of the lower portion of this forest was selectively logged for podocarp species and all the lower flats were converted to sheep pasture in the late 19th century. The immediate vicinity of the estuary is rain forest to the east and west, sheep, and occasionally cattle, pasture to the south and open water to the north. The settlement of Duncan Bay has 74 houses, twelve of which are occupied for at least half of the year, the others are occupied during weekends and holidays. All houses are on quarter acre sections, back from the waters edge, with some sections designated or informal forest reserves. On many sections, and on the road margins, the dominant vegetation is regenerating forest. Nearly all the catchment, except for the settlement and the margin of pasture owned by a local farmer, is publicly owned native forest reserve, administered by the N.Z. Department of Conservation. The catchment is drained by a permanent stream which rises to flood levels during high intensity rain storms.
2.3 ESTUARY HABITAT, PLANTS AND ANIMALS
The estuary substrate is a matrix of alluvial gravels and small stones with a covering of silt, and occasionally sand, along the stream channels. Seagrass (Zostera), averaging 74 mm in length, grows on the silty edges of feeder streams. There have been very occasional green algal blooms, but the only sustained period, when about 10% of the estuary was covered with sea lettuce (Enteromorpha – G. McRaild 1981, p.c.) was in September 1980 to January 1981.
The intertidal part of the estuary margin is covered with salt meadow rushes (Leptocarpus and Juncus) and low ground plants (Samolus and Sellieria). The surrounding margin above the tidal zone contains Manuka (Leptospermum), Coprosma, and Plagianthus shrubs, mixed with sedges (Mariscus) and pasture plants. These coastal communities are described in Morton & Miller (1968) and Moore & Adams (1963).
Shellfish present include Cockle (Chione stutchberyi ), Pipi (Paphies australe) and the non-native Pacific oyster (Crassostrea gigas) which arrived around 1986 and covered about half of the rocky substrate. The native Blue mussel (Mytilus edulis), and Green mussel (Perna canaliculus), seeded in by a local marine farmer in the 1980s, are both in very low numbers now, perhaps due to recreational harvesting or to oyster competition. The oyster population, with its white shells, is very evident still, but appears less robust than a decade ago. The Mud flat crab (Helice crassa) is common and found in the gut of fish such as Snapper and Flounder.
Resident estuarine fish, all of which are too small to be caught in our net, include rock fish (Acanthoclinus quadridactylus) and triplefins (Tripterygion spp). In season, there are Galaxias spp in the stream, and these are found in the gut of fish such as Kahawai. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 7
Birds seen in the estuary while we were clearing the net, in order of decreasing numbers, were hybrid ducks, Spur-winged plover, White-faced heron, Variable oystercatcher (black phase), Paradise shelduck, Black-backed gull, Pied shag, Kingfisher, Welcome swallow, Black swan, White-fronted tern, Australasian gannet, Black-fronted tern, Black shag, Caspian tern, Australasian harrier and Weka (names from Heather & Robertson, 1997).
Three aquatic mammals, Dusky dolphin, Orca and New Zealand fur seal have appeared in Duncan Bay, the Dusky dolphin nearly every year, one Orca mother and calf, for one day, and two male seals, one very large, for a residence of about four months. When the seals were present, eaten remains of fish in our net increased and there were large holes in the net on three occasions. These large holes were almost certainly the result of the seals, since no such holes had occurred in the net before they came and none have appeared since they left. Terrestrial mammals in the estuary are humans, dogs, rats, sheep and cattle with the pasture margin extensively grazed by sheep.
2.4. PHYSICAL ENVIRONMENT
Rainfall is the major weather factor causing changes in the estuary. The mean annual rainfall in the past twenty years (1984-2003) is 2460 mm with a range from 1620 mm (1985) to 3600 mm (1998). Three high intensity events resulting in stone, sand and silt deposition as well as change in stream courses happened in November 1994 (863 mm) and twice in October 1998 (191 and 536 mm). As far as we could determine, there was no noticeable long-term effect on bird or fish life in the estuary after these events.
Air temperatures vary from –5° C to +35° C, but rarely reach these extremes. Ground frosts occur between 26 March and 2 October, with a mean of 37 per year (Bray, 1991). Frosts occur less frequently in the estuary than on the surrounding low open slopes.
2.5 SAMPLING METHODS
A monofilament 27.4 m set net, stretched mesh size 124 mm, 16 units high was placed in the same position in the estuary at about one third the distance from the lowest low tide. The water depth at the net at high tide varied from approximately 1 m to 2.8 m. The net was cleared at all low tides, except when the low tide occurred in the middle of the night. Fish caught on these "double tides" could often be assigned to a single tide by their condition, and double tides which included only one fish could also be used in summary calculations. There were only two of our quantitative calculations, night and day tides and percent occupied tides, for which the remaining double tides could not be used.
All fish caught were weighed and measured. In the early years, 1971-1978, weights were in pounds and ounces with an accuracy of 150 grams and smaller fish weighed together for an average measurement. After 1978, weights were to 10g accuracy and fork length measured to 1mm accuracy. Some lengths prior to 1978 were estimated from weight/length graphs of fish caught 1978-1990. From 1971-1989, sampling was done throughout the year but on an irregular basis dictated by factors unrelated to the study. Since 1990, at least 8 tides per month were sampled (except January and February 1994). In the 34 years of the study 2832 tides and 1518 fish were sampled. http://www.oceansatlas.org 8
3. Te Mako fish species and their New Zealand distribution
3.1. FISH SPECIES IN TE MAKO ESTUARY
Twenty-three fish species were netted and six species stranded in Te Mako estuary from 1971-2004. Our species symbols, together with common, scientific and family names as in Ayling & Cox (1984), are shown in Table 1, with the order of species based on the number of individuals sampled. There were four cartilaginous species netted, Rig, Eagle ray, Elephant fish and Carpet shark, and nineteen bony species, Kahawai, Warehou, Yellow-eyed mullet, Flounder, Snapper, Blue mackerel, Barracouta, Spotty, Dab, Grey mullet, Jack mackerel, Blue cod, Trevally, Red mullet, Gurnard, Red cod, Red snapper, Skipjack tuna and Spotted stargazer. The six stranded species, in chronological order of arrival, were Leatherjacket, Estuarine stargazer, Porcupine fish, Ihi (garfish), Short-tailed stingray and Sunfish.
3.2. TE MAKO SPECIES IN OTHER NEW ZEALAND ESTUARIES AND NEAR COASTAL WATERS.
Presence of the 23 netted Te Mako species is shown for five New Zealand estuaries (E) and six coastal and offshore waters (W) in Table 2. Te Mako estuary is approximately 600 km from the farthest south site, Otago, and 570 km from the farthest north site of Leigh. The sites are listed from south to north with each site followed by its reference(s): Otago (W), Graham (1956); central Canterbury (W), Beentjes et al,. (2001); Christchurch, Avon-Heathcote (E), Webb (1972, 1973), Owen (1992); Nelson, Waimea (E), Davidson & Moffat (1990), Struik (1975); Nelson (W), Grange et al. (2003); Te Mako (E), this study; Porirua (E), Jones & Hadfield (1985), Leach & Davidson (1976); Southern North Island, New Plymouth, Palliser Bay, Whakatane (W), Hickford et al. (1997); Napier, Ahuriri (E), Kilner & Akroyd (1978); Auckland, Manukau (E), Morrison et al. (2002); Hauraki Gulf (W), Kendrick & Francis (2002); and Leigh (W), Thompson (1981).
No species was present in all of the eleven non- Te Mako sites of Table 2, but Kahawai, Yellow-eyed mullet, Barracouta, Dab, Gurnard, Red cod and Spotted stargazer were present in ten sites and Flounder, Spotty and Trevally were present in nine sites, so that nearly half the Te Mako species could be regarded as widely distributed. Snapper, Blue mackerel and Rig, three other major Te Mako species, occurred in eight of the sites, while Warehou was present in six of the eleven other sites. Grey mullet was in four of the sites around Nelson and northwards, but not further south in Canterbury or Otago. Three species, Red snapper, Skipjack tuna and Sunfish, which occur in the far north of New Zealand appeared only once, in each case in mid summer, and all since 2001.
The percentages by which the 23 Te Mako species appeared in the eleven estuarine and coastal sites in Table 2 from south to north were 70, 70, 48, 70, 78, 78, 74, 57, 26, 78 and 61 with a mean percentage of 65 %. Overlap for only the eleven major Te Mako species with the eleven estuaries and coastal waters was substantially higher, with percentage values of 82, 82, 64, 91, 91, 100, 64, 82, 36, 87 and 54 with a mean of 75 %. There was an expected close percentage affinity of 91, 91 and 100% for the three nearest geographic Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 9
Table 1. Fish species symbols, common, scientific and family names of the 23 netted and 6 stranded species in Te Mako estuary 1971-2004, listed in order of abundance. Eleven major species, from Kahawai to Grey mullet (in bold type), and 12 minor species, from Jack mackerel to Spotted stargazer, were netted. The remaining 6 species (in italics) were stranded. Nomenclature Ayling & Cox 1984.
Symbol Common name Scientific name Family name K Kahawai Arripis trutta Arripidae W Warehou Seriolella brama Centrolophidae Y Yellow-eyed mullet Aldrichetta forsteri Mugillidae F Flounder, Yellow-belly Rhombosolea leporina Pleuronectidae S Snapper Chrysophrys aureus Sparidae Bm Blue mackerel Scomber australasicus Scombridae R Rig Mustelus lenticulatus Triakidae B Barracouta Thyrsites atun Gemphylidae Sp Spotty Pseudolabrus celidotus Labridae D Dab, Sand flounder Rhombosolea plebeia Pleuronectidae G Grey mullet Mugil cephalus Mugillidae J Jack mackerel Trachurus declivus Carangidae C Blue cod Parapersis colias Mugiloididae T Trevally Caranx georgianus Carangidae Rm Red mullet Upeneichthys lineatus Mullidae E Elephant fish Callorhynchus milii Callorhinchidae Gu Gurnard Chelidonichthys kumu Triglidae Rc Red cod Pseudophycis bachus Moridae Er Eagle ray Myliobatis tenuicaudatus Myliobatididae Rs Red snapper Centroberyx affinis Berycidae St Skipjack tuna Katsuwonus pelamis Scombridae Cs Carpet shark Cephaloseyllium isabella Scyliorhinidae Ss Spotted stargazer Genyagnus novaezelandiae Uranoscopidae L Leatherjacket Parika scaber Balistidae Es Estuarine stargazer Leptoscopus macropygus Leptoscopidae P Porcupine fish Allomycterus jaculiferus Diodontidae I Ihi, Garfish Hyporhamphus ihi Hemiramphidae Su Short-tailed stingray Dasyatis brevicaudatus Dasyatidae Su Sunfish Mola mola Molidae http://www.oceansatlas.org 10
Table 2. Distribution data for the 23 Te Mako species. Location of Te Mako species in other New Zealand estuaries (E) and in coastal waters (W): in Otago (O), Canterbury (C), Nelson (N), Te Mako (T), Porirua (P), southern North Island (S), Ahuriri, Napier (A), Manakau (M), Hauraki Gulf (H) and Leigh (L), listed from south to north. Eleven major species are in bold type.
Spp Habitat ( W or E) and Location (O, C, N, T, P , S, A, M, H, L) Sym W W E E W E E W E E W W O C C N N T P S A M H L K x x x x x x x x x x x W x x x x x x Y x x x x x x x x x x x F x x x x x x x x x x S x x x x x x x x x Bm x x x x x x x x x R x x x x x x x x x B x x x x x x x x x x x Sp x x x x x x x x x x D x x x x x x x x x x x G x x x x x J x x x x x x x C x x x x x x x x T x x x x x x x x x x Rm x x x x x E x x x x x x Gu x x x x x x x x x x x Rc x x x x x x x x x x x Er x x x x x x x Rs x x St x Cs x x x x x x Ss x x x x x x x x x x x
Data sources: WO-Graham 1956; WC-Beentjes et al. 2002; EC-Webb 1972, 1973, Owen 1992; EN-Davidson & Moffat 1990, Struik 1975; WN-Grange et al. 2003; ET- this study; EP-Jones & Hadfield 1985; Leach & Davidson 1976; WS-Hickford et al. 1997; EA-Kilner & Akroyd 1978; EM-Morrison et al. 2002; WH-Kendrick & Francis 2002; WL-Thompson 1981. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 11 sites to Te Mako: Nelson’s Waimea estuary and Nelson coastal waters on the South Island and Porirua estuary on the North Island. From the south to north, the percentage overlap was 62% southern South Island, 69% northern South Island, 70% southern North Island, 55% northern North Island. For the eleven major Te Mako species the mean percent overlap from south to north was 76, 91, 82 and 58. These patterns show little difference in the mean overlaps south or north of Te Mako for the 23 species present, but a closer affinity for the sites immediately south of Te Mako for the eleven major species. The Te Mako estuarine species were not closely similar with New Zealand estuaries at a great distance as shown by percent overlap values of 48 for the Avon-Heathcote estuary, Christchurch, South Island and 26 for the Manukau estuary in Auckland northern North Island. The two closest estuaries to Te Mako, Nelson’s Waimea and Porirua both had percent overlap values of 78, nearly twice the mean percentage for the three most distant sites. The six major Te Mako species that are shallow bottom feeders, Yellow-eyed mullet, Flounder, Dab, Grey mullet, Spotty and Rig occur in 89% of the estuaries and 67% of the coastal waters in Table 2, while the five species that are mainly pelagic, Kahawai, Warehou, Snapper, Blue mackerel and Barracouta occur in 67% of the estuaries and 87% of the coastal waters. http://www.oceansatlas.org 12
4. Fish species data summary
4.1. YEAR SPECIES FIRST APPEARED
Six original species, Kahawai, Warehou, Yellow-eyed mullet, Snapper, Rig and Barracouta were caught in 1971 and together with Spotty, which was first caught in 1974, comprised 30 percent of the twenty-three species (Table 3, column 2). Five early invader species Flounder, Blue mackerel, Dab, Gurnard and Grey mullet were first caught between 1977 and 1981 and eleven later invaders were caught in two batches, Blue cod, Elephant fish, Trevally and Jack mackerel in 1986–1994 and Red mullet, Red cod, Eagle ray, Skipjack tuna , Red snapper, Carpet shark and Spotted stargazer from 1996-2002. Six species stranded in the estuary were Leatherjacket in 1972, Estuarine stargazer in 1974, Porcupine fish in 1974, Ihi in 1995, Short-tailed stingray in 1996 and Sunfish in 1996.
4.2 TOTAL YEARS SPECIES CAUGHT
The twenty-three species caught at Te Mako separate into two distinct groups: eleven major species which appeared in twelve or more of a possible thirty-four years and twelve minor species which were present in three or less years. The major species were present in from 33 to 97 percent of the 34 years while the minor species varied from 3 to 9 percent (Table 3, column 3).
Of the eleven major species Kahawai, Yellow-eyed mullet and Snapper were netted in 30 to 33 years, Flounder and Barracouta in 25 years, Blue mackerel in 20 years, and Rig, Spotty, Warehou, Dab and Grey mullet in 12 to 15 years. Of the twelve minor species, seven were netted in 2 or 3 years, and five in only one year.
4.3 NUMBER OF SEASONS SPECIES CAUGHT
The total number of times in which a species occurred in the summer (January, February, March), autumn (April, May, June), winter (July, August, September) and spring (October, November, December) is shown in Table 3, column 4. There were a total of 136 seasons for the period 1971-2004. Yellow-eyed mullet occurred in 87 seasons, Kahawai in 82, Flounder in 70 and Snapper in 65 seasons, which were between 64 % and 48 % of the total possible seasons. Barracouta, Spotty, Warehou and Blue mackerel were in 26 to 18 percent of the total possible seasons, Rig, Dab and Grey mullet were around 12 % and the remaining twelve species occurred in less than 3% of the total seasons.
4.4. NUMBER OF TIDES SPECIES CAUGHT
The number of tides on which each species was netted varied from 1 to 228 (Table 3, column 5). Of the 2832 tides for which the net was set, at least one fish was caught on 724 tides, a total of 26 % occupied tides. The five most common species, Kahawai, Warehou, Yellow-eyed mullet, Flounder and Snapper occurred in 228 to 93 tides for a total of 779 tides which was 79 % of the total number of tidal occurrences of 986 by all Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 13
Table 3. Summary data for 23 netted Te Mako species plus 6 stranded species, 1971- 2004. Species symbol, first year caught or stranded, number of years caught, number of seasons caught, number of tides caught, number of fish caught (Abundance), total fish weight (kg) and total length of fish (cm). Species listed in order of abundance. Eleven major species are in bold type, the six stranded species are in italics.
Species 1st year Number Number Number Number Weight Length Symbol caught of years seasons of tides* of fish (kg) (cm) K 1971 30 82 228 367 338 13730 W 1971 15 28 93 304 113 7761 Y 1971 32 87 170 227 35 5335 F 1977 25 70 168 193 81 6405 S 1971 33 65 120 151 192 5746 Bm 1980 20 25 29 55 25 1768 R 1971 12 17 25 55 208 5652 B 1971 25 35 45 46 114 3633 Sp 1974 14 31 44 45 9 1019 D 1980 13 15 20 26 9 786 G 1981 15 16 17 20 38 1030 J 1994 34672 185 C 1986 33331 72 T 1990 3333<1 60 Rm 1996 22333 145 E 1990 22222 113 Gu 1980 22221 59 Rc 1999 12222 87 Er 2001 222211 163 Rs 2002 11122 67 St 2001 11112 47 Cs 2002 11111 57 Ss 2002 1111<1 18 L 1972 1111 - - Es 1974 1111 - - P 1974 1111 - - I 1995 1111 - - Sh 1996 1111 - - Su 1996 1111 - -
*Double tides counted as a single tide when more than one fish of a species caught. There were 88 such tides. http://www.oceansatlas.org 14
species. The other six major species from Barracouta to Grey mullet were present in from 17 to 45 tides, a total of 180 occurrences, 18% of the total. The remaining twelve minor species had 27 tidal occurrences, less than 3% of the total.
The number of tidal occurrences for the Te Mako species is closely related to the number of fish caught as shown by a rank correlation between the number of tidal occurrences and the number of individuals (Rs +0.87, p<.01) for the eleven most common species. The first five major species do not have a significant correlation between number of tidal occurrences and individuals (Rs +0.40 p>.05) because of the low number of tidal occurrences of Warehou which has a ratio of 93 tides to 304 individuals (31 %) compared with 228 tides and 367 individuals for Kahawai (62 %).
4.5. NUMBER OF FISH CAUGHT
Fish abundance for the 23 netted species (Table 3, column 6) ranged from 367 to 1, with a well spread distribution between 24.1% and 0.1 % of the total. The five most common species, Kahawai with 367, Warehou with 304, Yellow-eyed mullet with 227, Flounder with 193 and Snapper with 151 made up 1242 or 82 % of the total catch while the six intermediate species from Blue mackerel with 55 to Grey mullet with 20 made up 16% of the total catch. The twelve minor species with 7 or less individuals made up the remaining 2 %. The well spread abundance values of Te Mako fish numbers contrasts with many estuaries and on-shore fisheries summarised in Chapter 10.
4.6. WEIGHT OF FISH CAUGHT
The total weight of the 23 fish species ranged from less than 1 kg to 338 kg (Table 3, column 7) with a total catch of 1188 kg. Five species exceeded a total weight of 100 kg, Kahawai 338, Rig 208, Snapper 192, Barracouta 114 and Warehou 113 kg. Flounder, Grey mullet, Yellow-eyed mullet and Blue mackerel were between 81 and 25 kg, and all the other species were 11 kg or less, with eleven species 3 kg or less.
4.7 LENGTH OF FISH CAUGHT
Total length of fish caught varied from 18 cm for Spotted stargazer to 13730 cm for Kahawai with six species, Kahawai, Warehou, Flounder, Snapper, Rig and Yellow-eyed mullet over 5000 cm. In spite of large differences in species shapes, the total length of the eleven major species (Table 3, column 8) was correlated with their total weight (Rs +0.79, p<.01).
Yellow-eyed mullet had the highest length (cm) over weight (kg) ratio of 152 and Grey mullet, Rig, Snapper and Barracouta had the lowest ratios from 26 to 32. Kahawai, Warehou and Flounder, which had the three highest total lengths had intermediate length over weight ratios of from 41 to 79. The lowest ratio for the minor species was 15 for the broadly shaped Eagle ray. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 15
5. Species numbers, weights and lengths
5.1 MAXIMUM, MINIMUM AND MEAN WEIGHTS AND LENGTHS
Maximum weight per fish (Table 4) varied from 200g for Trevally to 7755g for Eagle ray. Mean weights were from 113g for Trevally to 5690g for Eagle ray and minimum weights were from 25g for Yellow-eyed mullet to 3625g for Eagle ray. Minimum lengths varied from 110 to 730 mm, maximum from 230 to 1150 mm and means from 180 to 1028 mm. A comparison of Te Mako species maximum lengths with maxima quoted in the literature shows nine of our species exceeded 75% of the quoted maxima including Rig, Kahawai, Flounder, Blue mackerel, Spotty, Dab, Grey mullet, Red mullet and Eagle ray. Only two very low abundance species, Blue cod and Trevally, were less than 50% of their maximum quoted lengths.
5.2 NUMBER, WEIGHT AND LENGTH PER TIDE
The three major productivity variables, fish number per tide, weight per tide and length per tide are shown in the last three columns of Table 4. Number per tide x 10000 for the major species varied from 4 to 1296, with Kahawai and Warehou more than 1000, seven species from 159 to 802, two species from 71 to 92 and the remainder less than 25. There was a less even distribution for weight per tide due to greater differences in weight per fish. Eight species had more than 10 g per tide and 10 species were less than 1 g per tide. Eighty-one percent of the total weight per tide was concentrated in five species, Kahawai, Rig, Snapper, Warehou and Barracouta. Length per tide was dominated by six species which had over 18mm per tide, Kahawai, Warehou, Yellow-eyed mullet, Flounder, Snapper and Rig. Length per tide data were closely related to weight per tide (Rs +0.94, p<.01).
Mean species weight per fish was 1147g, weight per tide 18.2g, length per fish 418 mm, length per tide 8.3 mm and number per tide 0.233. For the ecosystem as a whole, the weight per fish was 783g, weight per tide 420g, length per fish 355 mm, length per tide 190 mm and number per tide 0.54. http://www.oceansatlas.org 16
Table 4. Maximum, mean and minimum weight and length per fish, number per tide times 10000, weight per tide in grams and length per tide in mm, for 2832 tides, 1971- 2004. Literature maximum lengths from Ayling & Cox 1984, Graham 1956 and Paul 2000. Eleven major species are in bold type
Weight per fish Length per fish Number Weight Length grams mm. per tide per per Spp Max. Mean Min. Max. Max. Mean Min. x 10000 tide tide lit. grams mm. K 3180 921 75 1000 840 374 165 1296 119.4 48.5 W 2724 370 90 750 377 255 165 1073 39.7 27.4 Y 430 154 25 500 305 235 110 802 12.3 18.8 F 790 420 70 500 405 332 160 681 28.6 22.6 S 6580 1270 140 >1000 725 381 225 533 67.7 20.3 Bm 2050 449 84 550 554 321 205 194 8.7 6.2 R 7140 3779 1240 1500 1150 1028 650 194 73.4 20.0 B 4100 2486 400 1500 970 790 290 162 40.4 12.8 Sp 300 190 125 270 255 226 180 159 3.0 3.6 D 670 346 130 450 410 302 210 92 3.2 2.8 G 3000 1924 266 750 640 515 293 71 13.6 3.6 J 390 245 120 500310 264 220 25 0.6 0.7 C 240 212 180 600251 240 225 11 0.2 0.3 T 200 113 40 600230 200 150 11 0.1 0.2 Rm 1470 1053 840 400 542 483 440 11 1.1 0.5 E 1000 1000 1000 1200628 565 500 7 0.7 0.4 Gu 900 471 42 600 440 295 150 7 0.3 0.2 Rc 950 770 590 800 470 435 404 7 0.5 0.3 Er 7755 5690 3625 1000 900 815 730 7 4.0 0.6 Rs 825 823 820 600 345 335 320 7 0.6 0.2 St 1900 1000 473 4 0.7 0.2 Cs 1400 2000 570 4 0.5 0.2 Ss 400 450 180 4 0.1 0.1 Means 1147 418 233 18.2 8.3 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 17
6. Species Importance Index.
6.1 RELATIVE FREQUENCY
Quantitative data on fish frequency, abundance and dominance were converted to a relative basis to allow direct comparisons between species and to facilitate the construction of a single index of the relative importance of the different species (Table 5).
Three relative frequency values for years caught, for seasons caught and for tides caught are summarised in Table 5, columns 2 to 4, and the means are shown in column 5 as a single relative frequency value. The distribution of relative frequency values in columns 2, 3 and 4 of Table 5 shows Kahawai, Yellow-eyed mullet, Flounder and Snapper had the highest values in each of the three frequency distributions. Relative number of years caught (column 2) was strongly dominated by Snapper, Yellow-eyed mullet and Kahawai with values of 11.7 % to 12.9 %. The other major species varied from 4.7 % to 9.8 % and the minor species from 0.4 % to 1.2 %. The 23 values for percent years caught had a well spread distribution with no large breaks or dominance by a single species, possible evidence of a stable and mature fish population.
The relative number of seasons caught had the same five major species as did the number of years caught, but with slightly higher relative values. Relative number of seasons caught had a highly significant rank correlation with relative number of years caught (Rs +0.97, p<.01) and with the number of tides caught (Rs +0.99, p<.01).
The relative number of tides on which the five major species were caught had a slightly closer rank correlation with number of seasons caught (Rs +0.99, p<.01) than with the number of years caught (Rs +0.96, p<.01).
6.2 RELATIVE ABUNDANCE
Relative abundance values in Table 5, column 6, again demonstrate the importance of the five most numerous species which have a total relative abundance of 82 percent. The five most abundant species include four species, Kahawai, Yellow-eyed mullet, Flounder and Snapper, which are in common with the five highest mean relative frequency species in column 5. Three of the most abundant species, Kahawai, Snapper and Warehou are in common with the highest relative dominance species in column 9.
6.3 RELATIVE DOMINANCE
The two relative dominance data sets (Table 5, columns 7, 8) of weight per tide and length per tide have a significant positive rank correlation with each other (Rs +0.73, p<.05), but vary in their relationships with the various frequency and abundance distributions. Weight per tide is significantly correlated with weight per fish (Rs +0.73, p<.05), but not with abundance (Rs +0.50, p>.05) while length per tide and abundance have a highly significant correlation (Rs +0.95, p<.01), but length per tide is not correlated with length per fish (Rs +0.19, p>.05). http://www.oceansatlas.org 18
Table 5. Relative data for the 23 netted species: 1) three frequency values: Years caught (YC), Seasons caught (SC) and tides caught (TC); 2) Percent frequency (%F) which is the mean of YC plus SC plus TC; 3) Percent abundance (%A), which is the relative number of individuals per species; 4) Two dominance values, Weight per tide (W/T) and Length per tide (L/T); 5) Percent dominance (%Dom) which is the mean of W/T plus L/T. The Importance Index (I. I.) in column 10 is the sum of %F, %A and %Dom (columns 5, 6,and 9) divided by three. Eleven major species are in bold type.
Species % % % % % % % % I. I. YC SC TC F A W/T L/T Dom K 11.7 16.4 23.0 17.0 24.1 28.5 25.5 27.0 22.7 W 5.7 5.6 9.4 6.9 19.9 9.5 14.4 12.0 12.9 Y 12.5 17.4 17.1 15.7 14.9 2.9 9.9 6.4 12.3 F 9.8 14.0 16.9 13.6 12.7 6.8 11.9 9.4 11.9 S 12.9 13.0 12.1 12.7 9.9 16.1 10.6 13.4 12.0 Bm 7.8 5.1 2.9 5.3 3.6 2.1 3.3 2.7 3.9 R 4.7 3.4 2.5 3.5 3.6 17.5 10.5 14.0 7.0 B 9.8 7.1 4.5 7.1 3.0 9.6 6.7 8.2 6.1 Sp 5.5 6.3 4.4 5.4 3.0 0.7 1.9 1.1 3.2 D 5.1 3.0 2.0 3.4 1.7 0.8 1.5 1.2 2.1 G 5.9 3.2 1.7 3.6 1.3 3.2 1.9 2.6 2.5 J 1.2 0.8 0.6 0.9 0.5 0.1 0.3 0.3 0.6 C 1.2 0.6 0.3 0.7 0.2 0.1 0.1 0.1 0.3 T 1.2 0.6 0.3 0.7 0.2 t 0.1 0.1 0.3 Rm 0.8 0.4 0.3 0.5 0.2 0.3 0.3 0.3 0.3 E 0.8 0.4 0.2 0.5 0.1 0.2 0.2 0.2 0.3 Gu 0.8 0.4 0.2 0.5 0.1 0.1 0.1 0.1 0.2 Rc 0.4 0.4 0.2 0.3 0.1 0.1 0.2 0.2 0.2 Er 0.8 0.4 0.2 0.5 0.1 1.0 0.3 0.6 0.4 Rs 0.4 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 St 0.4 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.2 Cs 0.4 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 Ss 0.4 0.2 0.1 0.2 0.1 t t t 0.1 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 19
The three summary indexes in Table 5 (columns 5, 6 and 9) relative frequency, relative abundance and relative dominance have significant rank correlations with each other. There is a very high correlation (Rs +0.97, p<.01) between relative frequency and relative abundance. Relative frequency is also significantly correlated with relative dominance (Rs +0.88, p<.01) and relative abundance has a significant, but lower correlation with relative dominance (Rs +0.56, p<.05).
6.4 IMPORTANCE INDEX
An Importance Index (Table 5, last column) for each of the 23 Te Mako species was constructed by calculating the mean of the sum of relative frequency in column 5, relative abundance in column 6 and relative dominance in column 9. The distribution of Importance Index values shows that Kahawai, with an Importance Index of 22.7, is nearly double the index for the next four highest species, Warehou, Yellow-eyed mullet, Snapper and Flounder, which varied from an index of 12.9 to 11.9. The remaining six major species varied in Importance Index from 7.0 to 2.1, with a mean of 4.1. The twelve minor species numbered between seven and one fish with Importance Indexes from 0.6 to 0.1. The two highest minor species had indexes of 0.6 and 0.4 as a result of their relative frequency (Jack mackerel) and their relative dominance (Eagle ray) values.
The distribution of the Importance Index values in Table 5 is skewed by Kahawai, which exceeds each of the next four species indexes by nearly double. Comparison of the distribution of the Importance Index values in the last column of Table 5, in descending order, with the distribution of relative abundance and weight profiles for the 122 global studies in Table 15.1, indicates the Te Mako Importance values are closer to a theoretic multitypic model than to a less diverse monotypic model. http://www.oceansatlas.org 20
7. Intraspecific and interspecific association
7.1 INTRASPECIFIC ASSOCIATION AND SCHOOLING
The distribution of the number of tides for each species in which one or more of its individuals were caught is shown in Table 6. All eleven of the major species had at least one tide with two individuals, seven species had at least one tide with three individuals and six species had one or more tides with four individuals. Only Yellow-eyed mullet, Kahawai, Blue mackerel, Rig and Warehou had at least one tide with 5 or more individuals up to a maximum of twenty-two.
A Schooling Index, the number of individuals divided by the number of tides, is shown in the last column of Table 6. The index varies from 1.022 to 3.27 with the eleven species in three groups: Barracouta and Spotty 1.022 to 1.023, Flounder, Grey mullet, Snapper, Dab and Yellow-eyed mullet 1.15 to 1.34, and Kahawai, Blue mackerel, Rig and Warehou 1.61 to 3.23. Of the four species with a high Schooling Index, Blue mackerel is probably anomalous since it has only two instances of a tide with more than two individuals and one of these tides had twenty-two individuals, the highest number of individuals per species during the study. The Schooling Index has a highly significant negative rank association (Rs –0.98, p<.01) with the percentage of times an individual of a given species is solitary and appears only once in a tide, as shown in the second to last column of Table 6. The two distributions are almost mirror images of each other.
7.2. INTRASPECIFIC ASSOCIATION AND WEIGHT DISTRIBUTION PER SCHOOL
The distribution of weights for Warehou, in the 24 tides with four or more Warehou shown in Table 7, ranged from 145 to 1120 grams. The mean weight ratio between the highest and lowest weight in each tide was 1.69 and ratios for the 24 tides varied from 1.17 to 2.42. These low ratios probably reflect an even aged composition for each tide, with most tides showing a well spread distribution between their heaviest and lightest fish.
The distribution of Kahawai weights in Table 8 is similar to the distributions shown for the presumed uniformly aged Warehou, but only for tides with weights under around 650 grams. For Kahawai, tides with heavier weights to the right of tide number 2117 had an increasing tendency for high over low weight ratios to increase to above 2 and range up to over 14. Along with this increase in weight ratio, there are examples of a bimodal spreading of the distribution with one or a few fish at the top, a few if any fish in between and the remainder, usually the majority, at the bottom of the weight range. Given the large difference between the heaviest and lightest fish and the cannibalistic habit of large Kahawai, (Baker, 1971) it is possible the top fish were feeding on the very small ones. All the other weight distributions in Table 8, for Blue mackerel, Yellow-eyed mullet, Flounder, Snapper and Rig, have well distributed weight ranges or are too small to analyse.
Fish populations of aNew Zealand estuary from 1971 to 2004. Bray & Struik (2006)
Table 6. Intraspecific association: Percent singles and Schooling Index. Column 2 shows the number of individuals; Column 3 is the number of tides of occurrence; Columns 4 to 19 list the number of tides with 1 to 22 individuals. The last two columns show the percent of tides with single individuals and the Schooling Index, which is the number of fish of a given species divided by its number of tidal occurrences.
Spp No. No. 1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 22 % School ind. tides Single Index B 46 45 44 1 95.6 1.022 Sp 45 44 43 1 95.5 1.023 F 193 168 149 14 4 1 87.0 1.15 G 20 17 14 3 70.0 1.18 S 151 120 98 14 7 1 64.9 1.26 D 26 20 14 6 53.8 1.30 Y 227 170 130 32 3 2 1 1 57.7 1.34 K 367 228 169 28 16 4 5 4 1 1 46.0 1.61 Bm 55 29 25 2 1 1 45.5 1.90 R 55 25 17 4 1 1 1 1 30.9 2.20 W 304 93 40 19 8 8 2 4 4 2 1 1 2 2 41.9 3.27
21 http://www.oceansatlas.org 22
Table 7. Distribution of fish weights for Warehou in schools with four or more individuals.
Weight Tidal number. Lower number indicates earlier date. grams 11 1 1 1 1 1 1 1 1 1 2 1 2 2 2 1 2 1 1 1 2 2 1 6 6 6 8 6 6 6 6 6 6 6 4 6 3 3 3 6 2 3 3 1 5 5 1 9 9 9 0 8 8 7 8 7 8 6 3 7 2 8 2 6 9 7 7 3 4 5 2 4 1 2 6 6 4 7 9 8 0 3 9 2 2 2 0 5 6 3 1 7 8 0 6 1101-1200 1 1001-1100 1 901-1000 0 801-900 1 0 701-800 0 0
* 676-700 2 1 0 1 651-675 0 0 0 0 626-650 0 1 0 0 601-625 1 1 1 1 0 576-600 1 1 1 1 0 0 1 551-575 0 0 2 0 0 1 526-550 0 0 1 0 0 0 501-525 0 0 0 0 0 0 476-500 0 1 4 1 0 0 451-475 0 0 1 0 1 426-450 1 2 0 0 401-425 1 1 1 1 0 2 1 376-400 2 1 0 0 0 2 1 1 351-375 1 1 2 0 0 0 0 1 1 0 326-350 1 1 0 2 0 0 1 0 4 2 1 301-325 1 0 0 1 2 0 0 1 3 0 0 1 276-300 2 3 4 2 3 3 1 0 1 2 1 2 0 2 251-275 2 2 1 0 2 2 5 0 2 0 0 1 1 0 1 0 0 226-250 1 2 2 1 4 6 5 5 3 1 1 1 1 2 2 1 2 201-225 5 1 0 1 1 1 2 2 0 1 2 176-200 1 1 2 2 0 0 1 151-175 0 2 126-150 1 *weight in column changes from units of 100 grams to units of 25 grams. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 23
Table 8. Distribution of fish weights on tides with schools of four or more individuals, for Kahawai, Blue mackerel, Yellow-eyed mullet, Flounder, Snapper and Rig.
Species K Bm Y F S R Tidal number Weight 7 1 2 9 2 2 2 2 2 2 2 1 1 6 1 4 9 2 2 3 5 4 4 3 grams 0 8 0 4 1 2 1 2 1 2 2 1 5 1 1 5 8 7 5 1 3 5 0 4 4 9 4 5 1 5 1 5 2 9 7 5 7 3 5 1 1 2 7 2 3 7 9 5 7 3 5 1 1 0 2 8 4 6 1 1 5001-6000 1 3 4001-5000 0 1 3001-4000 12 6 2201-2300* 1 2101-2200 2 0 2001-2100 1 0 1901-2000 1 0 1801-1900 0 1701-1800 2 0 1601-1700 1 0 0 1501-1600 1 0 0 0 1401-1500 0 0 0 0 1301-1400 3 0 0 0 1201-1300 1 1 0 0 0 1101-1200 0 1 0 0 0 1 1001-1100 0 5 0 0 0 0 901-1000 1 1 0 3 0 0 0 0 801-900 0 0 0 0 1 0 0 701-800 1 0 0 0 0 0 0 1 676-700* 0 0 0 0 0 0 0 2 651-675 0 0 0 0 0 0 0 2 626-650 1 0 0 0 0 0 0 0 0 601-625 1 1 1 0 0 0 0 0 0 1 576-600 1 0 0 0 1 1 0 0 1 0 551-575 0 0 0 0 0 0 0 0 0 0 526-550 1 0 1 0 0 0 0 0 0 0 501-525 1 0 0 0 1 1 0 0 0 0 476-500 0 0 0 0 1 0 0 0 0 451-475 1 1 0 0 0 1 0 0 0 0 426-450 0 0 0 1 0 0 0 0 0 401-425 1 1 0 0 0 0 0 0 0 0 376-400 0 1 0 0 0 1 0 0 0 1 0 351-375 3 0 0 0 0 0 0 0 0 0 0 0 326-350 2 1 0 0 0 0 0 0 0 0 0 2 0 301-325 0 4 0 2 0 0 1 0 0 0 0 1 1 276-300 1 2 2 0 1 1 0 0 2 0 3 1 251-275 2 4 0 1 1 0 0 0 0 8 1 226-250 1 1 1 1 1 0 1 1 2 6 201-225 1 3 0 1 1 1 176-200 0 1 1 0 0 151-175 0 0 0 4 2 126-150 1 1 1 2 1 101-125 0 1 76-100 2 4 *weight intervals change, from 1000g to 100g to 25 grams. From 3001 g to 2300 g there were no data. http://www.oceansatlas.org 24
7.3. INTRASPECIFIC ASSOCIATION AND THE NUMBER OF SPECIES PER TIDE
The number of species per tide at Te Mako varied from zero to four. The number of tides in which each of the eleven major species occurred with no other, one other, two other and three other species is shown in Table 9, columns B, C, D and E. The sum of the number of other species which occurred in the occupied tides, calculated by multiplying column C by 1, D by 2 and E by 3 for the eleven major species, is shown in Table 9, column F. When this sum is multiplied by the percent of the occupied tides for each of the eleven major species in column G of Table 9, then an index is available in column H which indicates the degree of association of each of the eleven species with all other species. This index in column H is based solely on joint tidal occurrence without regard to expected joint occurrences which is the basis of calculations of indexes of interspecific association. When the index in column H is compared with the number of significantly associated species pairs in common seasons from Table 10, shown in column I of Table 9, there is a highly significant rank correlation coefficient of +0.93 (p<.01). Since both the sum of the other species which occur in the occupied tides of a given species and the percent of occupied tides with other species are aspects of intraspecific association, as measured by the schooling index and by the percentage of times a species is solitary, the highly significant correlation of the data in columns H and I in Table 9 is evidence for a substantial influence of these aspects of intraspecific association on the interspecific association values shown by the species pairs in Table 10.
Of the two data sets which contribute to the index in column H of Table 9, the sum of the other species in occupied tides in column F is positively associated with the number of significantly associated species pairs in column I (Rs +0.87, p<.01), but the percent of occupied tides with other species in column G is not associated with the number of significantly associated species pairs (Rs –0.10, p>.05). The reason for this lack of association by the percent of occupied tides is that three major species, Kahawai, Yellow-eyed mullet and Flounder have high tidal occurrences, but a low percentage of occupied tides while Rig has a low tidal occurrence but a very high percentage of occupied tides. Nevertheless, the index in Column H combines the two data sets in columns F and G to produce a higher level of rank correlation with the number of positive significantly associated species pairs than do either the indexes in columns F or G separately. We conclude that, for our data, interspecific association can not be understood without considering the effects of intraspecific association on interspecific pair formation.
7.4 INTERSPECIFIC ASSOCIATION FOR FIFTY-FIVE SPECIES PAIRS
Two by two contingency tables were used to calculate Chi-square values for interspecific association for the fifty-five species pairs of the eleven major species with the results shown in Table 10A columns 2 to 4. Column 2 lists association for species pairs which appeared in common seasons only, as recommended by Ogburn & Allan (1993) and Sheaves (2001), a convention which prevents any distortion caused by a species prolonged absence from yearly periods. The third column in Table 10A is for inter- specific association in common years only and the fourth column is for all years. There Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 25
Table 9. Eight aspects of intraspecific association, of the eleven major species, compared with the number of their significantly associated species pairs. Column (A) is the number of occupied tides, (B) the number of tides with no other species, (C) the number of tides with one other species, (D) the number of tides with two other species, (E) the number of tides with three other species, (F) the sum of the other species in occupied tides, and (G) is the percent of occupied tides with other species. The product of columns G and F is shown in column H. Columns A to H are compared with the number of significantly associated species pairs listed (I), using rank correlation, with results shown in the last row of the table.
A B C D E F G H I Spp Number Number Number Number Number Sum of Percent F x G Number occupied tides tides tides tides other occupied signif. tides with no with with with species tides assoc. other one two three in with species species other other other occupied other pairs species species species tides species K 228 124 78 22 4 134 45.6 61.1 7 S 120 57 38 15 10 98 52.5 51.4 5 Y 170 98 50 18 4 98 42.4 41.6 6 W 93 44 33 13 3 68 52.7 35.8 4 F 168 108 48 10 2 74 35.7 26.4 2 B 45 20 16 7 2 36 55.6 20.0 3 R 25 7 12 5 1 25 72.0 18.0 3 Bm 29 13 8 7 1 25 55.2 13.8 2 Sp 44 23 15 5 1 26 47.7 12.4 2 D 20 10 5 4 1 20 50.0 10.0 1 G 17 9 6 2 0 10 47.1 4.7 1 A vs I B vs I C vs I D vs I E vs I F vs I G vs I H vs I Rs +.92 Rs +.04 Rs +.84 Rs +.90 Rs +.89 Rs +.87 Rs -.10 Rs +.93 p<.01 p>.05 p<.01 p<.01 p<.01 p<.01 p>.05 p<.01 http://www.oceansatlas.org 26
Table 10. Interspecific association and four environmental variables for the 55 species pairs of the eleven major species. A. Interspecific association chi-square values for common seasons, for common years and for all years. B. Rank correlation x 1000 between the order of the 55 species pairs and the four seasonal periods, the four temperature periods, the three precipitation periods and the four lunar periods in Table 25.
Species A. Interspecific Association B. Environmental Variables pairs Chi-square values Rank correlation x 1000 Common Common All years Seasons Tempera- Precipita- Lunar Seasons years ture tion K S 30.6 6.1 18.0 29 600 500 400 K B 22.0 35.1 23.0 86 400 1000 200 Y F 17.3 14.7 10.4 -29 -400 -500 400 K Y 14.5 12.4 14.4 -257 -400 -500 600 S R 9.6 29.0 16.7 200 0 500 200 Y S 9.6 13.2 12.5 600 400 500 400 W F 9.3 14.7 15.9 471 600 500 0 Y Bm 8.4 0.8 0.3 -143 -600 500 400 Y R 7.2 10.1 7.7 886 800 500 0 K Bm 7.0 2.7 2.1 429 400 500 400 W S 7.0 2.0 0.5 -329 -400 -500 -800 R Sp 6.7 16.2 3.3 314 400 500 200 Y B 6.1 4.8 2.5 -657 -1000 -500 -200 K W 5.2 8.9 12.4 586 400 500 -200 K Sp 5.0 6.1 4.2 -143 -200 -1000 400 K D 4.8 3.3 0.6 371 1000 -500 400 S G 4.6 0.0 0.1 429 0 500 0 B W 4.4 10.3 0.5 643 1000 500 600 Bm Sp 3.7 0.1 0.6 -600 0 -500 1000 K F 3.2 4.3 3.2 943 1000 1000 -400 S Sp 2.9 1.7 0.0 -371 200 -500 -600 S Bm 2.8 2.3 0.8 429 -400 1000 -600 F Bm 2.6 1.4 1.4 371 400 1000 -400 Y Sp 2.0 1.3 0.3 86 800 500 400 G D 1.9 -1.7 0.0 -600 -800 -500 800 B D 1.9 2.6 -0.2 257 400 -500 800 W Sp 1.2 0.3 0.4 -471 -800 -500 800 Y D 1.0 0.1 0.0 -429 -400 500 400 Y G 1.0 0.7 2.9 656 800 1000 800
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 27
Table 10 (continued) Interspecific association between 55 species pairs.
Species A. Interspecific Association B. Environmental Variables pairs
Chi-square values Rank correlation x 1000 Common Common All years Seasons Tempera- Precipita- Lunar seasons years ture tion F D 0.9 -0.0 0.1 257 1000 -500 -400 S F 0.9 0.8 0.6 86 600 500 400 B F 0.9 0.3 1.5 -86 400 1000 -800 W R 0.8 0.4 0.0 -814 -800 -1000 -400 K G 0.8 0.0 0.5 -771 -800 -500 800 W D 0.5 -0.2 0.1 214 400 500 800 W Y 0.4 0.1 0.4 -786 -1000 -1000 200 S B 0.3 0.0 0.4 143 -400 500 -800 W Bm 0.2 0.0 0.5 557 600 -500 800 S D 0.2 -0.1 -1.6 -200 600 -1000 -600 F Sp 0.2 7.7 0.1 -257 -200 -1000 -400 K R 0.2 -0.0 0.3 -486 -800 -500 800 B G 0.1 2.6 0.1 -314 -800 -500 400 R D 0.0 -2.0 1.0 -543 -800 -500 200 F R -0.0 0.0 -0.9 -257 -800 -500 -800 Bm R -0.2 -0.0 -0.2 -371 -800 500 200 R G -0.2 -2.0 -0.1 714 1000 1000 400 Bm G -0.2 -0.9 -0.1 -143 -800 500 800 Sp G -0.2 -0.1 0.0 257 400 500 800 Sp D -0.2 -0.2 0.0 143 -200 500 1000 W G -0.3 -2.0 -0.5 -786 -800 1000 400 Bm D -0.4 -0.2 -0.1 -429 400 -1000 1000 B Sp -0.4 -0.3 -0.3 -600 -800 -1000 800 B Bm -0.5 -0.0 -0.4 486 600 500 800 R B -0.6 -0.0 -0.4 -829 -800 -500 400 F G -0.7 -0.3 -0.9 -714 -800 -500 -200 http://www.oceansatlas.org 28 were 18 species pairs with a significant positive interspecific association (p<.05) in common seasons only, 15 species pairs were significant for common years and 10 species pairs were significant for all years. Ten species pairs were significant for all three time frames, three additional pairs for two time frames and seven pairs were significant only in one time frame. Of the 18 species pairs which were positively significantly associated in common seasons only, seven pairs included Kahawai, six pairs Yellow-eyed mullet, five Snapper, four Warehou, three Barracouta, three Rig, two Flounder, two Blue mackerel, two Spotty and one each Grey mullet and Dab. There were no significant negative associations and no negative values that even approached significance.
7.5 INTERSPECIFIC ASSOCIATION AND ENVIRONMENTAL VARIABLES
An attempt was made to analyse the relationship between the interspecific association of the fifty-five species pairs shown in Part A of Table 10 and the behaviour of each species in relation to the four environmental variables in Table 25. A rank correlation value was calculated for each of the species pairs for their number of fish per tide in relation to the six seasons, the four temperature periods, the three precipitation periods and the four lunar periods in Table 25. The resulting 220 rank correlation values are listed in Part B of Table 10. The order of species in this table is from the highest to the lowest interspecific association value as judged by their chi-square values in the first column of Table 10, Part A.
The rank correlation values (x 1000) for the significance of the relationships between the level of interspecific association and the four environmental variables in Part B (Table 10) showed a decline in mean rank correlation as the chi-square for the level of interspecific association declined. There are 18 species pairs with a significant X2 (>3.84). For associations less than 3.84 X2, but greater than 0.99, there were 11 species pairs and for associations with a chi-square less than 1.0, there were 26 species pairs. Mean rank correlation (x 1000), relative to interspecific association in the above three X2 classes declined from 254 to 67 to –111 for the seasonal correlations, from 300 to 273 to 46 for the temperature data, from 194 to 136 to –83 for the precipitation correlations and from 200 to 309 to 92 for the lunar correlations. These values showed a consistent decline in mean rank correlation with declining interspecific association for the first three of the four environmental variables and an increase and then a substantial decline for the lunar variable, a total of seven declines and one increase (X2 4.5, p<.05). The greatest decline was 365 for the seasonal variables followed by 277 for precipitation, 255 for temperature and 108 for lunar. When the difference between the environmental variables for species pairs which have significant interspecific association chi-squares of 3.84 or larger and species pairs with chi-squares of less than 1.0 were compared, using the median test for two samples (Walker and Lev, 1953), the results showed there was a significant difference for the seasonal variables (X2> 6.0, p<.02), but non-significant differences for the other three environmental variables. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 29
8. Stomach content and fullness, food preference and interspecific association
8.1. STOMACH CONTENT AND FULLNESS
Stomach content and fullness were noted for thirteen species from a total of 251 randomly selected fish (Table 11), and a literature survey was made of stomach contents from other New Zealand studies. There were seven species of Te Mako fish which had fish or fish pieces in their gullets or stomachs; Kahawai, Barracouta, Yellow-eyed mullet, Rig, Snapper, Red cod and Red mullet. The data shown in Table 11 are for percent stomach frequency only, no quantitative amounts were included in the table. Kahawai was the top fish predator with a diet of 89% fish, followed by Barracouta 78%, Rig 34% and Yellow-eyed mullet 14%. Warehou and Blue mackerel had empty stomachs, except for one Warehou with silt, and may have entered the estuary for purposes other than feeding. Digested material in some Warehou guts may be tunicates, a major dietary component (Paul, 2000). The two major predators were cannibalistic with 23% of Kahawai prey and 10% of Barracouta prey comprising their own species. Unknown fish were eaten by six species, Kahawai by three species, Yellow-eyed mullet by two species and Galaxias, Barracouta and Warehou by one species each.
Consumption by the nine invertebrate eating fish was mainly of Arthropoda, mostly crabs, with a mean consumption per fish species of 52% and of Mollusca, mainly cockles, mussels, periwinkles and snails with a mean consumption of 28%. Seven species consumed plants.
Snapper, Kahawai and Yellow-eyed mullet consumed food from the four major food groups, fish, invertebrates, sediments and plants, while Flounder and Barracouta consumed in three of the groups. Kahawai, Barracouta and Red mullet (one sample only) were predominantly fish eaters. Red cod was half fish and half invertebrate eating. Flounder, Snapper, Rig, Spotty and Dab were mainly invertebrate eaters. Yellow-eyed mullet and, especially, Grey mullet consumed mainly invertebrates in the bottom sediments plus plants.
Food consumption by the thirteen Te Mako species in Table 11 is similar, with a few exceptions, to consumption by these species throughout New Zealand, shown in part C of Table 11. Yellow-eyed mullet at Te Mako consumed more fish and less invertebrates than shown in the literature survey. For the species with only one or two food samples, Grey mullet and Red cod were similar to the New Zealand means, but Red mullet at Te Mako ate only fish and Dab ate no sediments.
Our identification of food materials was limited to groups we could easily identify, but there were many smaller organisms, especially those that were quickly digested, that we could not identify. Nearly all the food sources available at Te Mako appeared to be consumed by the incoming fish. Prey fish species were limited to the five species listed in Table 11, although it is likely most of the species in the estuary were occasionally consumed. The major invertebrates present, including the various crabs, snails, bivalves http://www.oceansatlas.org 30
Table 11. Percent stomach content and fullness of eleven major and two minor fish species. A. Percent stomach contents, Te Mako, B. Percent stomach contents summary, Te Mako, C. Percent stomach contents, New Zealand literature, D. Percent stomach fullness, Te Mako
Species K W Y F S Bm R B Sp D G Rc Rm Number of fish 42 24 22 52 52 27 5 12 8 2 2 1 2 A. Te Mako, stomach Fish-unknown 45 14 3 17 39 50 Kahawai 20 17 100 Yellow-eyed mullet 7 31 Galaxias 13 Barracouta 8 Warehou 1 Lower chordate 2 Invertebrate-unknown 5 Arthropoda 2 62 60 50 38 50 50 Annelida 2 15 Mollusca 18 20 13 17 50 50 Echinodermata 1 Silt, mud, sand 7 t 59 11 13 14 50 Plants,algae & higher 2 5 8 10 8 50 B. Te Mako summary Fish 89 14 0 4 33 77 0 0 0 50 100 Invertebrates 4 23 82 73 67 15 86 100 0500 Sediment, Plants 7 64 19 23 0 8 14 0 100 00 C. N.Z. literature Fish 59 30 3 0 5 40 1 70 5 0 0 65 8 Invertebrates 39 70 21 86 90 60 99 30 94 47 >2 34 92 Sediment, plants 2 0 76 14 5 0 0 0 1 53 <90 10 D. Te Mako, fullness Full 28 0 53 79 17 0 20 50 38 0 60 100 0 Part full 44 4 29 12 79 0 80 29 50 100 00100 Empty 28 96 18 9 4 100 0 21 13 0 40 0 0
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 31 and periwinkles were consumed with the exception of the Pacific oyster. We were impressed by the number of higher plant fragments in addition to the staple Zostera and algae that we found in the fish stomachs.
Stomach fullness, shown in Table 11, part D, had a mean of 34 % of individuals with a full stomach, 40% with a part full stomach and 26 % with an empty stomach, with Warehou and Blue mackerel nearly always empty. Red cod had a full stomach while Snapper, Rig, Dab and Red mullet were part full over half the time. There were not sufficient data on stomach fullness in other New Zealand studies to make a comparison with our study.
8.2 FOOD PREFERENCE AND INTERSPECIFIC ASSOCIATION
Of the eleven major species, Kahawai and Barracouta are mainly fish eating (P), Warehou, Snapper and Blue mackerel are invertebrate and fish eating (O), Rig and Spotty mainly invertebrate eating (I), and Yellow-eyed mullet, Flounder, Dab and Grey mullet feed mainly on invertebrates and algae in the bottom waters and in detritus and sediments (D). A comparison was made (Table 12) of the species in each food preference class with their number of significant interspecific associations in Table 10. The mainly fish eating species were significantly associated with each other (100%) and with the invertebrate and fish eaters (67%), with lower associations with bottom feeders (38%) and the mainly invertebrate eaters (25%). The invertebrate and fish eaters were significantly associated with each other (67%) and with the fish eaters (67%), but were less associated with the other two food preference groups than were the fish eaters. The invertebrate eaters were associated with each other (100%), but their mean percent of significant association with the other three groups was only 17%. The bottom feeders, unlike the other three groups, had a low association with each other (33%), barely higher than their mean significant association with the other groups (29%). The mean percent of significantly associated pairs was 50% for the mainly fish eaters, 41% for the invertebrate and fish eaters, 29% for the bottom feeders and 25% for the invertebrate eaters.
The two mainly fish eating species, Kahawai and Barracouta, are significantly associated with each other and with 44% of the other species including 37% of the mainly bottom and detritus species. One of these species is Yellow-eyed mullet, a major prey species. Interspecific association between Kahawai and Barracouta was X2 22.0 (p<.001), between Kahawai and Yellow-eyed mullet was X2 14.5 (p<.001) and between Barracouta and Yellow-eyed mullet was X2 9.9 (p<.01). These three interspecific association values are three of the five highest values in our study and reflect the high level of predation of Barracouta and Kahawai on Yellow-eyed mullet, thus providing a possible explanation for the high interspecific association between Barracouta and Kahawai. Another reason for this high association may be that they come in on the same tides, perhaps in hunting packs. We have observed tightly formed hunting packs of several fish species, with rarely more than ten individuals and often led by a single large predator, in the tropical Rarotonga lagoon. These tight packs were clearly different from the large mixed schools of Kahawai, Blue mackerel and Jack mackerel which can be seen from the air in New Zealand as described in Paul (2000). Whether Barracouta and Kahawai ever hunt http://www.oceansatlas.org 32
Table 12. The percentage of significantly positively associated species pairs for species which are mainly fish eating (P)*, mainly invertebrate and fish eating (O), mainly inver- tebrate eating (I), and mainly invertebrate and plant eating in detritus and sediments (D).
Food Number Percent of Significantly associated preference possible significantly fish pairs group pairs associated pairs P-P 2 100.0 K-B, B-K P-O 6 66.7 K-W, K-S, K-Bm, B-W P-I 4 25.0 K-Sp P-D 8 37.5 K-Y, K-D, B-Y
O-P 6 66.7 S-K, W-K, W-B, Bm-K O-O 3 66.7 W-S, S-W O-I 6 16.7 S-R O-D 12 33.3 W-F, S-Y, S-G, Bm-Y
I-P 4 25.0 Sp-K I-O 6 16.7 R-S I-I 2 100.0 R-Sp, Sp-R I-D 8 12.5 R-Y
D-P 8 37.5 Y-K, Y-B, D-K D-O 12 33.3 Y-S, Y-Bm, F-W, G-S, D-I 8 12.5 Y-R D-D 6 33.3 Y-F, F-Y
* P - Kahawai and Barracouta: eat mainly fish. O - Warehou, Snapper, Blue mackerel: eat mainly invertebrate and fish. I - Rig, Spotty: eat mainly invertebrates. D - Yellow-eyed mullet, Flounder, Dab, Grey mullet: eat mainly invertebrates and plants on the bottom and in detritus and sediments. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 33 together cannot be determined from our net data, but if they do, the very low schooling index for Barracouta would indicate that only one Barracouta would be involved per pack. We have seen large schools of young Barracouta in our bay, but they are much smaller than the Barracouta in our net. Evidence for mixed schools of Kahawai, Blue mackerel and Jack mackerel entering Te Mako estuary is the highly significant interspecific association between Kahawai and Blue mackerel (X2 7.0, p<.01) and a Chi-square interspecific association value of 11.1 between Kahawai and Jack Mackerel which could not be assessed for significance because the expected joint occurrence was too low.
The three species that are mainly invertebrate and fish eaters, Warehou, Snapper and Blue mackerel were associated with two-thirds of their own class, but with only 28% of the species in the invertebrate and the bottom feeding classes. The two species which were mainly invertebrate feeders, Rig and Spotty, were significantly associated with each other, but with only one species in each of the other three food preference classes. Since only two of the four bottom-detritus feeding species, Yellow-eyed mullet and Flounder, were significantly associated with each other, it seemed likely there might be some significant negative associations within the bottom feeding class, but this is not the case. The only negative association was between Flounder and Grey mullet and it was not significant.
In summary, the more a fish depends on other fish for food, the higher its percentage of significant interspecific associations, and the more it depends on invertebrates, detritus and sediments, the lower its percentage of significant associations. Species which depend partly or wholly on invertebrates are more highly associated with each other, than those that rely on detritus and sediments. This demonstration of the importance of food preference in determining interspecific association between species, when considered together with the existence of the strong correlation between intra- and interspecific association in Chapter 7.3 and the significant correlation between an environmental variable and interspecific correlation in Chapter 7.5, illustrates the difficulty in interpreting the complex relationships which underlie the interspecific association of fish species. http://www.oceansatlas.org 34
9. Species number, diversity and presence
9.1 SPECIES NUMBER AND DIVERSITY
The number of species, the mean number per year and the number of new species per year are shown in Table 13 in five yearly intervals. The number of species and the mean number of species per year increased from the first two to the last two pentads, while the number of new species per year decreased from the initial peak in 1971-1975 to 1981- 1985, but then increased to 2001-2004 as a result of the arrival of new invader species. Since comparisons of species diversity over a period of time are only meaningful when the search effort to determine the number of species is the same over time and because tide numbers varied greatly, we did a square root transformation of the number of tides, shown in column D of Table 13. The results are shown in columns A/D, B/D and C/D in which the number of species in columns A, B and C were each divided by column D. The results, in C/D, show a decline in new species per year to 1981-85 followed by an increase to the present. This pattern contrasts with a lack of direction in both the transformed number of species, in A/D, and in the transformed mean number of species per year in B/D.
The changes in the number of new species per year and in the transformed number of new species since 1981-85 (Table 13) show nearly consistent increases of five fold and two and a half fold. This is probably a result of decreased competition from the eleven major species, each of which over the same time interval, showed moderate to strong declines in every, or nearly every, one of their six population variables. These population declines, which occurred during a period of increasing diversity shown by an increase in the number of species, in the mean number of new species and in the transformed number of new species, support the conclusion that the use of species diversity values as an index of ecosystem stability or productivity should be questioned.
9.2 SPECIES PRESENCE IN THREE ELEVEN YEARLY PERIODS
The number of years in which a species is present relative to the square root of the number of tides is shown in Table 14 for three eleven yearly periods, 1971-81, 1982-92 and 1993-2003. Between the first and last of these periods, Rig, Snapper, Yellow-eyed mullet, Kahawai and Barracouta declined in relative years present, Grey mullet, Flounder, Dab and Blue mackerel increased and then declined, while Spotty and Warehou consistently increased. In the future, yearly appearance of most of the eleven species in Table 14 will probably continue to decline relative to the square root of the number of sampled tides. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 35
Table 13. The number of species, the mean number of species per year and the mean number of new species per year by five yearly intervals relative to the square root of the number of tides.
Year No. Mean Number Square Ratios Species number new root species species number per year per year tides A B C D A/D B/D C/D 1971-75 9 5.0 1.8 12.08 0.745 0.41 0.149 76-80 9 4.6 0.8 14.42 0.624 0.32 0.055 81-85 10 5.4 0.2 12.41 0.806 0.44 0.016 86-90 14 8.8 0.6 20.95 0.668 0.42 0.029 91-95 16 8.6 0.6 25.57 0.625 0.34 0.023 96-00 16 10.4 0.8 27.02 0.592 0.38 0.030 2001-05 17 9.6 1.00 24.57 0.692 0.39 0.041
Table 14. Number of years a species is present relative to the square root of the number of tides by 11 yearly intervals.
Species Number years present / square root number of tides 1971-81 1982-92 1993-2003 S 0.56 0.39 0.25 Y 0.56 0.32 0.28 K 0.41 0.36 0.28 B 0.30 0.29 0.25 R 0.30 0.07 0.08 F 0.15 0.29 0.28 D 0.10 0.18 0.15 G 0.05 0.21 0.18 Bm 0.05 0.32 0.23 W 0.05 0.18 0.20 Sp 0.05 0.11 0.23 http://www.oceansatlas.org 36
10. Diversity and global abundance-weight profiles
10.1 ABUNDANCE AND WEIGHT PER CATCH PROFILES
In the analysis of species diversity in Tables 13 and 14, the number of individuals, or their weight per catch, were not considered. As a result, species with a single individual had the same importance as species with many individuals. To remove this inequality, we constructed abundance and weight profiles for the Te Mako species by ranking them in descending order of their percent abundance or weight values. The results, in the first two rows of Part B of Table 15.1, show that for the twenty highest species, relative abundance declined from 24.1 for the highest species to 0.1 for the mean of the sixteenth to twentieth species and relative weight declined from 29.3 to 0.1. The distributions for percent abundance and percent weight for Te Mako were then compared with a global survey of 52 studies from both hemispheres which we made of estuaries and onshore and a few offshore fisheries, together with the 62 values from 31 years of Te Mako abundance and weight per tide data and with 10 yearly values for Chilean fish larval data (Loeb & Rojes, 1987) to give a total of 122 data sets. The 52 studies, from both hemispheres, were from a review we had previously made to familiarise ourselves with global fish community studies and it occurred before we analysed the abundance and weight profiles. These 52 studies are from 29 references, which are indicated by an asterisk.
10.2 GLOBAL ABUNDANCE AND WEIGHT PROFILES
The rank order of the 122 relative abundance and relative weight studies noted in section 10.1 above is shown in Part A of Table 15.1 with the data divided into ten equal size classes from relative values of 0.1-10 percent to 90.1-100 percent. The number of studies in the ten size classes consistently increase from one to thirty-three and then consistently decline to one. This distribution is skewed by peaking towards the lower size classes, with a peak in the 30.1- 40 percent class and with most studies concentrated in the four size classes between 20.1 and 60.0. Maximum mean relative abundance or weight for the highest species is 91.5% in the 90.1-100% size class. The second and third highest species peak at 25.4 and 16.3% in the 30.1- 40% size class, the fourth and fifth highest species peak at 10.7 and 7.1% in the 20.1-30% size class and the sixth to the 16 to 20th highest species peak from 6.5% to <3.6% in the 0.1-10% size class. These distributions show the extent to which the highest two to three species dominate the abundance and weight profiles. This dominance is further illustrated by the highest species having a percentage abundance or weight of greater than 50 in 24.6% of the 122 data sets while the first and second highest species have a percentage greater than 50 in 74.5% of the studies and the top three species have a percentage greater than 50 in 93.4% of the studies. After the top three species, relative values quickly decline so that by the tenth highest species, relative values are less than 5.8 and by the twentieth species, relative values are nearly always less than 1.0.
The profiles in Table 15.1A can be compared with the two extremes of theoretical community diversity, maximum multitypic, in which each species has the same abundance or weight per catch and maximum monotypic which contains only one Fish populations of aNew Zealand estuary from 1971 to 2004. Bray & Struik (2006)
Table 15.1. Relative abundance and weight profiles for 122 Global studies showing mean percent values from the ten highest to the 16- 20 highest species in comparison with a Multitypic and a Monotypic model. The number of studies is designated by n. Part A . Global relative abundance and weight. Part B. Te Mako (T) relative abundance (A) and relative weight (W) values compared with relative Global (G) and Chilean (C) data.
Part A n Relative abundance and weight for the 20 highest species in decreasing rank order Classes 1 2 3 4 5 6 7 8 9 10 11-15 16-20
Multitypic 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 0.1 - 10 1 9.2 7.7 7.3 6.9 6.8 6.5 6.5 6.0 5.7 5.7 4.2 <3.6 10.1 - 20 7 13.9 11.1 9.5 7.4 5.8 5.5 4.8 3.9 3.6 3.2 2.2 1.3 20.1 - 30 28 26.2 20.9 15.5 10.7 7.1 4.5 3.6 2.9 1.9 1.4 0.6 <0.3 30.1 - 40 33 34.6 25.4 16.3 9.1 6.4 3.4 2.0 1.4 0.7 0.4 0.1 <0.1 40.1 - 50 23 43.0 23.3 12.4 6.5 4.0 2.6 1.7 1.4 1.1 0.8 0.4 <0.1 50.1 - 60 15 55.3 17.6 9.1 5.3 3.6 2.6 1.8 1.1 0.8 0.6 0.4 <0.1 60.1 - 70 6 65.7 16.4 8.9 4.3 1.5 1.0 0.7 0.4 0.3 0.2 0.1 <0.1 70.1 - 80 5 72.2 13.3 5.0 3.7 2.4 1.0 0.8 0.6 0.4 0.3 <0.1 <0.1 80.1 - 90 3 87.7 4.2 2.2 1.3 1.1 0.8 0.7 0.5 0.3 0.2 <0.1 <0.1 90.1 -100 1 91.5 2.9 1.3 1.2 1.0 0.6 0.4 0.3 0.3 0.2 <0.1 0 Monotypic 100.0 0 00000 000 0 0
Part B. 37 T A* 24.1 19.914.9 12.7 9.9 3.6 3.6 3.0 3.0 1.7 0.5 0.1 T W 29.3 17.716.5 9.9 9.8 7.2 3.0 2.8 2.2 1.0 0.3 0.1 G-T A 36.0 18.111.2 6.6 5.0 3.6 3.0 2.5 2.0 1.6 0.8 0.4 G A 40.5 20.514.1 7.7 5.4 3.2 2.0 1.8 1.8 1.2 0.4 <0.1 T bR W 62.6 21.67.8 5.5 1.8 0.6 0 0 0 0 0 0 T aR W 37.4 25.513.1 8.0 5.7 4.7 3.0 1.7 0.4 0.3 <0.1 0 C bAn A 81.9 6.5 3.0 2.0 1.8 1.5 1.0 0.8 0.5 0.3 <0.1 <0.1 CaAn A 72.0 13.84.1 3.8 1.9 1.0 0.8 0.6 0.5 0.4 0.1 0
*T A-Te Mako abundance, T W-Te Mako weight, G-T A- Global minus Te Mako and Chilean abundance data, G A-Global abundance, TbR W- Te Mako weight profile for 1971-74 before Rig collapse, TaR W- Te Mako weight profile for 1975-78 after Rig collapse, C bAn A- Chilean abundance profile 1970-73 before Anchovy collapse, CaAn A-Chilean abundance profile 1983 after Anchovy collapse. http://www.oceansatlas.org 38 species, as shown in the first and last rows of Table 15.1A. Between the maximum multitypic and the maximum monotypic profiles there is an increasingly steep downward slope for the relative abundance and weight profile values which vary from 9.2 to <3.6 in the 0.1-10 class to 91.5 to 0.0 for the 90.1-100 class. With each increase in size class, the profile of the global studies of the twenty highest species changes from nearly resembling the maximum mulititypic profile in the 0.1-10 size class to nearly resembling the maximum monotypic profile in the 90.1-100 size class. To reach 50% of the cumulative total in the 0.1-10 profile class of this table, it is necessary to include the sum of the seven highest species. In the 10.1- 20 profile class, the six highest species are required to reach 50% of the total and in the 20.1-30 profile class, three species are required. The 30.1-40 and 40.1-50 class require two species and from the 50.1-60 to the 90.1-100 class only one species is needed. The higher profile classes from 80.1 to 100 are so dominated by the high relative values of the top species that there is virtually no upward slope in the cumulative sum from the top species to the top ten species.
10.3 COMPARISON OF TE MAKO WITH GLOBAL DIVERSITY PROFILES
If diversity is judged by its approach to a system of great complexity, as represented by the multitypic model in which each species has the same abundance or relative weight, then the abundance profile and the weight profile for Te Mako, which both occur in the 20.1-30 percent profile class in the first two lines of Part B Table 15.1, are closer to the multitypic model than 70% of the global values.
10.4 CUMULATIVE TOTALS FOR THE TEN HIGHEST SPECIES, GLOBAL COMPILATION
The change from multitypic to monotypic complexity is clearly seen in the patterns of cumulative totals for the ten highest species of the 122 global studies shown in Table 15.2. The ten highest cumulative totals in the 0.1- 10 percent profile class in Table 15.2 vary from 9.2 to 68.3% compared with a distribution of 91.5 to 99.7% for the highest profile class from 90.1- 100 percent. If the mean deviation of the profile classes from the multitypic and montypic models is calculated, the results show a mean deviation of 12.7 from the multitypic model and of 59.8 from the monotypic model for the 0.1- 10 percent profile class, of 20.0 and 52.9 for the 10.1- 20 percent profile class and of 46.3% from the multitypic model and 25.7% from the monotypic model for the 20.1 to 30 percent profile class. From the 20.1 to 30 percent profile class onwards, the distributions in Table 15.2 increasingly resemble the monotypic model with the 90.1 to 100 percent profile class having a deviation of 69.7% from the multitypic model and of 2.8% from the monotypic model. These results indicate that only those communities in which the highest species has a percent abundance or weight of less than around 20% are more closely related to the multitypic model than to the monotypic model. Fish populations of aNew Zealand estuary from 1971 to 2004. Bray & Struik (2006)
Table 15.2. Cumulative totals for the ten highest species, in descending order of their relative abundance or weight for the 122 Global studies in Table 15.1, Part A, in comparison with a multitypic and a monotypic model
Profile class Rank order of species 1 2 3 4 5 6 7 8 9 10 Multitypic 5 10 15 20 25 30 35 40 45 50 0.1 – 10 9.2 16.9 24.2 31.1 37.9 44.4 50.9 56.9 62.6 68.3 10.1 – 20 13.9 25.0 34.5 41.9 47.7 53.2 58.0 61.9 65.5 68.7 20.1 – 30 26.2 47.1 62.6 73.3 80.4 84.9 88.5 91.4 93.3 94.7 30.1 – 40 34.6 60.0 76.3 85.4 91.8 95.2 97.2 98.6 99.3 99.7 40.1 – 50 43.0 66.3 78.7 85.2 89.2 91.8 93.5 94.9 96.0 96.8 50.1 – 60 55.3 72.9 82.0 87.3 90.9 93.5 95.3 96.4 97.2 97.8 60.1 – 70 65.7 82.1 91.0 95.3 96.8 97.8 98.5 98.9 99.2 99.4 70.1 – 80 72.2 85.5 90.5 94.2 96.6 97.6 98.4 99.0 99.4 99.5 80.1 – 90 87.7 91.9 94.1 95.4 96.5 97.3 98.0 98.5 98.8 99.0 90.1-100 91.5 94.4 95.7 96.9 97.9 98.5 98.9 99.2 99.5 99.7 Monotypic 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
39 http://www.oceansatlas.org 40
11. Biyearly fish population variables
11.1 REASONS FOR BIYEARLY ANALYSIS
Partly due to the low number of tides sampled in 1973 and 1985, and to the eight years when the number of fish caught was 15 or less, statistical analysis based on annual sampling was limited by these small sample sizes. Using a biyearly sample, the lowest numbers of fish caught were 23, 24 and 28, adequate numbers for most non-parametric statistical analysis. Biyearly data for six population variables are shown in Part A of Tables 16 to 21, together with mean values for the first eight biyearly periods in which the species occurred (1971-86) and the means of the second eight biyearly periods of occurrence (1987-02) in Part B.
11.2 TEMPORAL PATTERNS
The sums for all twenty-three netted species shown in the last columns of Tables 16, 17, 19 and 21 for percent of occupied tides, number per tide, weight per tide and length per tide had the same patterns of successive biyearly maxima, all of which peaked in 1971- 72, 1979-80, 1985-86, 1991-92, 1995-96 and 1999-2000. The basis of this similarity amongst the four variables was the significant positive correlations between weight per tide and length per tide (Rs +0.81, p<.01), between weight per fish and length per fish (Rs +0.92, p<.01), between weight per tide and number per tide (Rs +0.64, p<.01), and between number per tide and length per tide (Rs +0.88, p<.01). A further contribution to the shared pattern of successive peaks amongst the above four variables is the correlation of number per tide with the percent of occupied tides (Rs +0.95, p<.01) and the significant correlations of the percent of occupied tides with weight per tide (Rs +0.80, p<.01) and, especially, with length per tide (Rs +0.99, p<.01). Peaks for weight per fish and length per fish (Tables 18 and 20) resembled the patterns of successive maxima for percent sum of occupied tides, number per tide, weight per tide and length per tide in 1971-72, 1991-92 and 1999-2000, but immediately preceded the other variables in 1977- 78 and followed the other variables in 1987-88.
11.3 PERCENT OF OCCUPIED TIDES
Ten of the eleven major species peaked in percent of occupied tides from 1971-86 and declined in the second half of the study, 1987-02, by from 10% to 89% as shown in Part B of Table 16 and Flounder increased by 3%. Percent of occupied tides varied from 1.7% to 11.9% (mean 4.4%) from 1971-86 and from 0.6% to 8.6% (mean 3.2%) in the second half of the study. The greatest declines were by Rig 89%, Spotty 80%, Snapper 77% and Dab 74%, and the least declines by the predators Kahawai 10% and Barracouta 48%.
11.4 NUMBER PER TIDE X 1000, BIYEARLY
The sum of the number of fish per tide x 1000 (Table 17, last column) peaked at 1929 in 1971-72, with the subsequent maxima of 607, 894, 452, 838, and 805 declining, but not consistently. Intervening minima of 224, 364, 376, 330 and 431, in contrast to the Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 41
Table 16. Percent of tides occupied by the eleven major species, biyearly, columns 3 to 13. The sum of the percent of occupied tides for all species is listed in the last column with peak values in bold type. 16A. Biyearly data, 1971-2004. 16B. The mean of the first eight biyearly periods in which the species occurred (1971-86) and the mean of the second eight biyearly periods of occurrence (1987-2002).
16A Eleven major species All Years Tides R Y B S K W Sp Bm F D G spp. * 1971-72 46 17.4 21.7 6.5 21.7 19.6 13.0 0.0 0.0 0.0 0.0 0.0 100 1973-74 36 13.9 11.1 2.8 8.3 8.3 0.0 8.3 0.0 0.0 0.0 0.0 53 1975-76 84 2.4 6.0 2.4 6.0 9.5 0.0 0.0 0.0 0.0 0.0 0.0 26 1977-78 105 1.9 5.7 1.0 9.5 2.9 0.0 0.0 0.0 1.0 0.0 0.0 22 1979-80 60 0.0 10.0 0.0 10.0 11.7 0.0 0.0 3.3 5.0 5.0 0.0 47 1981-82 64 0.0 6.3 3.1 7.8 3.1 0.0 0.0 0.0 7.8 1.6 1.6 31 1983-84 77 0.0 5.2 3.9 14.3 3.9 1.3 0.0 5.2 9.1 1.3 1.3 47 1985-86 46 0.0 2.2 0.0 17.4 17.4 0.0 0.0 2.2 13.0 4.3 2.2 61 1987-88 163 0.0 9.2 5.5 3.7 9.2 0.6 1.2 1.8 6.1 0.0 1.2 39 1989-90 227 0.9 7.5 0.4 3.1 6.2 1.3 0.4 0.9 6.6 0.9 0.4 30 1991-92 228 0.0 3.9 2.2 3.1 6.6 2.2 0.4 1.3 10.5 0.4 0.4 32 1993-94 284 0.0 3.5 0.7 5.3 2.8 3.5 0.7 1.1 3.9 0.4 0.4 23 1995-96 264 0.4 6.1 1.5 3.0 11.8 6.5 2.3 0.0 6.1 1.9 0.4 41 1997-98 324 0.0 6.2 0.6 1.5 6.8 2.2 3.7 1.5 7.7 0.6 0.9 32 1999-00 243 1.7 3.7 1.7 0.8 19.0 8.7 3.7 0.8 7.9 0.0 0.8 51 2001-02 244 0.0 3.3 1.2 1.2 6.1 5.7 0.8 0.8 10.6 0.8 0.4 33 2003-04 249 0.8 6.4 1.2 4.0 6.8 2.0 2.4 0.8 1.2 0.0 0.8 27 16B 1971-86 8.9 8.5 3.3 11.9 9.6 7.2 8.3 3.6 7.2 3.1 1.7 29.7 1987-02 1.0 5.4 1.7 2.7 8.6 3.8 1.7 1.2 7.4 0.8 0.6 25.6
*Doubles tides with more than one individual of a species were counted as one tide, which resulted in a total of 2744 tides. http://www.oceansatlas.org 42
Table 17. Number of fish per tide x 1000 of the eleven major species, biyearly, columns 2 to 12. The sum of the number of fish per tide x 1000 for all species is listed in the last column with peak values in bold type. 17A. Biyearly data, 1971-2004. 17B. The mean of the first eight biyearly periods in which the species occurred (1971-1986) and the mean of the second eight biyearly periods of occurrence (1987-2002).
17 A. Eleven major species All Years R Y B S K W Sp Bm F D G spp. 1971-72 607 357 54 268 268 375 0 0 0 0 0 1929 1973-74 216 135 27 81 81 0 81 0 0 0 0 622 1975-76 11 75 32 86 97 0 0 0 0 0 0 301 1977-78 19 56 9 94 37 0 0 0 9 0 0 224 1979-80 0 213 0 98 131 0 0 33 49 66 0 607 1981-82 0 91 30 76 30 0 0 0 91 30 15 364 1983-84 0 52 39 208 39 39 0 325 91 13 13 831 1985-86 0 21 0 234255 0 0 21 255 64 21 894 1987-88 0 121 52 46 155 12 12 35 69 0 12 511 1989-90 9 92 4 35 96 22 4 13 66 18 4 376 1991-92 0 61 22 40 114 61 4 13 114 9 4 452 1993-94 0 35 7 63 28 112 7 11 42 4 7 330 1995-96 4 66 15 33 169 434 22 0 63 18 4 838 1997-98 0 78 6 15 127 36 36 18 93 6 12 431 1999-00 15 49 15 8 378 199 37 8 75 0 8 805 2001-02 0 56 12 8 71 139 8 8 111 8 4 456 2003-04 12 112 12 64 84 36 24 8 12 0 12 382 17B 1971-86 213 125 32 143 117 207 81 126 99 43 16 643 1987-02 9 70 17 31 142 127 16 15 79 11 7 526 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 43 maximum values, have a nearly consistent increase. Rig, Yellow-eyed mullet, Barracouta and Snapper all peaked in 1971-72, with some revivals in the 1980s and, again in the early 2000s. Kahawai and Warehou peaked in 1971-72, with Kahawai on a secondary peak in 1985-86 and a top maximum in 1999-2000. Warehou was nil after 1971 until 1983-84 after which it increased consistently to a top maximum in 1995-96 before declining to the present. Graham (1956) described Warehou as having high numbers in some years and disappearing in other years in Otago Harbour and a similar pattern occurred in the present study, but this pattern disappeared at Te Mako after 1986 with Warehou being consistently present in every biyearly period thereafter. The invader species, Dab, peaked in 1979-80 and the other three invaders peaked in 1983-84 or 1985- 86 with Flounder having lesser peaks in 1991-92 and 2001-02. Ten species decreased in Part B, Rig by 96%, Blue mackerel 88%, Spotty 80%, Snapper 78%, Dab 74%, Grey mullet 56%, Barracouta 47%, Yellow-eyed mullet 44%, Warehou 39% and Flounder 20%. Kahawai increased by 21%. The greatest declines were for Rig, Blue mackerel, Spotty, Snapper and Dab, the same species which had the highest declines in percent tidal occupancy.
11.5 MEAN WEIGHT PER FISH, BIYEARLY
A decline in mean weight per fish over time is one of the surest indicators of fishery decline (McGoodwin 1990). Eight of the eleven major species in Part B of Table 18 declined in weight per fish between the first and second half of the study, but two of the increases had too few values to be significant. The sum of mean weights for all species peaked at 2100g in 1971-72 with the subsequent maxima of 1254g, 962g, 776g and 645g consistently declining.
Rig mean weight increased from 2739g to 4539g between the first and second halves, but the values are based on only a few fish and are not significant. Yellow-eyed mullet declined in mean weight by 25% between 1971-86 and 1987-02, as shown in Part B. Its mean weight went from 224g to a biyearly minimum in 1983-84 and has since recovered to 73% of its peak weight. Except for two biyearly periods in which it was not netted, Barracouta maintained a high level of weight per fish until 1995-96 and has since declined to 58% of its biyearly maximum. In Part B, Barracouta had a 29% weight decline. Snapper declined from 2087g in 1971-72 to 767g in 1981-82, and then fluctuated between 971g and 1723g and fell after 1997-98 to 35% of its original biyearly maximum. Comparing the first 8 biyearly value with the second in Part B, Snapper’s decline was 13%. Kahawai also declined from a maximum of 2117g in 1971-72 to 731g in 2003-04, 35% of its peak and had a 30% decline in Part B. Warehou mean weight declined to nil after its peak of 863g in 1971-72 and later reappeared and reached a mean weight of 631g in 1991-92 and then declined to 249g, which is 31% of its peak weight. Spotty, after a hiatus of over 12 years, never regained its early maximum mean weight of 220g and is now 95% of this maximum, with Part B showing a 22% decline. Blue mackerel mean weight increased in Part B by 21% from 532g to 645g. After a peak of 1125g in 1979-80, it declined to 0 and then increased to a maximum of 1458g in 1993-94 and has since declined to 27% of this peak. Flounder declined from its original peak of 675g, has since varied from 517g to 306g and is now 62% of its maximum weight with a decline in Part B of 10%. Dab, with a low sample size, peaked at 600g per fish and has declined to 403g in 2001-02 and nil in 2003-04. In Part B it declined 25%. Grey mullet http://www.oceansatlas.org 44
Table 18. Mean weight per fish, in grams, of the eleven major species, biyearly, columns 2 to 12. The sum of the mean weights for all species is listed in the last column with peak values in bold type. 18A. Biyearly data, 1971-2004. 18 B. The mean of first eight biyearly periods in which the species occurred (1971-1986) and the mean of the second eight biyearly periods of occurrence (1987-2002).
18 A . Eleven major fish species All Years R Y B S K W Sp Bm F D G spp. 1971-72 3872 224 3180 2087 2117 863 0 0 0 0 0 2100 73-74 3234 220 3630 787 2077 0 220 0 0 0 0 1733 75-76 1240 205 2952 1199 1242 0 0 0 0 0 0 1154 77-78 2608 215 2945 1276 1803 0 0 0 675 0 0 1254 79-80 0 166 0 1011 1076 0 0 1125 517 233 0 607 81-82 0 141 3065 767 240 0 0 0 374 585 1300 667 83-84 0 86 3329 1367 1122 163 0 261 306 565 266 713 85-86 0 225 0 1723 929 0 0 210 432 383 2497 942 87-88 0 137 3013 1451 1190 115 138 313 413 0 2270 962 89-90 4720 126 3775 1204 1053 256 140 277 402 290 2025 714 91-92 0 114 2382 971 1210 631 170 1103 383 600 2360 776 93-94 0 162 2950 1272 1189 438 195 1458 347 130 1595 719 95-96 3260 105 2588 1133 946 258 157 0 402 244 2290 497 97-98 0 134 2190 1474 407 382 192 453 458 330 2115 462 99-00 5636 163 535 665 730 341 208 698 439 0 2225 645 2001-02 0 164 767 760 673 407 165 210 469 403 2170 513 03-04 2870 163 1793 730 731 249 208 400 417 0 1643 672 18 B. 1971-86 2739 185 3184 1277 1326 513 220 532 461 442 1354 1294 1987-02 4539 138 2275 1116 925 352 171 645 414 333 2131 626
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 45
increased from 1300g to 2497g in 1985-86 and is now 66% of its peak, The increase in Grey mullet weight shown in Part B is not significant (X2 0.25, p>.5) due to low catch numbers.
11.6 MEAN WEIGHT PER TIDE, BIYEARLY
The most critical dynamic measure of change with time, for both individual fish species and for the ecosystem as a whole, is the mean weight per tide (Table 19). All eleven major species declined in weight per tide as shown in Part B (X2 11.0, p < .001), with a mean decline of 64%. Rig declined by 94%, Spotty and Snapper by 83%, Dab 81%, Blue mackerel 79%, Warehou 74%, Barracouta 62%, Yellow-eyed mullet 58%, Grey mullet 44%, Kahawai 27% and Flounder 20%. Snapper and Rig, the two most valuable commercial species in the 1970s, declined in weight per tide by 88%, while the less valued species, Warehou and Kahawai, declined by 51%, a percent which could possibly increase since both species are now more actively fished than in the 1970s.
11.7 MEAN LENGTH PER FISH, BIYEARLY
Eight of the eleven major fish species declined in length in Part B of Table 20. Temporal patterns were similar to those for weight per fish, but less extreme, as shown by declines of 17% for Kahawai, 15% Barracouta, 13% Dab, 11% Warehou, 10% Yellow-eyed mullet, 8% Spotty, 7% Snapper and 3% Blue mackerel. Rig increased by 2%, Flounder by 4% and Grey mullet by 21%.
11.8 MEAN LENGTH PER TIDE, BIYEARLY
All eleven major species decreased in length per tide in Part B of Table 21 with declines similar to those for weight per tide (Rs +0.91, p<.01); Rig by 96%, Blue mackerel 87%, Snapper 81%, Spotty and Dab 79%, Barracouta 55%, Warehou 53%, Grey mullet and Yellow-eyed mullet 50%, Flounder 13% and Kahawai 9%. The pattern of these decreases was similar to the changes in weight per tide (R 0.81 p<.01) with Rig declining to nil and narrowly reappearing, while Yellow-eyed mullet, Barracouta, Snapper and Spotty showed a strong, often erratic, downward trend. Kahawai and Warehou consistently declined from their maxima in 1971-72, but then, reflecting their recent increase in numbers per tide, recorded secondary maxima in 1999-2000 (Kahawai) and 1995-96 (Warehou). All the late arrival (invader) species reached their maximum length per tide between 1983 and 1986 and have since declined.
11.9. SUMS OF THE SIX POPULATION VARIABLES FOR ALL SPECIES, 1971-86 AND 1987-2002 AND 1971-1974 to 2001-2004.
The discussions in sections 11.2 to 11.8 were based on the temporal patterns of the six population variables calculated as a mean for each of the eleven major species. For the ecosystem as a whole, the values for each of the population variables were summed for all twenty-three netted species in each biyearly period and then divided by the total number of individuals to give a mean sum for each period. These sums are shown in the last columns of Tables 16 to 21. Between 1971-86 and 1987-2002, there were declines http://www.oceansatlas.org 46
Table 19. The mean weight of fish per tide, in grams, of the eleven major species, biyearly, columns 2 to 12. The sum of the weight of fish per tide for all species is listed in the last column with peak values in bold type. 19A. Biyearly data, 1971-2004. 19B. The mean of the first eight biyearly periods in which the species occurred (1971- 1986) and the mean of the second eight biyearly periods of occurrence (1987-2002).
19 A. Eleven major fish species All Years R Y B S K W Sp Bm F D G spp. 1971-72 2351 80 170 559 567 324 0 0 0 0 0 4051 73-74 699 30 98 64 168 0 18 0 0 0 0 1077 75-76 13 15 95 103 120 0 0 0 0 0 0 347 77-78 49 12 28 119 67 0 0 0 6 0 0 281 79-80 0 35 0 99 141 0 0 37 25 15 0 368 81-82 0 13 93 58 7 0 0 0 34 18 20 242 83-84 0 4 130 284 44 6 0 85 28 7 3 592 85-86 0 5 0 403 237 0 0 4 110 24 53 842 87-88 0 17 156 67 185 1 2 11 28 0 26 492 89-90 41 12 16 42 92 6 1 4 26 5 9 268 91-92 0 7 52 38 138 39 1 15 44 5 10 350 93-94 0 6 21 80 33 49 1 15 15 <1 11 237 95-96 12 7 38 37 160 112 3 0 25 4 8 417 97-98 0 10 13 22 52 14 7 8 43 2 25 199 99-00 84 8 8 5 276 68 8 5 33 0 17 520 2001-02 0 9 9 6 48 57 1 2 52 3 9 234 03-04 35 18 22 47 62 9 5 3 5 0 20 256 19B. 1971-86 778 24 102 211 169 165 18 42 41 16 25 832 1987-02 46 10 39 37 123 43 3 9 33 3 14 329 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 47
Table 20. Mean length per fish, in mm., of the eleven major species, biyearly, columns 2 to 12. The sum of the lengths per fish for all species is listed in the last column with peak values in bold type. 20A. Biyearly data, 1971-2004. 20B. The mean of the first eight biyearly periods in which the species occurred (1971-1986) and the mean of the second eight biyearly periods of occurrence (1987-2002).
20A Eleven major fish species All Years R Y B S K W Sp Bm F D G spp. 1971-72 1054 271 906 486 626 337 0 0 0 0 0 627 73-74 1015 264 960 333 554 0 240 0 0 0 0 599 75-76 800 236 853 370 401 0 0 0 0 0 0 413 77-78 960 251 870 398 506 0 0 0 0 0 0 445 79-80 0 258 0 375 408 0 0 400 350 270 0 331 81-82 0 233 885 332 275 0 0 0 308 380 480 353 83-84 0 216 883 376 418 220 0 294 309 390 293 341 85-86 0 285 0 417 381 0 0 385 310 308 580 364 87-88 0 229 848 385 399 168 203 283 332 0 523 379 89-90 1055 228 952 353 384 237 210 287 332 310 520 337 91-92 0 221 761 354 395 303 215 422 324 368 640 353 93-94 0 214 883 375 431 270 233 466 312 210 485 328 95-96 944 207 791 377 369 237 220 0 331 264 580 290 97-98 0 221 863 383 288 252 224 341 342 293 546 301 99-00 1078 244 488 318 352 253 232 376 336 0 525 332 2001-02 0 241 495 320 340 263 222 259 350 310 533 312 03-04 827 240 776 332 349 216 233 301 340 0 476 331 20B 1971-86 957 252 893 386 446 279 240 360 319 337 451 462 1987-02 976 226 760 358 370 248 220 348 332 293 544 323
http://www.oceansatlas.org 48
Table 21. Mean length of fish per tide, in mm, of the eleven major species, biyearly, columns 2 to 12. The sum of the lengths per tide for all species is listed in the last column with peak values in bold type. 21A. 1971-2004. 21B. The mean of the first eight biyearly periods of occurrence (1971-1986) and the mean of the second eight biyearly periods of occurrence (1987-2002). ` 21 A Eleven major fish species All Years R Y B S K W Sp Bm F D G spp. 1971-72 640 97 49 130 168 126 0 0 0 0 0 1209 73-74 219 36 26 27 45 0 19 0 0 0 0 372 75-76 9 18 28 32 39 0 0 0 0 0 0 124 77-78 18 14 8 37 19 0 0 0 4 0 0 100 79-80 0 55 0 37 54 0 0 13 17 18 0 201 81-82 0 21 27 25 8 0 0 0 28 12 7 128 83-84 0 11 34 78 16 9 0 96 28 5 4 283 85-86 0 6 0 97 97 0 0 8 79 20 12 325 87-88 0 28 44 18 62 2 2 10 23 0 6 194 89-90 9 21 4 12 37 5 1 4 22 5 2 127 91-92 0 14 17 14 45 19 1 6 37 3 3 159 93-94 0 8 6 24 12 30 2 5 13 1 3 108 95-96 3 14 12 12 62 103 5 0 21 5 2 243 97-98 0 17 5 6 36 9 8 6 32 2 7 130 99-00 16 12 7 3 133 50 9 3 25 0 3 267 2001-02 0 13 6 3 24 37 2 2 39 2 2 143 03-04 10 27 9 21 29 8 6 2 4 0 6 126 21 B 1971-86 222 32 29 58 56 68 19 39 31 14 8 297 1987-02 9 16 13 11 51 32 4 5 27 3 4 170 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 49 in Part B of Tables 16 to 21 of 27% for percent of occupied tides, of 18% for number per tide, of 52% for mean weight per fish, of 54% for weight per tide, of 30% for mean length per fish and of 43% for length per tide. These rates of decline are lower than the declines when the first four survey years from 1971-74 are compared with the last four years from 2001-2004. In these comparisons, percent of occupied tides declined 46% from 40% to 22%, the number of fish per tide declined 70%, from 1.408 to 0.419, the mean weight per fish declined 71% from 2036g to 585g, mean weight per tide declined 91% from 2868g to 245g, mean length per fish declined 48% from 622 mm to 321 mm, and mean length per tide declined 85% from 876 mm per tide to 134 mm per tide. http://www.oceansatlas.org 50
12. Percent changes in the six biyearly population variables
The extensive changes shown by each of the eleven major species for the six population variables, over time, in Chapter 11 are summarised in relative terms in Table 22. Percent values for data in Tables 16-21 are shown for each variable, by comparing the first half, 1971-86, with the second half, 1987-2002 (Part B). There were 58 decreases and 8 increases between the first and second halves of Part B (X2 37.8, p<.001). Four species had one or two increases, Kahawai by 21% in number per tide, Flounder by 3% in percent tidal occurrence and 4% in length per fish, Blue mackerel by 21% in weight per fish, Rig by 66% in weight per fish and 2% in length per fish and Grey mullet by 57% and 21% for weight and length per fish. All eleven major species had mean percent declines with the lowest mean decline 9% for Flounder, 12% for Kahawai, 23% for Grey mullet, and greater mean declines of 39% for Yellow-eyed mullet, 43% for Barracouta and Warehou, 51% for Blue mackerel and Rig, 57% for Snapper, 58% for Dab and 59% for Spotty.
Mean percent declines for the six population variables in Table 22 ranged from 4% for weight per fish and 5% for length per fish to 54% for percent of occupied tides, 55% for number per tide, 59% for length per tide and 64% for weight per tide. The mean change for both the six variables and the eleven species was a decline of 40%.
Table 22. Percent changes for the six population variables in Tables 16 to 21 for the eleven major species between the early and later division in Part B (1971-86, 1987- 2002). Column 1 lists six population variables: weight per tide, length per tide, number per tide, percent occupied tides, weight per fish, length per fish, and the mean of the six values. The last column lists the mean for the six variables, the last row lists the mean for each of the eleven major species.
Variables Fish species K W Y F S Bm R B Sp D G Mean Part B Wt/tide -27 -74 -58 -20 -83 -79 -94 -62 -83 -81 -44 -64 Lgt/tide - 9 -53 -50 -13 -81 -87 -96 -55 -79 -79 -50 -59 No./tide +21 -39 -44 -20 -78 -88 -96 -47 -80 -74 -56 -55 % tides -10 -47 -37 + 3 -77 -67 -89 -49 -80 -74 -65 -54 Wt/fish -30 -31 -25 -10 -13 +21 +66 -29 -22 -25 +57 - 4 Lgt/fish -17 -11 -10 + 4 - 7 - 3 + 2 -15 - 8 -13 +21 - 5 Mean-B -12 -43 -39 - 9 -57 -51 -51 -43 -59 -58 -23 -40
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 51
13. Biyearly running means for weight per tide and length per tide
The patterns of moderate to quite substantial changes with time for weight and length per tide shown in Tables 19 and 21 for the eleven major species can be more clearly visualised when the biyearly values are smoothed by a three biyearly period, non- weighted, running mean, as shown in Table 23.
13.1 CHANGES IN WEIGHT PER TIDE.
There are six patterns of smoothed temporal changes in weight per tide (Part A): (1) Rig and Yellow-eyed mullet declined steeply from highs in 1971-72 to troughs in 1983-84 and 1993-94, after which they had minor recoveries to 2001-02 or 2003-04. (2) Barracouta and Snapper had a sustained decline which was initially less steep than that for Rig and Yellow-eyed mullet and was interrupted by a substantial recovery in the mid 1980s followed by a decline to 1999-2000. Snapper has increased slightly since 1999-2000. (3) Kahawai, after its major peak in 1971-72, consistently declined to 64 g/tide in 1981- 82, then had substantial lesser peaks in 1987-88 and 1997-98 followed by an all time low in 2003-04. (4) Warehou and Spotty, after their peaks in 1971-72, declined to nil in 1975-80 for Warehou and to nil in 1977-84 for Spotty, and then both increased to a secondary peak in 1997-98 and declined to the present. (5) The four early invader species started at nil and reached peaks in 1981-82 for Blue mackerel, 1983-84 for Flounder and Dab and 1987-88 for Grey mullet, after which Blue mackerel and Dab declined to nearly nil in 2003-04. Flounder and Grey mullet declined to lows in 2003-04 which were less than half their peak values. (6) The pattern for the sum of the twelve other species increased from 0 in 1971-76 to a minor peak in 1981-82, declined in 1983-86, but then slowly and consistently increased to 2003-04, when it had its highest smoothed weight per tide value.
13.2 CHANGES IN LENGTH PER TIDE
The smoothed temporal changes in length per tide in Table 23 Part B are similar to those for weight per tide, but with a few variations. The decline in length per tide for Rig was greater than for weight per tide and the slight recovery not as substantial. Yellow-eyed mullet length per tide declined to troughs in 1983-84 and 1993-94, and has since consistently increased. Barracouta and Snapper declined from peaks in 1971-72 to 1979- 80, then increased to secondary peaks in 1983-86, declined to troughs in 1999-2002, with Snapper increasing to the present. Kahawai greatly declined to 1981-82, increased to peaks in 1987-88 and 1997-98, then declined to a record low in 2003-04. Warehou length per tide declined to nil from 1971-72, then substantially increased with peaks in 1993-94 and 1997-98, which nearly equaled its maximum in 1971-72, and has since declined. Spotty also declined to nil, increased to 1997-98, and has currently stabilised. Blue mackerel had a somewhat “normal” distribution, which gradually increased from nil to a maximum in 1985-86, then consistently declined to nearly nil at present. Dab also had a near “normal” distribution with a peak in 1981-82 followed by a decline to nearly http://www.oceansatlas.org 52
Table 23. Weight per tide in grams (A) and length per tide in cm (B) in three period running means of all biyearly periods, 1971-2004, for eleven major fish species and twelve minor (Other) species. Peak values for each species are in bold type.
Fish species R Y B S K W Sp Bm F D G Other Sum A.grams 1971-72 1525 55 134 312 368 162 9 0 0 0 0 0 2565 73-74 1021 42 121 242 285 108 6 0 0 0 0 0 1825 75-76 254 19 74 95 118 0 6 0 2 0 0 0 568 77-78 21 21 41 107 109 0 0 12 10 5 0 12 338 79-80 16 20 40 92 72 0 0 12 22 11 7 12 304 81-82 0 17 74 147 64 2 0 41 29 13 8 13 408 83-84 0 7 74 248 96 2 0 30 57 16 28 2 560 85-86 0 9 95 251 155 2 1 33 55 10 27 2 640 87-88 14 11 57 171 171 2 1 6 55 10 29 4 531 89-90 14 12 75 49 138 15 1 10 33 3 15 3 368 91-92 14 8 30 53 88 41 1 11 28 4 10 4 292 93-94 4 7 37 52 110 67 2 10 28 3 10 5 335 95-96 4 9 24 46 82 58 4 8 28 2 15 6 286 97-98 31 8 20 21 163 79 6 4 34 2 17 7 392 99-00 27 9 10 11 125 46 5 5 43 2 17 16 316 2001-02 45 12 13 19 129 45 5 3 30 1 15 26 343 03-04 12 26 10 33 55 42 5 2 25 1 10 29 250 B. cm. 1971-72 430 67 38 79 107 63 10 0 0 0 0 0 794 73-74 290 50 34 63 84 42 6 0 0 0 0 0 569 75-76 82 22 21 32 34 0 6 0 1 0 0 0 198 77-78 9 29 12 35 37 0 0 4 7 6 0 2 141 79-80 6 30 12 33 27 0 0 4 16 10 2 2 142 81-82 0 29 20 47 26 3 0 36 24 12 4 3 204 83-84 0 13 20 67 40 3 0 35 45 12 8 2 245 85-86 0 15 26 65 58 4 1 38 43 8 7 2 267 87-88 3 18 16 43 65 2 1 7 41 8 7 3 214 89-90 3 21 22 15 48 9 1 7 27 3 4 2 162 91-92 3 14 9 17 31 18 1 5 24 3 3 4 132 93-94 1 12 12 17 40 51 3 4 24 3 3 4 174 95-96 1 13 8 14 37 47 5 4 22 3 4 4 162 97-98 7 14 8 7 77 54 7 3 26 2 4 4 213 99-00 5 14 6 7 64 32 6 4 32 1 4 6 181 2001-02 9 17 7 4 62 32 6 2 23 1 4 7 174 03-04 3 25 5 14 24 33 6 1 18 1 3 8 141 Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 53 nil. Grey mullet length per tide peaked in 1983-84 and then declined. Length per tide for the sum of the other invader species has slowly increased from zero to 8 cm per tide. The “normal” distributions in length per tide in Table 23 for Blue mackerel and Dab, as well as for Flounder and Grey mullet, are all skewed with a gentle decline to the present.
13.3 COMPARISON OF WEIGHT AND LENGTH PER TIDE
The sums of smoothed weight per tide and length per tide in the last column of Table 23 are similar in their patterns of peak and trough values. Weight per tide has consecutive peaks of 2565, 640 and 392 g/tide and troughs of 304, 286 and 250 g/tide, while length per tide has peaks of 794, 267 and 213 cm/tide and troughs of 141, 132 and 141 cm/tide. Each subsequent peak or trough is lower than the preceding one except the last trough for length per tide which is not yet confirmed since a lower length per tide for the years 2005-06 could lower the present smoothed value. http://www.oceansatlas.org 54
14. Early, Mid and Later Dominants and Invaders
14.1 DOMINANTS AND INVADERS
The changes in the smoothed distributions for weight per tide and length per tide in Table 23 show a clear division into the seven original species, from Rig to Spotty, and the four invader species, Blue mackerel, Flounder, Dab and Grey mullet. This division, augmented by an analysis of species arrival times, were used to establish a distribution of Te Mako species into four groups, Early, Mid and Later Dominants and Invaders, as shown in Table 24. The Early Dominants, Rig and Yellow-eyed mullet, had the highest biyearly values in number per tide in 1971-74 and 1979-80 and in weight per tide in 1971-74. Mid Dominants, Snapper and Barracouta, had the highest number per tide in 1975-78 and in weight per tide in 1975-78, 1981-88 and 1993-94. Later Dominants, Kahawai, Warehou and Spotty had the highest number per tide in 1987-2005 and in weight per tide in 1979-80, 1989-92, 1995-96, and 1999-2005. The sixteen Invader species had the highest smoothed biyearly values in number per tide in 1981-86 and in weight per tide in 1997-98.
The patterns in Table 24 show a decline from 1971-72 to 1977-78 of first the Early then the Mid and then the Later Dominants which was accompanied, after 1975-76, by a rapid increase in the number and weight per tide of the Invader species, which peaked in 1983- 86. Thereafter, the Invader species declined and, except for a short revival around the new millenium, are still declining. This Invader decline was accompanied by an increase in the Later Dominants, with both the Early and Mid Dominants showing a lesser increase in number and weight per tide beginning in 1998-2002. The recent increases in the number and weight per tide of the Later Dominants has occurred during a period of overall decline in the Te Mako catch and it suggests there is a swing back to the original species which may reflect a decline in fishing pressure. The two biyearly periods when the Invaders reached 41 and 44 percent of the total weight per tide were the result of a chance peak for both Flounder and Grey mullet in 1997-98 and the anomalous appearance of three large Invaders in 2001-02. In the future, these large Invaders, Skipjack tuna, Eagle ray and Carpet shark may reappear, but the present trend, as shown by the percent invader values in Table 24, indicates a return to a higher percentage of original species.
There is a categorisation of grassland and other terrestrial ecosystems which divides species within communities under stress into decreasers, increasers and invaders, and which seems applicable to the divisions in Table 24. The Early Dominants, Rig and Yellow-eyed mullet are early decreasers, the Mid Dominants, Snapper and Barracouta are later decreasers. Both the Early and Mid Dominants have declined to low levels, but are slowly increasing at present. The Later Dominants, Kahawai, Warehou and Spotty are the increasers, though their most recent values are lower than their peaks in 1995-96 and 1999-2000. The Invaders at Te Mako consistently increased from 0 to 55% of the number of fish per tide from 1971-72 to 1983-84 and have since declined to 7%. Invader species, as a percent of total weight (Table 24), increased to 1981-82 and then declined except for the two anomalous periods of 1997-98 and 2001-02 noted above. Invader species may again increase, or even predominate in the future, if the decreaser and increaser species are more stressed than they are at present. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 55
Table 24. Biyearly values for number per tide x 1000 and weight per tide in grams, for seven original species divided into Early, Mid and Later Dominants (ED, MD, LD) and for sixteen later arriving Invader species (I)*. The highest value in each biyearly period is shown in bold type.
Year Number per tide x 1000 Weight per tide in grams ED MD LD I % I ED MD LD I % I 1971-72 964 322 643 0 0 2431 729 891 0 0 1973-74 351 108 162 0 0 729 162 186 0 0 1975-76 86 118 97 0 0 29 198 120 0 0 1977-78 75 103 37 9 4 61 147 67 6 2 1979-80 213 98 131 164 27 35 99 141 92 25 1981-82 91 106 30 136 37 13 151 7 71 29 1983-84 52 247 78 455 55 4 414 50 123 21 1985-86 21 234 255 382 43 5 403 237 197 23 1987-88 121 98 179 116 23 17 223 188 65 13 1989-90 101 39 122 114 30 53 59 107 50 27 1991-92 61 62 179 149 33 7 90 178 76 22 1993-94 35 70 147 77 23 6 101 83 47 19 1995-96 70 48 625 96 11 19 75 275 46 11 1997-98 78 21 199 132 31 10 35 72 81 41 1999-00 64 23 614 105 13 92 13 352 63 12 2001-02 56 20 218 163 36 9 15 106 104 44 2003-04 124 76 144 36 9 53 69 76 59 23 2005 243 78 340 49 7 44 43 114 34 14
* Early Dominants (ED) are Rig and Yellow–eyed mullet. Mid Dominants (MD) are Snapper and Barracouta. Later Dominants (LD) are Kahawai, Warehou and Spotty. Invaders (I) are Blue mackerel, Grey mullet, Flounder, Dab, Gurnard, Jack mackerel, Blue cod, Trevally, Red mullet, Elephant fish, Red cod, Eagle ray, Skipjack tuna, Carpet shark, Spotted stargazer and Red snapper. http://www.oceansatlas.org 56
15. Environmental variables and fish populations.
15.1 SEASONS, TEMPERATURES, PRECIPITATION, MOONS, PERCENT NIGHT TIDES AND FISH POPULATIONS.
The number of fish per tide x 1000 is shown in Table 25 for: 1) six consecutive bimonthly seasonal periods starting with July- August, which is the coldest bimonthly period, 2) four intervals of increasing temperature from the winter months, July-August, to the four circumwinter months, September-October and May-June, to the four circumsummer intervals, November-December and March-April, to the summer months of January-February, 3) three periods of increasing bimonthly precipitation from January, February, March and April, which had 174-194 mm (mean 180mm) to May, June, July, August which had 183 to 226 mm (mean 201mm) to September to December which had 209 to 294 mm (mean 240 mm), 4) four lunar quarters and 5) percent of fish occurrence on night tides.
15.2 SEASONS
Seasonal occurrences in Table 25 show strong modal patterns for all but two species. These two were Kahawai and Flounder, which peaked in May-June, but had lesser peaks in September-December, around six months after the May-June period. Every bimonthly period had at least one species which was dominant with the exception of March-April. Warehou peaked in July-August, Blue mackerel and Barracouta in September-October, Yellow-eyed mullet and Snapper in November-December, Rig, Spotty and Grey mullet in January-February and Kahawai, Flounder and Dab in May-June. Three species, Warehou in January-February and March-April, Rig in September-October, and Grey mullet in May-June failed to appear in one or two of the bimonthly periods. Eight of the eleven major species had peaks, or in the case of Kahawai, a near peak, in the half year from September-February, in which temperatures rose to a maximum during January- February. Kahawai, Yellow-eyed mullet, Snapper and Barracouta peaked in September- December while Rig, Spotty and Grey mullet made up a distinctive high summer component in January and February. Warehou was the only species peaking in the coldest months of July-August, while Kahawai peaked to either side of these coldest months along with the flat fish Flounder and Dab, which peaked in May-June, with a secondary peak for Flounder in November-December.
15.3 TEMPERATURE
Temperature data in Table 25 resemble the trends for seasonal data, but with more ordered and consistent patterns. From colder to warmer intervals, Warehou and Barracouta consistently declined, Warehou decreasing from 266 (x 1000) to 0 fish per tide. Kahawai, Flounder, Blue mackerel and Dab peaked in the second coldest interval, Snapper in the second warmest and Rig, Spotty, Yellow-eyed mullet and Grey mullet in the warmest interval. All the species’ peaks are clear-cut and five species declined to lows of five to zero fish (x1000) per tide. Fish populations of aNew Zealand estuary from 1971 to 2004. Bray & Struik (2006)
Table 25. Environmental variables, 1971-2004. A. The number of fish per tide x 1000 in relation to; 1) bimonthly seasons, 2) temperature from colder to warmer by bimonthly periods, 3) mean precipitation from lower to higher bimonthly periods, 4) four lunar phases. B. Percent occurrence in night tides. Bold-face numbers are the highest for each species in each category.
A. Fish Seasons Temperature Mean monthly Moons Percent spp colder to warmer to cooler colder to warmer precipitation night 180 202 240 Last Quarter, New Moon, tides mm mm mm First Quarter, Full Moon Jul Sep Nov Jan Mar May July Sep Nov Jan Jan My Sep LQ NM FQ FM Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Feb Jun Oct My Mar Mr July Nov Jun Apr Apr Aug Dec CS* CW* W 266 151 51 0 0 147 266 149 26 0 0 207 102 243 52 80 102 90.1 Bm 21 59 22 17 2 6 21 33 12 17 10 9 41 49 14 9 15 85.7 B 23 >23 16 5 17 12 23 18 17 5 11 18 20 19 18 10 25 78.8 Y 43 47 133 119 96 54 43 51 115 119 108 49 90 111 91 83 67 83.9 S 12 91 113 38 40 32 12 62 77 38 39 22 102 52 74 61 48 78.8 R 2 0 58 62 >2 2 2 1 30 62 32 2 29 12 44 10 18 67.6 Sp 12 13 11 26 15 20 12 17 13 26 21 16 12 22 17 13 18 63.6 57 G 2 >2 4 19 17 0 2 1 11 19 18 1 3 16 8 3 5 66.7 K 93 185 182 57 46 209 93 197 114 57 52 151 184 157 165 121 129 63.0 F 58 70 78 57 56 86 58 78 67 57 57 72 74 78 61 99 60 63.3 D 7 8 4 2 13 18 7 13 92813 6 13 >9 9 10 63.6 Sum 539 649 672 402 304 586 539 620 491 402 356 560 663 772 553 498 497 No. tides 515 471 450 420 478 498 515 969 928 420 898 1013 921 714 653 676 789 * CW – Circumwinter, two months before and after the two winter months, July and August. CS – Circumsummer, two months before and after the two summer months, January and Febuary. > indicates slightly higher value for rank correlation, when two values are very close. http://www.oceansatlas.org 58
15.4 PRECIPITATION
The precipitation patterns in the three intervals in Table 25 show that with increasing rainfall, the number of fish per tide (x 1000) increased from 356 to 560 to 663. Yellow- eyed mullet, Rig, Spotty and Grey mullet peaked in the lowest rainfall period, and of these Yellow-eyed mullet and Rig were nearly as numerous in the highest rainfall period. Blue mackerel, Barracouta, Snapper, Kahawai and Flounder peaked strongly in the highest rainfall period, making up 64% of the total, while Warehou and Dab peaked in the intermediate rainfall period and are nil or nearly nil in the lowest rainfall period. If seasonal and precipitation data in Table 25 are combined, then with increasing rainfall, there is a shift from species with warm and dry peaks, Yellow-eyed mullet, Rig, Spotty and Grey mullet, to cold and moist peaks, Warehou and Dab, to cool and wet peaks, Blue mackerel, Barracouta, Snapper, Kahawai and Flounder.
15.5 LUNAR
Between the Last Quarter and the Full Moon the number of fish species with a peak number per tide declined from six to one and the sum of the fish per tide x 1000 declined from 772 to 497. Warehou, Blue mackerel, Yellow-eyed mullet, Spotty, Grey mullet and Dab peaked in the Last Quarter, Kahawai, Snapper and Rig peaked on the New Moon, Flounder in the First Quarter and Barracouta on the Full Moon. Nine of the eleven major species peaked on a New Moon or a waning moon, with only two species peaking on a waxing or Full Moon, which may mean avoidance of light was a large factor in species lunar occurrence. Another factor may be a division of the prey fish by the two major predators, Barracouta, which peaked on the Full Moon, and Kahawai, which peaked on the New Moon.
15.6 PERCENT NIGHT TIDES
The distribution of values for percent species occurrence on a night tide (Table 25, Part B), has a strong division between the five species, Warehou, Blue mackerel, Barracouta, Yellow-eyed mullet and Snapper, which appear from 79 to 90% on night tides, and the remaining six species which occur on night tides from 63 to 68%, with four species Kahawai, Flounder, Dab and Spotty present from 63 to 64%. All six species with a lower night tide occurrence are bottom feeders, with the exception of Kahawai, which may imply that light is necessary in searching for invertebrates. Of the two flatfish, Flounder hunts by touch and taste and Dab by vision and taste (Paul, 2003), so the higher day tide presence of Flounder may be related to avoidance of the night tide hunter, Barracouta, with its large night adapted eyes. Barracouta and Kahawai seem to divide the prey with Barracouta targeting its favourite, Yellow-eyed mullet, the species with the third highest night tide occurrence. It is also noteworthy that there is a highly significant rank correlation between the rank order of species starting with the coldest months and the rank order of percent night tide occurrences (Rs +0.84, p<.01). This correlation may be the coincidental result of all the low night tide species peaking during the cooling from January-February to May-June, whereas the high night tide species all peak during the warming from July-August to November-December. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 59
16. Yearly fish population variables and precipitation
16.1 POPULATION VARIABLES AND PRECIPITATION
Yearly change in seven variables, weight per fish, length per fish, diversity, weight per tide, length per tide, number per tide and percent tidal occupancy is shown in Table 26 together with the number of sampled tides and yearly precipitation values for Te Mako. Precipitation was based on measurements by Mrs. Gail Archer in Tuna Bay, 2.5 km from our rain gauge at Te Mako, from 1984-1994, and by us in Te Mako, Duncan Bay from 1994 to the present. Precipitation in 1994 at Tuna Bay was 2746 mm and at Te Mako was 2725 mm, a difference of less than one percent. Because of this small difference and the similar elevations of the rain gauges in the two bays, it seemed reasonable to combine the rain data into a single series from 1984 to the present. Data from three nearby weather stations in Pelorus Sound, namely Titirangi Bay, Manaroa and Pokokini, were then used to extend the precipitation data back to 1971 by taking a mean ratio between the 1984 to 2000 Te Mako-Tuna Bay data and data from the mean of the three additional stations for the same period and extrapolating to the years before 1984.
16.2 OSCILLATION MODELS AND POPULATION VARIABLES
The patterns of successive maxima in Table 26 for weight per fish have 10 one year intervals, 1 two year, 1 three year and 1 four year interval, while length per fish has 8 one year, 1 two year, 2 three year and 1 four year intervals. These patterns strongly resemble the classical oscillations of Lesmoir-Gordon et al.(2001) in which overpopulation in one year is overcompensated in the next year resulting in peaks every two years. The thirteen peaks for length per fish occur in the same years as thirteen of the fourteen peaks for weight per fish. These fourteen peaks are followed sixteen times in the same year, thirty- nine times in the next year and two times in the two remaining years by the peaks in weight per tide, length per tide, percent tidal occupancy, number per tide and diversity. When this distribution is compared with expected values of 25.9, 24.2 and 6.9, the result is a highly significant X2 of 16.3 (p<.001) which indicates that the peaks in the remaining population variables are concentrated in the first year following the peaks in weight and length per fish.
16.3 POPULATION PATTERNS AND ENVIRONMENTAL VARIABLES
The close relationship noted above and the predominance of two yearly intervals between the similar weight per fish and length per fish peaks raises the possibility that these peaks, and the peaks for the other population variables which closely follow, might be related to one or more environmental variables. This relationship was tested by comparing the dates of the yearly precipitation peaks in Table 26 with the dates of peaks for weight per fish and for length per fish and with the dates of peaks for weight per tide and for length per tide. None of these comparisons were significant. This lack of a significant tendency for the major fish population variables to peak in the same or in the following years as the yearly precipitation peaks does not rule out other models for a relationship with precipitation. Since there is a correlation between rainfall induced nutrient flushing from the land to the ocean and the increased growth of farmed mussels http://www.oceansatlas.org 60
Table 26. Population variables, yearly; precipitation in mm., weight per fish in grams, length per fish in mm., diversity (number of species / square root of the number of tides) x 100, weight per tide in grams, length per tide in mm., number per tide and percent tidal occupancy. Successive peaks in bold-face.
Year No. of Ppt.* Weight Length/ Diver- Weight Length No./ Tidal tides mm. / fish fish sity x / tide / tide tide occup’y grams mm. 100 grams mm % 1971 39 2152 1743 540 96 3307 1025 1.90 46 72 17 1934 2879 816 121 5758 1631 2.00 47 73 11 1914 1689 591 120 1853 645 1.09 55 74 26 2475 1770 608 98 749 257 0.42 19 75 53 2416 1074 407 55 324 123 0.30 17 76 40 2288 1260 421 79 378 126 0.31 15 77 49 2487 1016 444 71 290 127 0.29 22 78 58 2291 1586 446 39 273 77 0.17 14 79 30 2743 522 312 67 418 250 0.80 40 80 41 2777 671 345 109 344 177 0.51 34 81 34 2301 556 321 86 147 85 0.26 18 82 32 1793 733 372 88 344 174 0.47 31 83 39 2574 434 310 128 467 334 1.08 31 84 38 2175 1245 399 114 721 231 0.58 32 85 11 1619 1986 493 60 361 90 0.18 18 86 36 1834 890 357 117 989 397 1.11 53 87 81 2138 1077 406 89 731 276 0.68 27 88 93 2137 776 335 83 284 123 0.37 22 89 88 2521 815 365 86 204 91 0.25 19 90 141 2927 679 328 101 308 149 0.45 27 91 126 1943 839 370 88 306 135 0.37 25 92 102 2468 725 339 89 405 190 0.56 26 93 140 2452 854 395 59 122 56 0.14 13 94 145 2725 683 310 91 348 158 0.51 28 95 141 3101 894 371 67 266 111 0.30 18 96 131 2714 407 272 114 579 386 0.42 40 97 188 2332 383 282 73 191 141 0.50 25 98 144 3602 615 333 83 209 113 0.34 22 99 149 3155 704 341 98 463 224 0.66 35 2000 118 2424 596 324 82 591 321 0.99 31 01 133 2434 540 323 87 227 136 0.42 21 02 119 2493 487 302 119 241 150 0.50 24 03 134 2064 628 311 78 192 95 0.31 22 04 115 2936 703 346 93 293 144 0.42 20 05 103 1818 318 256 69 241 194 0.76 31
* 1971-1984 precipitation extrapolated from three Pelorus Sound weather stations. 1985-2004 data from Te Mako and nearby site. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 61 in Pelorus Sound (R. Schuckard, p.c.), then peaks in precipitation, and especially peaks in the maximum spring precipitation season of October to December which occur during the crucial spring warming period, might also be reflected in increased fish growth and increases in the other population variables. To test this possibility, we compared peaks for seasonal precipitation in spring (October-December), summer (January-March), autumn (April-June) and winter (July-September) with the peaks in the population variables. When the Te Mako peaks were extrapolated back to 1971, based on the three Pelorus Sound weather stations (Table 27), then there was a very highly significant correlation between spring precipitation and both weight per fish and length per fish ( X2 15.1, p<.001), but there were no significant correlations with summer, autumn or winter precipitation. For weight and length per tide, there was a highly significant correlation with summer precipitation (X2 8.3, p<.01), but not with precipitation in the other seasons. Summer precipitation peaks were not related to peaks in fish length and weight (X2 0.25, p>0.5), perhaps because the summer months, with their minimum precipitation, would have produced a much lower nutrient flush and a shorter period for the fish to react to the nutrient flush. Also, the summer months occur after the main vernal spawning period in which there is a substantial loss in fish weight. For number per tide, diversity and percent of occupied tides, there was also a highly significant correlation with summer precipitation (X2 10.8, p<.01), but not with the other seasons.
When similar comparisons between precipitation and fish population variables were made for only the non-extrapolated period (1984 to present) in which precipitation was measured at Te Mako, then these non-extrapolated comparisons gave the same results as the extrapolated data above. There was a significant correlation between spring precipitation and both weight and length per fish (X2 4.2 p<.05). Summer precipitation was significantly correlated with weight and length per tide (X2 9.4, p<.01) and with the other three variables (X2 10.8, p<.01).
16.4 SPRING PRECIPITATION AND POPULATION VARIABLES
The significant relationships between seasonal precipitation and the fish population variables may be the result of a discrete causal sequence from the precipitation peaks to peaks in weight and length per fish and then to peaks in weight and length per tide and the remaining population variables. These sequences can be seen in the fifteen rows of Table 27 in which the 42 spring precipitation peaks are followed in 0.88 years by peaks in weight per fish and in 0.86 years by peaks in length per fish. The peaks in weight per fish are then followed in 0.88 years by peaks in number per tides, in 0.73 years by peaks in diversity and in 1.1 years by peaks in percent tidal occupancy. Similarly, the peaks in length per fish are followed in 1.1 years by peaks in length per tide, in 0.88 years by peaks in number per tide, in 0.91 years by peaks in diversity and in 1.2 years by peaks in percent tidal occupancy. These results suggest that peaks in weight per fish and length per fish follow a year after peaks in spring precipitation and are then followed in around a year by peaks in weight per tide and length per tide, number per tide, diversity and percent tidal occupancy. To test the discreteness of these sequences in Table 27, the number of times the end of each sequence was in the same year, or before the beginning of the next sequence (twelve instances), was compared with the number of times the end of the sequence occurred after the beginning of the next sequence (two instances). There http://www.ocean
Table 27. Years from 1971 to 2005 with peak values for seven population variables in relation to peak spring and peak summer precipitation for four Pelorus Sound stations: Manaroa (M), Pokokini (P), Titirangi Bay (T) and Te Mako (TM). Diversity is the number of species divided by the square root of the number of tides times 100. Years in parentheses were extrapolated from Manaroa, Pokokini satlas.org and Titirangi Bay data.
Peak Years Spring (Oct,Nov,Dec) Summer (Jan,Feb,Mar) Population variables M P T TM M P T TM Weight Length Weight Length No./ Diver Tidal 62 per fish per fish per tide per tide tide sity occ % 71 71 71 (71) 72 72 (72) 72 72 72 72 72 72 73 73 73 73 (73) (74) 74 74 75 (75) 75 76 76 (76) 76 76 77 76 77 77 77 77 (77) (78) 78 78 79 79 79 79 79 79 79 (79) 80 80 80 (80) 80 80 81 81 81 (81) 82 82 (82) 82 82 84 83 83 83 84 83 83 83 (84) 84 85 85 86 86 86 86 86 85 86 86 86 86 87 87 88 88 88 88 89 89 89 89 90 90 90 90 90 90 90 90 91 90 91 91 92 92 92 92 92 92 92 92 92 92 92 93 93 94 94 94 94 94 94 95 95 94 95 95 95 96 95 95 96 96 96 96 96 99 99 98 98 98 98 99 99 99 99 00 00 00 00 00 01 01 01 01 02 02 02 02 02 02 02 02 02 04 04 04 04 04 (05) (05) 04 (05)
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 63 is a significant probability (X2 7.2, p<.01) that this temporal distribution of the sequences was not due to chance. The sequences in Table 27 also demonstrate a significant tendency for a spring Southern hemisphere precipitation peak in one year to be immediately followed, as could be expected, by a summer precipitation peak in the following year as shown by the summer peaks occurring in the same or a preceding year from the spring peaks in only three instances and occurring in the following year in thirteen instances (X2 4.6, p<.05).
If the sequences noted above are the result of peak spring nutrient enrichment, then the first indication of its effect on fish assemblages should be an immediate increase in weight per fish and length per fish. Since the spring period is the last season of the year, this effect should also result in an increase in fish length and weight in the following year as is clearly shown in the sequences in Table 27. These heavier and longer fish, which occur in a year after a vernal precipitation peak, would then result in an increased weight and length per tide. This would presumably increase fish fertility, which would then result in higher fish numbers and an increase in percent tidal occupancy and, perhaps, in diversity.
16.5. CONCLUSIONS
The flow sheet for the sequences from peaks in vernal precipitation to peaks in the seven population variables in Table 27, and the statistical relationships which test these sequences illustrate our conclusion that these variables might be mainly related to a single environmental factor. Such a possibility is in agreement with conclusions in Lloyd and May (1999) and with the correlation between sea temperature and 80% of the total production of commercial and estuarine species in Maine, U.S.A., over the period 1905- 1975 (Dow, 1986). It is also in agreement with the patterns in the Blasius et al.(1999) models which oscillate regularly, but not in agreement with their conclusions about chaotic peak amplitudes. Neither our peak precipitation nor fish population amplitudes seem to be chaotic. Precipitation peaks gradually increased in an ascending wave from 1971 (or earlier) to 1998 and then declined to 2004. Fish populations have apparently cyclical amplitudes with the sums of percent occupied tides, number per tide, weight per tide and length per tide all peaking in 1971-72, 79-80, 85-86, 91-92, 95-96 and 1999- 2000, as noted in Chapter 11.2.
Because a certain set of variables follow in ordered ranks from another data set does not prove a connection between the two data sets, except that there are possible environmental reasons why increased spring rainfall could trigger increases in weight and length per fish and the population variables related to fish weight and length. A second reason they might be related is that connections between environmental events and a wide range of New Zealand cyclical or less structured biologic events are being more widely recognized and critically studied at present (D. Kelly, p.c.2004).
Whether the fairly regular cyclical rhythms of the seven variables in Table 27 are related to their well defined maxima in Table 26, which occurred in 1972, 1985-86 and more diffusely in 1993-96, requires longer term study. http://www.oceansatlas.org 64
17. Te Mako fish population trends
17.1 THREE MAJOR TRENDS
Over the past 34 years, there have been three major dynamic trends in the fish populations of Te Mako estuary. The first is the decline in all six of the population variables from 1971-74 to 2001-04, calculated for the ecosystem as a whole (Chapter 11.9) of 46% for tidal occurrence, of 70% for number per tide, of 71% for mean weight per fish, of 91% for mean weight per tide, of 48% for mean length per fish and of 85% for mean length per tide. The second trend is the shift in maximum numbers per tide from the Early Dominants to the Mid Dominants to, briefly, the Early Invaders, and then to the Later Dominants and the shift in weight per tide from the Early to Mid to Later Dominants. During these changes, there have been shifts back to the Early Dominants in numbers per tide in 1979-80 and to the Mid Dominants in weight per tide in 1993-94 together with a shift to the Early Invaders in weight per tide in 1997-98, but the trend is clear. It is unlikely that there will be a future shift back to the Early Dominants without a large recovery in Rig numbers, nor to the Mid Dominants, which have suffered a large decline in both Barracouta and Snapper weight per tide, partly the result of a continuing fall in weight per fish. The third trend is the persistence through time of all the eleven major species. While Spotty, Rig and Warehou missed appearing in five or six consecutive biyearly periods, they have since returned, and all other major species have been present, since their initial appearance in all, or nearly all, the biyearly periods. All the major species except Rig were present in every biyearly period since 1989-90.
The compositional stability of the Te Mako fish populations may be partly a result of the opportunistic nature of their food preferences. Only Kahawai and Barracouta are nearly exclusively fish eating. All the other fish eat varying combinations of fish, invertebrates and plants, though Flounder, Dab and Grey mullet eat fish rarely or never. Snapper is omnivorous with one of the most varied of fish diets, including a substantial plant component. These opportunistic fish populations may favour stable community trophic structures. Another reason for the stability of the Te Mako fish populations may be that there are no species with a very high percent abundance or weight per tide so that there are a number of major species with moderate abundance and weight per tide available if there is a collapse in the dominant species. An example of such a collapse is shown in Table 28, which lists biyearly percent weight per tide for the major species. From 1971 to 1974, the dominant species, Rig, had 58% and 65% of the biyearly percent weight per tide, but it then collapsed and from 1979-80 to 1987-88 was absent. From 1989-90 onwards Rig has made occasional appearances with a low weight per tide. When Rig collapsed, Kahawai dominated the weight per tide results in nine biyearly periods with percent weight of from 24 to 53. Snapper dominated four biyearly periods with percent weight of 34 to 48 and Barracouta and Warehou each predominated in one biyearly period. Since the Rig collapse in 1971-74, no species has attained a similar level of dominance and the ratio between the percent weight per tide of the highest and next highest species has been low, from 1.1 to 3.6 with a mean of 1.7. These ratios indicate a much higher level of stability than the biyearly ratios for Rig in 1971-74 of 4.1 and 4.2. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 65
Table 28. The percent weight of fish per tide, biyearly, for the eleven major species and the twelve other species.
Years Fish species – percent weight per tide R Y B S K W Sp Bm F D G Other 1971-72 58.0 2.0 4.2 13.8 14.0 8.0 73-74 64.9 2.9 9.1 5.9 15.6 0.0 1.7 75-76 3.8 4.3 27.4 29.8 34.7 0.0 0.0 77-78 17.4 4.3 10.0 42.3 23.8 0.0 0.0 2.1 79-80 0.0 9.0 0.0 25.4 36.2 0.0 0.0 9.5 6.4 3.9 9.5 81-82 0.0 5.3 38.3 23.9 2.9 0.0 0.0 0.0 14.0 7.4 8.2 0.0 83-84 0.0 0.7 22.0 48.1 7.4 1.0 0.0 14.4 4.7 1.2 0.5 0.0 85-86 0.0 0.6 0.0 47.9 28.1 0.0 0.0 0.5 13.1 2.9 6.3 0.6 87-88 0.0 3.4 31.7 13.6 37.5 0.3 0.4 2.2 5.8 0.0 5.3 0.0 89-90 15.8 4.6 6.2 16.2 35.4 2.3 0.4 1.5 10.0 1.9 3.5 2.3 91-92 0.0 2.0 14.8 10.8 39.3 11.1 0.3 4.3 12.5 1.4 2.8 0.6 93-94 0.0 2.5 8.9 33.9 13.9 20.8 0.4 6.4 6.4 0.0 4.7 2.1 95-96 2.9 1.7 9.2 8.9 38.6 27.0 0.7 0.0 6.0 1.0 1.9 2.2 97-98 0.0 5.2 6.6 11.1 25.9 6.9 3.5 4.1 21.5 1.0 12.8 1.3 99-00 16.1 1.5 1.5 1.0 53.1 13.1 1.5 1.0 6.3 0.0 3.3 1.5 2001-02 0.0 3.8 3.8 2.6 20.5 24.4 0.4 0.8 22.2 1.3 3.8 16.2 03-04 13.6 7.0 8.6 18.3 24.1 3.5 1.9 1.2 1.9 0.0 7.8 12.1 http://www.oceansatlas.org 66
The collapse of Rig, and its replacement as a dominant species at Te Mako by four other fish species can be compared with the collapse of the Anchovetta fishery off Chile (Loeb & Rojas, 1987). Anchovetta declined from a percent weight of 100 in 1963-64, to 0 in 1979-80, and was replaced by Sardine, Jack mackerel and Mackerel. In this replacement, the ratio between the highest and next highest species varied from 1.2 to 9.6 with a mean of 3.6, over twice the variation that we observed at Te Mako following the collapse of Rig.
Another approach to estimating the potential for recovery following a top species collapse is to look at the abundance profile and estimate the number of species capable of replacing the collapsed species, but this is difficult without estimates of the minimum values required for replacement. What is certain is that fish ecosystems dominated by a single species may be unstable and this instability probably increases with the relative abundance of the top species. The top species which collapsed in Te Mako and Chile both did so in the sixth year following a percent abundance peak of 76 for Rig at Te Mako and in the eighth year following a period of 100% abundance for anchovetta in Chile, although this 100% abundance may have been the result of not reporting the bycatch until the collapse began.
17.2 TEMPORAL CHANGE – DECLINE IN FISH EATING BIRDS
The major fish eating bird in the estuary is the Pied shag whose numbers have declined from common to occasional over the past 34 years, a likely result of the decline in Te Mako fish productivity. During this time, Pied shag were seen twenty-three times near the net; five from 1971-81, eight from 1982-92 and ten from 1993-2004. The number per tide x 1000 decreased from 13 to 9 to 5, a decline of 59%. Another fish eater, the Australasian gannet was rare in the 1970s, but established a nearby breeding colony which grew from 5 pairs in 1975 to 185 pairs in 2001 (P. Gaze, p.c. 2004). During this expansion, visits to Te Mako increased, but have recently declined. We have kept records of all birds seen while clearing the net since 1995. The results from 1995-97 to 2003-05 show a decline in Pied shag from 19 to 3 per 1000 tides. Gannets also declined from 5 to 1 per 1000 tides as did the Kingfisher from 12 to 8 per 1000 tides.
Since 1995, birds which feed both in the water and on the estuary have also declined, Paradise shelduck from 129 to 46 per 1000 tides and hybrid ducks from 420 to 271 per 1000 tides. Both these species are the only game birds which feed on the estuary and their decline may reflect hunting pressure. Birds which feed or scavenge on the estuary have increased in numbers, from 118 to 146 per 1000 tides for White-faced heron, 49 to 203 per 1000 tides for Oystercatcher and 16 to 152 per 1000 tides for Black-backed gull. The increase in gull numbers was probably the result of the gull changing from a visitor to a resident. One species, Spur-winged plover, decreased from 225 to 128 per 1000 tides, a result of its recent arrival in the area and a possible overshooting of its resource base with a subsequent population decline. Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 67
17.3 FISH POPULATIONS, FISHING PRESSURE AND ENVIRONMENTAL DEGRADATION
Our study supports numerous warnings of decline in the diversity and productivity of the world’s fisheries (McGoodwin 1990, Pauly & Christensen 1995, Pauly et al. 1998, Kates & Parris 2003, Jackson et al. 2001).
Between the first and second halves of our study there were 58 declines and 8 increases in the six fish population variables for the eleven major species (Table 22). Weight per tide (Table 19) and length per tide (Table 21) declined for all eleven major species, number per tide (Table 17) and percent occupied tides (Table 16) declined for 10 species, and weight per fish (Table 18) and length per fish (Table 20) declined for 8 species. The only species that increased in a population variable were Flounder, Grey mullet and Rig twice each, and Kahawai and Blue mackerel, once each. It was the faster growing, shorter lived species like Flounder and Grey mullet that had more than one temporal increase. Both of the increases for Rig were, as noted, the result of a very small sample in the second half of the study. For the 11 species as a whole, all of the six population variables declined with time, often quite precipitously.
The weight per tide at Te Mako (Table 26), after the peak value of 5758 g per tide in 1972, consistently declined until 1978 when there were 273 g/tide. During this time there was a continuing increase in the New Zealand catch by Foreign Fishing Vessels (FFVs) from a reported catch of zero in 1967 to 20,000t in 1968 and 1969, 60,000t in 1971, 100,000t in 1972, 250,000t in 1974, 400,000t in 1976 (Struik, 1980) and 500,000t in 1977 (Sissenwine and Mace (1992). Foreign Fishing Vessels were observed in Tennyson Inlet until, in 1978, the New Zealand government declared an Exclusive Economic Zone (EEZ). In 1978 in Tennyson Inlet, Mr. R. Winstanley (p.c.1978) saw three FFV trawlers working near Maud Island and Dr. E. Twose (p.c.1978) saw a Japanese purse seiner near Maud Island with a net stretched to Richmond Bay. One possible effect of the FFVs was noted by Mr. N. Andrews (p.c.) of Tuna Bay, who had fished the area since 1955. He stated that before the early to mid 1970s you were guaranteed a Snapper meal whenever you went fishing, but not thereafter. Not all the pressure on the fish resource of Tennyson Inlet was from FFVs. We were told by a New Zealand man that he had taken part in illegal night time dynamiting of all the uninhabited bays in the Inlet in the late 1970s.
Increased fishing, foreign and domestic, from 1971 onwards, probably resulted in some, if not most, of the decline in the Te Mako catch. The history of N.Z. fishing regulation by Paul (2000) and Sissenwine and Mace (1992) notes that deregulation of the fisheries occurred in 1963 and was followed by a depletion of the inshore fisheries resulting from a “massive increase in fishing effort” from 1965-75 (Stevens 2001a). The decline in catch which occurred as a result of the EEZ declaration 1978 led to a sharp decrease in the N.Z. catch for a few years, followed by a recovery in the early 1980s (Sissenwine and Mace, 1992) which was enhanced by the N.Z. government announcement of a new fisheries act in 1983 which included quota management and transferable quotas. At Te Mako the massive decline in weight per tide from 5758g in 1972 to 147g in 1981 was likely the result of the unregulated FFVs working the New Zealand coastal waters. It is http://www.oceansatlas.org 68 notable that this decline was then reversed, probably as a result of the 1983 N.Z. Fisheries Act, with increased weights per tide at Te Mako and peaks of 721g in 1984 and 989g in 1986. This pattern is similar to the commercial fish recovery in N.Z. coastal waters in the early 1980s noted by Sissenwine and Mace (1992). Since these higher weight per tide peak values at Te Mako, there has been a continuing overall decline with only two subsequent minor peaks of 579 g/tide in 1996 and 591 g/tide in 2000 (Table 26).
The decline in weight per tide at Te Mako may be related to a decline in environmental quality, though this is difficult to assess. There has been no noticeable increase in eutrophication of the estuary over the past 34 years and little change in its physical appearance, except for a small increase in sandy depressions and more rock rubble, both the result of increased rainfall. The major changes have been the invasion of the Pacific oyster (Crassostrea gigas) and the various meanderings of the small streams that interbraid through the estuary. The benthic habitat of Pelorus Sound, into which Te Mako estuary discharges and which is the source of all the fish in this study, has been altered in the past 30 years by trawling and dredging which is viewed by Thrush and Dayton (2002) as a significant threat to structural and functional biodiversity. Mr. J.H. King-Turner of Canoe Bay, Tennyson Inlet has told us (p.c. 1993) he observed a long term decline in the bottom flora and fauna, including some fish species, of his bay over the previous 30 years. He thought the decline might be due to scallop dredging, bottom trawling, over fishing and the increased number of boats with anti-fouling chemicals on their hulls. Another possible factor in a declining environment may be the increased area and harvest (Paul 2000) in mussel (Perna canaliculus) farms in Pelorus Sound. These mussels feed on phytoplankton, zooplankton and bacteria (Paul 2000), as do related filter feeding mussels and Pacific oysters studied elsewhere by Davenport et al (2000) and Cognie et al. (2001). This feeding could lead to productivity declines for other invertebrates and for vertebrates in the food web. The benthic area beneath farms is also altered by mussel droppings.
17.4 RECOVERY SCENARIOS FOR TE MAKO FISH POPULATIONS
If possible traumas like overfishing and environmental degradation are reduced, then there is reason to believe that fish numbers, weights, lengths and total productivity could recover. If the precautionary approach (Essington 2001) were applied to Te Mako, then populations which have less than 40% of their original biomass would be regarded as overfished and populations with less than 10% biomass would no longer be fished. By these criteria, comparing biomass in 2003-04 with biomass in 1971-72 (or in the maximum biyearly period for the four invader major species), all the 11 species are overfished and fishing of Rig, Warehou, Snapper, Blue mackerel, Flounder and Dab should be stopped. Present Kahawai biomass of 10.9% of original and Barracouta of 12.8% of original are barely above the cut off point for fishing. If overfishing were reduced, the return to previous population levels may not always happen quickly. Hutchings (2002) analysed recovery of 90 marine fish at 10 and 15 year intervals after their populations had experienced at least a 45% decline in reproductive biomass. Of these 90 species, 41% continued to decline, 51% exhibited some recovery and 8% had fully recovered. We estimated population recovery periods for the Te Mako species based on observed short term recoveries following a decline of 45% or more of their Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 69 highest three year weight per tide running mean. The results in Table 29 show estimates for 20, 40, 60, 80 and 100% recovery. These results, compared with those of Hutchings noted above, showed 18% of our species had fully recovered after 10 years, 54% had some recovery and 27% had not yet reached 20% recovery. Three species, Dab, Grey mullet and Flounder recovered quickly, reaching 20% of maximum in one year, 40% in one to two years, 60% of maximum in 2 to 3 years and 80% in 2 to >15 years. Both Dab and Grey mullet reached 100% in 2 to 5 years. The medium recovery species, Kahawai, Yellow-eyed mullet, Barracouta, Snapper and Spotty recovered to 20% in 1 to 12 years, to 40% in 1 to 13 years, to 60% and 80% in 5 to >31 years and to 100% in 20 to >31 years. No medium recovery species recovered to 100% in 20 years, except Spotty. The slow recovery species Blue mackerel, Warehou and Rig recovered to 20% of maximum in 7 to >30 years and to 40% in 23 to >30 years with no species recovering to 60% in less than >19 years. Recovery periods to 100% varied from 2 to >15 years for the fast recovery species, 20 to >31 years for the medium recovery species and >19 to >32 years for the slow species. Mean recovery periods, shown in the last three rows of Table 29, ranged from 1.0 years for fast, 3.6 years for medium and >19.3 years for slow recovery species to reach 20% recovery and from >7.3, >27.2 and >27 years for fast, medium and slow species to reach 100% recovery.
The two fast recovery species, Dab and Grey mullet, together with medium recovery species Spotty, were the only species which achieved 100% recovery in the 35 years of our study and they were three of the four least commercially valuable of the 11 major Te Mako species. The most commercially valuable, Snapper and Rig, and the less valuable in terms of total catch, Barracouta, Kahawai, Blue mackerel and Warehou, all have long recovery periods, taking a mean of >26 years to reach 60% recovery and >27 years to reach 100% recovery. Besides having a low commercial value, the fast recovery species had a low mean age to reproductive maturity of 2.9 years and a low mean life span of 8.2 years. The medium recovery species had a higher age to maturity of 3.6 years and a longer life span of 23.7 years, while the slow recovery species had an age to maturity of 4.7 years and a mean life span of 17 years. The mean life span of the medium recovery species was skewed by the very high value of over 60 years for Snapper. http://www.oceansatlas.org 70
Table 29. Recovery period in years for Te Mako fish species to reach 20, 40, 60, 80 and 100% of their highest three year weight per tide running mean following a decline of 45% or more. A 45% decline was used so that results would be comparable with Hutchings (2002).
Species, Percent recovery following 45% decline Age of Life Recovery maturity span groups 20% 40% 60% 80% 100% years years Fast D 1 2 2 2 2 3 8 G 1 1 3 5 5 3 11.5 F 1 2 2 > 15 > 15 2 5 Medium K 1 1 > 31 > 31 > 31 4.5 > 26 Y 1 3 > 29 > 29 > 29 3 7 B 3 4 5 5 > 26 3 21.5 S 1 9 11 > 30 > 30 4 > 60 Sp 12 13 13 13 20 3.5 > 4 Slow Bm 7 > 19 > 19 > 19 > 19 5 15 W 21 23 > 32 > 32 > 32 4 16 R > 30 > 30 > 30 > 30 > 30 4.5 20 Means Fast 1.0 1.7 2.3 > 7.3 > 7.3 2.7 8.2 Medium 3.6 6.0 > 17.8 > 21.6 > 27.2 3.6 23.7 Slow > 19.3 > 24.0 > 27.0 > 27.0 > 27.0 4.7 17.0
Fish populations of a New Zealand estuary from 1971 to 2004. Bray & Struik (2006) 71
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