This file is part of the following reference:

St John, Jill (1995) Feeding ecology of the coral trout,

Plectropomus leopardus (Serranidae), on the Great

Barrier Reef, Australia. PhD thesis, James Cook

University.

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http://eprints.jcu.edu.au/24127/

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Feeding ecology of the coral trout, Plectropomus leopardus (Serranidae), on the Great Barrier Reef, Australia.

by Jill St John

(BSc. Hons, Sydney)

A thesis submitted for the degree of Doctor of Philosophy in the Department of Marine Biology

at James Cook University of North Queensland. August 1995

Third fisherman :

Master I marvel how the fishes live in the sea

First fisherman :

Why as men do a-land the great ones eat up the little ones

Shakespeare Pericles Act ii sc 1 I. 29 1608

STATEMENT OF ACCESS

I, the undersigned, the author of this thesis, understand that James Cook University of North Queensland will make it available for use within the University library and, by microfilm or other means, allow access to users in other approved libraries. All users consulting this thesis will have to sign the following statement:

In consulting this thesis I agree not to copy or closely paraphrase it in whole or in part without the written consent of the author; and to make proper public written acknowledgement for any assistance which I have obtained from it.

Beyond this, I do not wish to place any restriction on access to this thesis.

(Jill St. John) (Date)

STATEMENT ON SOURCES

DECLARATION

I declare that this thesis is my own work and has not been submitted in any form for another degree or diploma at any university or other institution of tertiary education. Information derived form the published or unpublished works of others has been acknowledged in the text and a list of references is given.

(Jill St. John) (Date)

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ACKNOWLEDGEMENTS

One of the major conclusions of this study is that PhD's are not accomplished alone. Mine is no exception. I extend my sincere thanks to all of you who helped me along the way.

I'd like to thank the people who made this all possible by providing a stipend. I am gratefully indebted to the members of the Australian Federation of University Women - Queensland, particularly those volunteers who run a business to fund women in academia. This thesis is dedicated to them.

I am very grateful to my supervisor, Garry Russ for his continuous support, guidance and advice throughout this study. Garry read and re-read all of my thesis except this section. I found Garry to be accessible, dependable and humorous, three qualities of a good supervisor. I was fortunate to have two associate supervisors. Hugh Sweatman provided invaluable logistical and statistical assistance. Hugh's answers to endless questions and his provision of important equipment such as access to statistical programs and a coffee machine has greatly improved this thesis. Bruce Mapstone supported me with sensible and logical advice on many matters including statistics and design. Bruce assisted me a lot in the early stages of this study. Thank you.

A special thanks to Lyle Squire who helped me get this thesis up and running. Lyle provided much needed practical assistance and advice as well as connections to the Townsville Spearfishing Club. Lyle gave me his time, his hospitality and a heap of specimens speared privately (by himself and by his sons) or in connection with his work.

Many thanks to the following people (mentioned in no particular order) for their various roles. • In the field, many trout were collected by the spearos from Townsville Skindiving Club, Bill Roberts, Todd Protheroe, Geoff Ramm, John Goebels, Dave Welsh, Angelo Angeli and John Robinson. Chris Robb was particularly helpful as he volunteered his spearing skills on other field trips. Others kept trout guts for me.... a smelly business, Lyle Squire Jnr and Snr, Campbell Davies, Jeff Bibby, Beatrice Ferreira, Phill Laycock, Hugh Sweatman and Melita Samoilys. Phill Light dissected 150 tiny stomachs for me. Data was provided by Peter Doherty (Australian Institute of Marine Science), Ian Brown and Melita Samoilys (Southern and Northern Fisheries Research Institute QDPI), Tony Ayling (Sea Research) and Bruce Mapstone (GBRMPA). At Orpheus Island Research Station helpful staff included Russell Bathard and Snow and Virginia Downton. These people and Jo. Goudie fed fish for me. Volunteer assistants included Dave Hocking, Dave Welsh, Debbie Rae and Carole Wallace. Bill Gladstone took some great photographs. Line fishing was done by Michael Osborne, Doug and John Clark and Jim Higgs. Laura Castell-Perez and Hugh Sweatman assisted me in the field. Technical help

was provided by Leigh Winsor, Don Booth, Leigh Owens, PhiII Osmond, Colleen, Anita, Ann Sharp, Savita and Jan Woodley, who also sent my mail on. Brett Molony, Hugh Sweatman, Garry Russ sent me references when I was in Japan. Kyoko Tamate and Keiichi Matsuura provided me with access to references in Japan. H. Hirakawa provided me the use of a microscope at the Onahama Fisheries Research Institute. Prey fish were identified by Jeff Leis, Doug Hoese, Dirk Zeller, Alison Green, Garry Russ, Hugh Sweatman, Ann Cabanban and Bill Gladstone. Cephalopods were identified by Natalie Moltschaniwskyj. John Collins and Danielle Johnston identified crustaceans.

Chapters were read by my supervisors, as well as Leigh Winsor, Melita Samoilys, Bill Gladstone, Bryce Stewart, Lin Schwarzkopf, Kathy Kavanaugh, Jean Deklerk, Brett Molony, Jo Beukers, Barry Goldman, Julian Caley, Ilona Stobbard and Ivana Noell. Maps and diagrams were drawn by Steve Clarke.

References were typed by Lishu Chen with the help of Suzy Dormer. The thesis was greatly improved by discussions with Geoff Jones, Beatrice Ferriera, Melita Samoilys, Campbell Davies, Julian Caley, Bryce Stewart, Jo Beukers and my supervisors.

Warm friendships and fun times were enjoyed with many kind people. People who have been particularly supportive include Jo Beukers, Suzy Dormer, Bea Ferreira, Jane Fromont, Bill Gladstone, Gina Mercer, Libby Moodie, Brett Molony, Debbie Rae, Mary Robertson, Carole Wallace and Jan Woodley. Thanks to Mary Robertson, Cheryl Hercus and Grant Whiteman, Carole Wallace and Chris Hill, Jane Fromont and Dave Sutton and Ivana NoeII for providing a place to live. Baby sitters include Mary Robertson, Mary Stafford-Smith, Jane Fromont and Dave Sutton, Julia Shand and Rob MacCauley, Gina Mercer, Suzy Dormer, Hugh Sweatman, Laura Castel! and Lishu Chen.

Finally, I thank my family. Both my parents, and played a vital role in all of this. Little P was born to this thesis and has had to share me with a computer for her first year of life. Her joy of life and thrill of learning has kept my work in perspective. lived this thesis with me from field work to editing. His endless support, encouragement and patience helped me the most.

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ABSTRACT

The Great Barrier Reef (GBR) is the largest coral reef system in the world and

the coral trout, Plectropomus leopardus (Pisces: Serranidae, Lacepede 1802)

is one of its most widespread and abundant piscivores (Ayling and Ayling 1986). Despite the importance of the coral trout fishery, very little is known

. about the feeding ecology of P. leopardus on the GBR.

The principal objective of this study was to describe the diet and feeding of P. leopardus on this large reef system. Variation in the diet of this piscivore

over a range of latitudes along the GBR and on reefs open and closed to fishing was examined. Seasonality of feeding by P. leopardus was

addressed by comparing rates of digestion, consumption of food and diet in the austral summer and winter. Within populations of P. leopardus, the

ontogenetic shifts in diet and individual feeding behaviour were examined.

Plectropomus leopardus is one of the major predators of adult coral reef fishes on the GBR. After their first year of life, P. leopardus are almost entirely

piscivorous oh adult fishes (99% of fish diet). The dominance of fish in the diet does not vary temporally or spatially.

Feeding studies have shown that P. leopardus are intermittent feeders,

consuming an average of one prey item daily. After 24 hours, approximately 90% of prey items were digested. Thus, contents of stomachs represented

daily feeding in P. leopardus. Seasonal rates of food consumption were not related to rates of digestion in P. leopardus.

In P. leopardus, the major dietary shift in the type, species composition, length

and shape of prey occurred at approximately 20 cm SL and corresponded to the onset of sexual maturity. Juvenile P. leopardus consumed a higher

proportion of benthic crustaceans, mostly penaeid prawns. Though families of prey fish in the diet varied ontogenetically, the breadth of the diet (in terms

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of family richness) did not increase with the size of predator . This shift in diet must be due to a change in feeding behaviour because juvenile P. leopardus

live in similar habitats to adults.

Overall, Pomacentridae were the dominant family in the diet of P. leopardus. The main families in the diet of P. leopardus were divided into two groups. Common families (Pomacentridae, Labridae, Caesionidae and Scaridae) constituted a consistent and substantial portion of the diet. Small schooling fishes (e.g. Clupeidae), were a highly variable component of the diet both temporally and spatially. Sporadic appearances of small schooling prey in the diet of P. leopardus were the main cause of the reduction in dietary overlap between reefs and times.

The abundance of prey in three common families varied latitudinally in the diet of P. leopardus . Pomacentridae and Caesionidae were consumed more in the northern GBR whereas Scaridae were eaten more in the south. There were no detectable effects of fishing on the diet of P. leopardus. Dietary overlap among reefs zoned for different levels of fishing were within natural spatial and temporal variations for .the GBR. Most of the major changes in the diet of P. leopardus occurred before the piscivores entered the fishery (current legal minimum size is 38 cm FL). Adult P. leopardus prey on juvenile Plectropomus spp. and thus high densities of adults may reduce abundances of juveniles.

The major seasonal difference in feeding was a higher consumption of food in winter that coincided with increased production of mesenteric fat that is stored in preparation for reproduction (Ferreira 1993). Family richness in the diet increased during winter months because rare prey were consumed to meet a higher requirement for food. Otherwise, composition of the diet did not vary seasonally. Plectropomus leopardus did not appear to respond to the seasonal influx of recruits to the reef during summer; newly recruited prey individuals were consumed rarely.

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Plectropomus leopardus appear to be selective feeders in terms of both families and species of prey. At a family level, 37 families were identified in

the stomachs of P. leopardus. Many abundant families of fishes on the GBR (e.g. Chaetodontidae, Holocentridae, Pomacanthidae and Siganidae) were rarely eaten while others (e.g. Haemulidae) were not identified in the diet at all. The choice of species among the Pomacentridae was examined. Approximately 25% of the 120 possible species of Pomacentridae were identified in the stomachs of P. leopardus. Some of these prey species were eaten in much higher proportions than expected when compared to their general abundance on reefs (e.g. Acanthochromis polyacanthus) whereas

other species were eaten in much lower proportions (e.g Pomacentrus amboinensis, Pomacentrus moluccensis and Neopomacentrus azysron).

The abundance of families (measured by digested weight) in the diet of P. leopardus was compared to the biomass of prey fishes on another midshelf reef on the Central GBR (Williams and Hatcher 1983). The top five ranked families in the diet of P. leopardus (Pomacentridae, Caesionidae, Scaridae, Labridae and Serranidae) were also the top five families in terms of biomass on reefs. Furthermore, these five families represented approximately 80% of both fishes on reefs and the total biomass of the prey in the diet. Prey in families were not eaten in exactly the same ranking as biomass on reefs. There was a large difference between the proportions of the two major prey species, Pomacentridae and Caesionidae, on midshelf reefs. Plectropomus leopardus consumed more Pomacentridae than Caesionidae even though the biomass of Caesionidae on the reef was more than double that of Pomacentridae. The modes of life of schooling, mobile Caesionidae and demersal, site attached Pomacentridae differ on coral reefs and this may explain the reversal of ranks of these two families in the diet of P. leopardus.

In general, the size of prey and composition of the diet P. leopardus was related to the size of the predator. As the gape of P. leopardus limits the size of prey ingested, the critical dimension of the prey is depth rather than length.

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When feeding on larger prey items, small to medium-sized P. leopardus (< 35 cm SL) appear to feed optimally as defined by Werner (1974) with respect to

body-depth of prey. This size-related feeding did not occur under all circumstances. Size of prey was decoupled from size of predators for all P.

leopardus larger than 35 cm (SL) and when the prey were small schooling

fishes. It appears that P. leopardus switch to feeding on small schooling

fishes when they are available.

Predatory behaviour of P. leopardus in tanks was highly variable among

individuals. Both the success of capture of Acanthochromis polyacanthus (F.

Pomacentridae) and the consistency of predatory abilities varied among individuals. Individual variation in feeding may help explain the high

variability in growth and fecundity of individuals observed within populations of P. leopardus (Ferreira 1993; Davies 1995).

The abundance and species richness of the families common in the diet of P. leopardus suggests that food for the coral trout fishery is sufficient on the GBR. Furthermore, feeding by P. leopardus appears to be unaffected by this

human activity at present levels of fishing. Other aspects of this study, however, suggest that fishery managers cannot be complacent. The effects

of cannibalism within the fishery need to be understood. Fishing may reduce predation on prey on reefs directly by removing predators and, in the case of

line fishing, indirectly by feeding predators. Data from the fishery should be treated with caution for two reasons. Firstly, Catch Per Unit Effort (fish per

fisher per hour) was considered a poor method to estimate or compare the population density of P. leopardus as more of the catch was taken by line

when a high proportion of the population was hungry. Secondly, line fished samples of P. leopardus should not be used for dietary studies as the

abundance and composition of prey was biased by regurgitation and the presence of bait. Line caught fish had fewer natural prey in their stomachs

and the composition of the natural diet differed from the sample caught by spear.

In conclusion, the major sources of variability in the diet and feeding behaviour of P. leopardus were the following:

ontogenetic shift from juveniles to adults, high individual variation in successful capture of prey, and regional variation in the diet.

In contrast, seasonality and varying levels of fishing had little effect on the diet of P. leopardus.

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TABLE OF CONTENTS

STATEMENT OF ACCESS iii

STATEMENT ON SOURCES iv

ACKNOWLEDGEMENTS ABSTRACT vii

1.0 GENERAL INTRODUCTION

2.0 GENERAL METHODS 8 2.1. THE STUDY SPECIES, CORAL TROUT 8 2.2 ANALYSIS OF STOMACH CONTENTS 10 2.3 PILOT STUDIES TO DETERMINE SOME GENERAL

METHODS 14 2.3.1. Comparison of fishing methods 14 2.3.2. The effect of delays in processing specimens 18 2.3.3. Weight changes in stomach contents due to

preservation 21 2.3.4. Quantifying the digestion scale 25

2.4. TANK EXPERIMENTS 29 2.4.1 Study sites 29 2.4.2 Collection of live predators and prey 29 2.4.3 Problems associated with life in captivity 30

2.5. GENERAL ANALYSES 31 2.5.1 Analysis of frequency data 31 2.5.2 Analysis of parametric data 32 2.5.3 Probability data 33 2.5.4 Indices of relative importance and dietary-overlap 33 2.5.5 Habitats of prey fishes 35

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3.0 SEASONAL RATES OF DIGESTION, DAILY CONSUMPTION OF FOOD AND GROWTH IN CAPTIVE P. leopardus 44

3.1 INTRODUCTION 44 3.2 METHODS 46

3.2.1 Daily consumption of food 46 3.2.2 Rates of Digestion 47 3.2.3 Growth rates of captive P. leopardus 50

3.3 RESULTS 50 3.3.1 Daily consumption of food 50 3.3.2 Rates of digestion 51 3.3.3 Growth rates of captive P. leopardus 52

3.4 DISCUSSION 52

4.0 ONTOGENETIC PATTERNS IN THE DIET OF P. leopardus 67 4.1 INTRODUCTION 67 4.2 METHODS AND RESULTS 69

4.2.1 Collection of P. leopardus 69 4.2.2 Measurement of specimens 70 4.2.3 Ontogenetic changes in the diet of P. leopardus 71 4.2.4 Ontogenetic changes in the size of prey fishes in the

diet of P. leopardus 72 4.2.5 The prey fishes of P. leopardus 75 4.2.6 Species composition of the diet of P. leopardus 78

4.3 DISCUSSION 79

5.0 TEMPORAL VARIATION IN THE DIET OF P. leopardus ON THE CENTRAL GBR WITH A COMPARISON TO THE ABUNDANCE OF POTENTIAL PREY FAMILIES 111 5.1 INTRODUCTION 111 5.2 STUDY SITES, SAMPLING DESIGN AND DATA USED IN

THE ANALYSES 114

5.3 RESULTS 115 5.3.1 Temporal variation in the diet of P. leopardus 115

5.3.2 Comparison of dietary composition with abundance • of potential prey families on the Central GBR 120

5.4 DISCUSSION 121 5.4.1 Temporal variation in the diet of P. leopardus 121 5.4.2 Comparison of diet to abundance of potential

families of prey fish on the Central GBR 126

6.0 REGIONAL VARIATION IN DIET OF P. leopardus 140 6.1 INTRODUCTION 140 6.2 STUDY SITES, SAMPLING DESIGN AND METHODS 141

6.2.1 Study Sites 141 6.2.2 Sampling Design 142 6.2.3 Intra- and inter-region comparison using other

studies 143 6.3 RESULTS 143

6.3.1 Differences among regions and between reefs in the catch of P. leopardus 143

6.3.2 Regional diet of P. leopardus 144 6.3.3 Diet of P. leopardus in the southern region 152 6.3.4 Diet of P. leopardus in the southern and central

regions 153 6.4 DISCUSSION 154

7.0 THE EFFECT OF PROTECTION FROM FISHING ON THE DIET OF P. leopardus 176 7.1 INTRODUCTION 176 7.2 STUDY SITES, SAMPLING DESIGN AND METHODS 181

7.2.1 Background to this study 181 7.2.2 Source of specimens 181 7.2.3 Collection of samples from different fishing zones . . 182

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7.3 RESULTS 185 7.3.1 The catch of coral trout from reefs within the two

zones 185 7.3.2 Differences between samples of P. leopardus

caught by line and spear 186 7.3.3 Effects of line fishing on the diet of P. leopardus 187 7.3.4 Variation in the diet of P. leopardus between reefs

within different fishing zones 187 7.3.5 Comparison of the diet of P. leopardus among three

fishing zones 190 7.3.6 Pomacentridae as prey of P. leopardus 190 7.3.7 Juvenile Plectropomus in the diet of P. leopardus 191

7.4 DISCUSSION 193

8.0 PREDATORY SUCCESS AND BEHAVIOUR OF INDIVIDUAL P. leopardus IN CAPTIVITY 218 8.1 INTRODUCTION 218 8.2 METHODS 223

8.2.1 Experimental preparations 223 8.2.2 Experimental design and protocol 224 8.2.3 Justification of methods used 228

8.3 RESULTS 229 8.3.1 Predatory experiment 229 8.3.2 Behaviour of P. leopardus in the experiment 230 8.3.3 Rates of predation by piscivores on coral reefs 233

8.4 DISCUSSION 233

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9.0 GENERAL DISCUSSION 254 9.1 P. leopardus are predators on a wide variety of fish prey on

the GBR 256 9.2 Potential impact of P. leopardus on coral reef fish

communities 257 9.3 Relevance to management of fisheries 259 9.4 Future directions for research 263 9.5 Conclusions 264

10.0 BIBLIOGRAPHY 265

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