Population Dynamics of Australasian Gannets (Morus Serrator)

Home , Australasian gannet, Cape gannet, Eddystone (Tasmania), Lawrence Rocks, Northern gannet, Pedra Branca (Tasmania)

Population dynamics of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria; competition with fisheries and the potential use of seabirds in managing marine resources.

Ashley Bunce

Thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy of The University of Melbourne

Department of Zoology The University of Melbourne

October 2000 DECLARATION

This is to certify that (i) this thesis comprises only my original work, except where indicated in the preface, (ii) due acknowledgement has been made in the text to all other material used, and · (iii) this thesis is less than 100,000 words in length, exclusive of tables, maps, bibliographies, and appendices .

October 2000 ABSTRACT

Increasing exploitation of pelagic fish populations worldwide has often resulted in overfishing and the collapse of commercial fisheries and associated serious declines in many marine predator populations, including seabirds. These events higMight the competing demands for limited marine resources. Recent emphasis for the ecologically- sustainable management of commercial fisheries has stimulated attempts to manage fisheries by incorporating knowledge of trophic interactions and ecosystem functioning, known as ecosystem-based fisheries management. Seabirds are often highly visible, wide-ranging upper trophic level consumers that aggregate in areas of increased ocean productivity and therefore be used as natural monitors of marine environmental conditions. Further, many seabirds commonly fed on commercially-exploited fish stocks (often targeting prey of similar size). In this study, the population dynamics of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, is investigated and competition between gannets and commercial fisheries is determined. In addition, the potential use of seabird reproductive and population parameters as indicators of the abundance of commercially exploited fish stocks, and pelagic conditions generally, is assessed. The breeding success of Australasian gannets declined significantly in the 1998- 1999 breeding period and the growth rates of chicks was reduced, following the spread of a large-scale mass mortality of pilchards (Sardinops sagax) in southern Australian waters, suggesting that food availability may influence the reproductive ability of gannets. Similarly, the provision of supplementary food to offspring resulted in increased nestling growth and survival rates whereas, at nests where artificial twins were created (and the level of food demand at the nest increased), offspring had reduced growth rates and survival. The Australasian gannet population in Australian waters has increased considerably in recent years, from some 6,600 breeding pairs in 1980 to approximately 20,000 pairs in 1999-2000, an increase of 6% per year, although the population may have been expanding somewhat earlier (i.e. before 1980). The reasons for the substantial increases in the Australasian gannet population are poorly understood, but it is suggested that a significant trend towards more frequent, and stronger El Nino Southern Oscillation events, warmer summer sea surface temperatures and changes in major commercial fisheries, are likely to have resulted in increased local productivity and therefore may account for at least some of the observed increase in the local Australasian gannet population. Australasian gannets are a major local marine predator feeding predominantly on pilchards and other inshore pelagic schooling fish and cephalopod species, including anchovy {Engraulis australis), garfish (Hyporhamphus melanochir), barracouta (Thyrsites atun), and mackerel {Scomber australasicus). Dietary changes of Australasian gannets between 1988 and 2000 may reflect changes in the abundance of commercially exploited fish stocks. The relative proportion of pilchard in the gannet diet was significantly correlated with annual commercial fishery landings; however, changes in the proportion of other prey in the gannet diet was not. In addition, the proportion of pilchard in the gannet diet declined considerably in 1998, following the spread of pilchard deaths in Victorian waters, presumably reflecting a consequent unavailability of pilchards. Commercial catches of pilchard in Victorian waters have also declined since 1998. It is considered that the abundance of important prey items in the gannet diet, such as pilchards, and commercial fisheries landings are proportional to stock abundance or size and the implications of this relationship for fisheries management is examined. Competition between gannets and commercial fishers is also apparent as both actively target and exploit similar prey (also taking similar age-size classes). It is estimated, based on daily energy requirements (4561 kJ day"') calculated from attendance patterns and activity-specific metabolic rates, that Australasian gannets breeding in Port Phillip Bay consume 228 tonnes of prey throughout the breeding period. The potential foraging range of gannets, determined from foraging trip durations, is within Port Phillip Bay and the nearby coastal waters of Bass Strait. Within this range a number of large commercial fisheries targeting major prey items of gannets operate; however, only pilchards are consistently taken as a major local commercial fishery, consequently the potential for this fishery to negatively affect gannets is apparent. It is considered that reproductive and population parameters, such as breeding success or population change, considered in this study, although likely to be influenced Ill by changes in food availability, vary little if at all when food conditions are favourable. Consequently, such parameters are not ideal indicators of the abundance of commercial fish stocks or pelagic conditions as a negative impact on seabird populations (e.g. a reduction in breeding success or adult survival) is necessary to detect changes in stock size. Alternatively, the prey consumption and changes in the diet composition of seabirds, such as gannets, may provide a more effective, and reliable indicator of commercial fish stocks and are likely to improve the effectiveness of ecosystem-based fisheries management models. However, at present the application of such models is limited and dependent on knowledge of the importance of other biotic and abiotic factors influencing pelagic conditions and fish populations. IV

PREFACE

The completion of this thesis has required some collaboration with others as indicated below, however, the information contained in this thesis is solely my own original work. In the case of chapter 1, chapter 2, and chapter 5, all material presented is solely my own original work and in the other chapters I have used other supporting material which I have not collected myself, as indicated below. In Chapter 3, data on the changes in numbers of Australasian gannets breeding in Tasmanian waters was obtained from Nigel Brothers and Rosemary Gales at the Parks and Wildlife Service in Tasmania. Patrick Coutin from the Marine and Freshwater Resources Institute, Queenscliff, provided information on commercial pilchard catches and catch per unit effort data for this fishery. Sea Surface temperature data for Bass Strait was provided by Steve Worley of the National Centre for Atmospheric Research, Comprehensive Oceanic-Atmospheric Data Set. Information on the Southern Oscillation Index was obtained from the Bureau of Meteorology (Australia) archives. In Chapter 4, data on the commercial fishery catches of selected species were obtained from Patrick Coutin and the Catch and Effort Database at the Marine and Freshwater Resources Institute, Queenscliff. In Chapter 6, data on the commercial fishery catches of selected species were obtained from Patrick Coutin and the Catch and Effort Database at the Marine and Freshwater Resources Institute, Queenscliff. Nutritional analysis of samples of major prey items in the gannet diet were conducted at the Department of Agricultural Sciences, LaTrobe University, Bundoora, under the supervision of Dr. Theresa Frankel. However, these contributions constituted the provision only of raw data and all interpretations and conclusions drawn from this data constitutes solely my own original work. Reference to all other material used, has been fully acknowledged in the text. The chapters presented in this thesis have been prepared as individual papers, consequently some repetition of material is inevitable and unavoidable, however attempts have been made to keep any repetition to a minimum. Each of these papers has been either submitted for publication or published as listed below. The inclusion of co-authors on these papers acknowledges a supervisory role or the provision of raw data (in the case of other authors) only. The interpretation drawn from this data and the ideas and conclusions as presented reflects solely my own original work. Publication details of the papers presented in this thesis are indicated below;

Chapter 1 Bunce, A., Norman, F. I. & Ward, S. J. (2000). Age-related variations in the breeding success of Australasian gannets {Moms serrator) during contrasting years of prey availability. Journal ofAvian Biology (submitted).

Chapter 2 Bunce, A. (2000). Effects of supplementary feeding and artificial twinning on nestling growth and survival in Australasian gannets {Moms serrator). Emu (in press).

Chapter 3 Bunce, A., Norman, F. I., Brothers, N. & Gales, R. (2000). Long-term trends in the Australasian gannet {Morus serrator) population in Australia: the effect of climate change and commercial fisheries. Marine Ornithology (submitted)

Chapter 4 Bunce, A. (2000). Dietary changes of Australasian gannets {Morus serrator) reflect variability in pelagic fish stocks. Marine and Freshwater Research (submitted).

Chapter 5 Bunce, A. & Norman, F. I. (2000). Changes in the diet of the Australasian gannet {Moms serrator) in response to the 1998 mortality of pilchards {Sardinops sagax). Marine and Freshwater Research 51, 349-353.

Chapter 6 Bunce, A. (2000). Prey consumption of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, south-east Australia and potential competition with commercial fisheries. ICFS Journal of Marine Science (submitted). VI

In addition, a collaborative joint-authored paper detailing changes in the status of Australasian gannets in Victoria has also been included as an appendix. This paper details research conducted by each of the authors and therefore does not constitute solely my own original work, however, this paper represents other research that I have been involved in during this project that is also directly related to this thesis and is consequently included as an appendix.

Appendix 1 Norman, F. I., Minton, C. D. T., Bunce, A. & Govanstone, A. P. (1998). Recent changes in the status of Australasian gannets Moms serrator in Victoria. Emu 98, 147-150. Vll

ACKNOWLEDGMENTS

Many people have generously and freely provided their time, effort, and support during the completion of this project and I have benefited greatly from this help. I have been extremely fortunate to have had such wonderful and amazing support from a large and terrific group of people. Unfortunately it is not possible to mention everybody who has contributed to the completion of this project, other than to say a collective, 'Thank you'. In particular, I would especially like to thank the people listed below.

Supervision I have been very fortunate to have Ian Norman (Department of Natural Resources and Environment) as a supervisor. Ian has given freely of his time and energy, and has provided excellent support, encouragement, guidance and motivation throughout all aspects of this project. He has been an excellent mentor, contributing enormously to my understanding of seabird biology and has helped me to think clearly, logically and critically - a gift for which I am extremely grateful. lan's willingness and eagerness to contribute and assist in so many ways can be described as nothing other than astounding and exceptional, and is greatly appreciated. In addition, Ian has also always shown concern for my own well-being and has been a good friend. Thank you. I would also like to thank Simon Ward (Department of Zoology, University of Melbourne) for his support and encouragement as a supervisor. Simon has always been friendly, good-humoured and available and open for discussions. He has guided me through this challenging and sometimes difficult process and his encouragement is much appreciated. Simon has also shown great patience and perseverance, assisting in various aspects of this project, from his efforts in the field to dealing with administrative guff, he has done so willingly.

Funding The completion of this project would not have been possible if not for the generous support provided by the following. Vlll

Research This project was conducted under Flora and Fauna permit BB-97-007 from the Department of Natural Resources and Environment. Ethics approval (Reg. No. 97073) was obtained from the Animal Experimentation Ethics sub-Committee. The following groups kindly provided financial support: University of Melbourne Research Award; Department of Natural Resources and Environment; Department of Zoology, the University of Melbourne; Holsworth Wildlife Research Fund; and the Phillip Island Penguin Parade.

Travel I have been very fortunate to be supported to attend a number of national and international conferences during my candidature. I have also received assistance to travel and meet with fellow scientists at international research institutions. For this experience I am grateful for the support offered by the Department of Zoology, University of Melboume; Melbourne Abroad Postgraduate Travelling Scholarship; F.H. Drummond

International Travel Award; Stuart Leslie Bird Research Award; Australasian Wildlife Management Society Travel Award; and Ecological Society of Australia Student Travel

Grant.

Fieldwork This research was conducted at Pope's Eye Marine Reserve, located near the entrance to Port Phillip Bay between Queenscliff and Portsea. Transport to this site was provided by Rod Watson from the Queenscliff Marine Station (QMS). Rod has made much of my fieldwork enjoyable with his good humour and antics, and also never failing to return and collect me from my desolated rocky outpost. His commitment and dedication to helping the students down at QMS is extraordinary and is appreciated by all (well almost all). The following people came to help torment the gannets and showed great courage, skill and determination; often rewarded with excellent snorkelling opportunity's. IX

1997-1998 Katharine Evans, Karen Blaakmeer, Ian Norman, Simon Ward, Pam Baker, James Norman, Kirstin Long, Jeremy Hatch, Jan Lind, Steven Floyd, and John Arnould.

1998-1999 Ian Norman, Simon Ward, Helena Bender, Katharine Evans, Lara Pugh, Mark Padgham, Femmie Kraaijeveld-Smit, and Ken Kraaijeveld.

1999-2000 Ian Norman, Katharine Evans, Simon Dyer, and Rod Watson.

Lab Work Nutritional Analyses of fish samples were conducted in the Department of Agricultural Sciences at LaTrobe University, Bundoora, under the supervision of Theresa Frankel. Theresa gave generously of her time and facilities, guiding me through a field I new little about and allowing me to use her equipment.

Travel In June 1999, I visited Jan Komdeur at the University of Groningen in the Netherlands to conduct molecular sexing on blood samples taken from gannet chicks, however, things did not go as planned and regrettably these analyses were not performed. Thanks especially to Karen Blaakmeer for her good friendship and also superb hospitality whilst in Groningen. In late June 1999, I also visited Rory Wilson at the Institut fiir Meerskunde, University of Kiel, Germany. Rory has been a great source of advice and support throughout my candidature and it was a terrific opportunity to meet with him personally. For this experience I am grateful, and wish to thank Rory and also Marie-Pierre and the kids for their kindness and generous hospitality whilst in Kiel and for making my brief stay an enjoyable one. Provision of advice and assistance This thesis has greatly benefited by the contributions of many. In particular, I would like to thank the following.

Data A number of people have provided data for me to use in this thesis, which has undoubtedly improved the story that I have been able to tell. In particular, I would like to thank Nigel Brothers, Rosemary Gales and Patrick Coutin (see Preface for details).

Fish samples Phil and Kim McAdam kindly provided fresh samples of pilchard, anchovy, garfish and barracouta for use in nutritional analysis. Frozen pilchards used for the supplementary feeding of young were provided by R.F. McLaughlin's Bait.

Advice This thesis has also greatly benefited from comments and advice provided by many people. In particular I would like to thank Rory Wilson, Peter Dann, Patrick Coutin, Bill Montevecchi, Rosemary Gales, Mike Francis, April Hedd and Jochim Lage. Florence Choo (Statistical Consulting Centre, University of Melbourne) and Mick Keough (Department of Zoology, University of Melboume) provided expert statistical advice when needed.

Comments on this thesis The quality of this thesis has undoubtedly been greatly improved by the comments kindly provided by Ian Norman, Simon Ward, Melissa Giese, Patrick Coutin, Nigel Brothers, Peter Dann, Norbet Klages, Bill Montevecchi, Rosemary Gales, and John Arnould.

Friends and Family This work has taken a long time to complete and I am extremely grateful to my friends and family for their understanding, support, encouragement and patience. At times aspects of this project have often made me less than sociable, yet you have all XI persevered and kept be motivated with your good will and kindness. I would especially like to thank my family for their love, support and patience: Peter, Jason and Chris and my parents Neville, Sandra and Ken. Thanks also to Rosalie, Geoff, Jan, Steven and Robyn. I would also like to thank all the friends that I have made in the Department over the years. In particular, my office companions Michelle, Bron, Nadine, Doug, and Iain, fellow lab members, including Helena, Kath, Femmie, Dan, Phil, Tani, and the guys down at QMS - Rod, Jeremy, and Melissa. Finally, I have difficulty in finding words to thank Sharon, who has been an unfailing source of strength, understanding, support, love and inspiration. Thanks... TABLE OF CONTENTS

GENERAL INTRODUCTION

CHAPTER 1

Age-related variations in the breeding success of Australasian gannets(Moms serrator) during contrasting years of prey availability

CHAPTER 2 23.

Effects of supplementary feeding and artificial twinning on nestling growth and survival in Australasian gannets(Moms serrator)

CHAPTER 3 ^

Long-term trends in the Australasian gannet(Morus serrator) population in Australia: the

/■ potential effect of climate change and commercial fisheries

CHAPTER 4 1 63

Dietary changes of Australasian gannets (Morus serrator) reflect variability in pelagic fish stocks.

CHAPTER 5 ^

Changes in the diet of the Australasian gannet (Morus serrator) in response to the 1998 mortality of pilchards (Sardinops sagax)

CHAPTER 6 92

Prey consumption of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, south-east Australia and competition with commercial fisheries GENERAL DISCUSSION n6

Ecosystem-based fisheries management and the use of seabirds in the management of marine resources

REFERENCES 123

APPENDIX 1 140

Recent changes in the status of Australasian gannets (Morus serrator) in Victoria GENERAL INTRODUCTION Increasing exploitation of pelagic fish populations worldwide has often resulted in overfishing and the collapse of commercial fisheries and associated serious declines in many marine predator populations, including seabirds, highlighting the competing demands for limited marine resources (Murphy, 1977; Crawford & Shelton, 1978; Nettleship et al., 1984). The lack of sustainability and current uncertainty of traditional fisheries management results from a dependence on stock assessments. Such assessments are presently generally based on past catches and catch rates and have little scope for environmental variability. Commercial harvests are additionally, often greatly influenced by technological improvements, market and political demands for increased harvests (Cairns, 1992a; Montevecchi, 1993; Botsford et al., 1997). Recently the relevance of ecosystem-based fisheries management, which uses knowledge of trophic interactions and ecosystem functioning for the effective sustainable management of commercial fisheries, has been widely recognised (Cairns, 1992a; Shelton, 1992; Botsford et at., 1997; Shannon et al., 2000; Hollowed et al., 2000). Ecosystem-based fisheries management requires a detailed understanding of the abundance of fish stocks and factors which may influence them including natural mortality, recruitment and environmental variability. Seabirds are often highly visible, wide-ranging upper trophic level consumers that aggregate in areas of increased ocean productivity and therefore may be natural monitors of marine environmental conditions (Croxall, 1987; Furness & Monaghan 1987; Burger 1988; Montevecchi 1993). In addition, many seabirds commonly feed on commercially- exploited fish stocks (often targeting similar size classes) and may be excellent indicators of pelagic fish stocks, providing reliable, simple and effective fisheries independent indications of pelagic conditions (Berruti & Cloclough, 1987; Montevecchi & Berruti, 1991; Cairns, 1992a; Montevecchi & Myers, 1996). Estimates of the predation of pelagic fish stocks by seabirds have been essential in developing ecosystem-based fisheries management procedures and have also enabled quantification and assessment of the level of competition between a number of seabird populations and commercial fisheries (Furness, 1978; Furness & Cooper, 1982; Wanless etal., 1998). Numerous studies have estimated the consumption of prey by seabirds, and investigated the relationship between seabird and commercial fishery harvests, in different marine environments (e.g. Weins & Scott, 1975; Fumess, 1978; Furness & Cooper, 1982; Croxall et al., 1984; Guinet et al., 1996; Wanless et al, 1998), however, there is little similar information available for seabirds in Australian waters (see Gales & Green, 1990). This is despite the existence of large numbers of breeding seabirds (Marchant & Higgins, 1990; Ross et al., 1995) in the cool temperate waters of southeastern Australia, and the presence of a number of major commercial fisheries in this region (Kailola etal., 1993). Australasian gannets {Moms serrator) breed at several localities in southeastern Australia and also around New Zealand (Wodzicki et al, 1984; Marchant & Higgins, 1990). In 1980-1981, the world population of about 53,000 breeding pairs included 6,600 pairs nesting in Australian waters (Marchant & Higgins, 1990); where gannets now breed at six colonies; Lawrence Rocks, Point Danger, and Port Phillip Bay along the southem coast of Victoria and Black Pyramid, Pedra Branca and Eddystone Rock off Tasmania (Fig. LI). Earlier this century another gannetry also existed at Cat Island in Bass Strait, although this colony has now all but disappeared, primarily a result of human persecution (Warham & Serventy, 1978; Phillipps, 1993; N. Brothers, pers. comm.). Australasian gannets are a major local marine predator feeding predominantly on pilchards (Sardinops sagax) and other inshore pelagic schooling fish species and previous studies have documented aspects of the feeding and breeding ecology of gannets and demonstrated variability in reproductive parameters and population parameters (see Waghom, 1982; Norman, 1992; Norman &Menkhorst, 1995; Gibbs et al., 2000). In this study, the population dynamics of Australasian gannets {Morus serrator) breeding in Port Phillip Bay, Victoria, competition with commercial fisheries and the possible use of seabirds in managing marine resources is examined in the following chapters (here prepared as and presented as papers, consequently some minimal repetition of material is inevitable and unavoidable). Variations in the breeding success and reproductive performance of Australasian gannets are investigated in relation to changes in prey availability (Chapter 1) and, using artificial twinning and supplementary feeding of young, the effects of alterations in the level of food demand on nestling growth and survival is determined experimentally (Chapter 2). Changes in the Australasian gannet population in Australian waters (see also Appendix 1) are documented and discussed in Port Phillip Bay

Point Danger ^

Lawrence Rocks

Black Pyramid•

Pedra Branca % Eddystone Rock

Figure I.l. Distribution of Australasian gannet(Morus serrator) breeding colonies in

Australian waters. relation to changes in local marine productivity, in particular changes in climatic and oceanographic conditions and the activity of commercial fisheries (Chapter 3). The relationship between seabird and commercial fishery harvests as a potential indicator of changes in pelagic fish stocks is assessed by comparing variation in the prey harvests of Australasian gannets breeding in Port Phillip Bay with annual commercial fisheries landings (Chapter 4). Changes in the diet of Australasian gannets in response to a mass- mortality of pilchards in 1998 (Chapter 5) are also examined. Information from these previous chapters (i.e. Chapters 1-5) is then used to construct a simple bioenergetics model to determine the prey consumption of gannets in Port Phillip Bay and potential competition with commercial fisheries (Chapter 6). Finally, the last chapter (General Discussion) draws together details from the preceding chapters and the possible use of seabirds as indicators of pelagic food conditions and in the management of marine resources is discussed. Although the primary focus of this thesis has been to document variability in reproductive and population parameters of gannets in relation to changes in prey availability and to assess the applicability of this information in developing effective fisheries management models, this knowledge is also essential for determining the potential impact of changes in commercial fisheries activity on Australasian gannets and other local marine predator populations CHAPTER 1 Age-related variations in the breeding success of Australasian gannets (Morus serrator) during contrasting years of prey availability

Abstract. The reproductive performance of Australasian gannets {Moms serrator) breeding at sites in Port Phillip Bay, Victoria, was investigated between 1997 and 2000, during years of contrasting prey availability. There was a large-scale mass mortality of pilchards {Sardinops sagax), a major prey item of gannets, in southern Australian waters in 1998, which is thought to have affected up to 70% of the local pilchard biomass. In the 1998-1999 breeding period, the reproductive performance of gannets declined considerably compared with the previous breeding period, with reduced hatching, fledging and breeding success and chick growth being recorded. In the subsequent breeding period (1999-2000), reproductive performance improved, but was still less than that recorded in 1997-1998. Whilst the average breeding performance of gannets was reduced in 1998-1999, that of individuals in different nest positions varied considerably, with younger peripherally nesting birds having a lower breeding success than older, centrally nesting birds. It is considered that this differential effect is due to age-related improvements in reproductive ability, including increased foraging efficiency and breeding experience, or effort of older breeders. Introduction

Australasian gannets {Moms serrator) breed at several localities in the cool temperate waters off southeastern Australia and also around New Zealand. In 1980-1981, the world population of about 53,000 breeding pairs included some 6,600 pairs nesting in Australian waters (Wodzicki et al., 1984), although local populations and those in New Zealand are known to have expanded considerably since 1980, if not before (Norman et al., 1998; Bunce et al., 2000). In southeastern Australia, gannets are a major local marine predator, feeding predominantly on pilchards (Sardinops sagax) and other inshore pelagic schooling fish and cephalopod species (Norman, 1992, Norman & Menkhorst, 1995; Bunce 2000a). In 1998, there was a large-scale mortality of pilehards in southem Australian waters, which is thought to have affected up to 70% of the local pilchard biomass (e.g. Gaughan et al., 2000). Deaths were first reported on 2 October near Spencer Gulf, South Australia; the front of dead pilchards then progressed east and west, often against prevailing currents, spanning almost the entire Australian range of the species. Masses of dead pilchards were found floating on the surface at sea, and washed up on beaches; in all cases gill lesions were found in affected fish and death appeared to have resulted from hypoxia (Murray, 1999; Gaughan et al., 2000). A similar mortality event in 1995, with an apparently identical epicentre, killed approximately 10 to 15% of pilchard stocks; that event also included New Zealand waters and both outbreaks are likely to have been caused by a novel (and exotic) herpesvirus (Fletcher et al., 1997; Griffin et al., 1997; Hyatt et al., 1997; Whittington et al., 1997). Before these large-scale pilchard mortalities in 1995 and 1998, there had been only one other localised mortality reported in Australian waters, at Hawley Beach in northem Tasmania (Copas, 1982).

It has been suggested that there were no significant changes observed in species (such as piscivorous fish, seabirds, or marine mammals) which take pilchards as food, in response to the 1995 pilchard mortality event (e.g. Hetcher et al, 1997; Whittington et al, 1997). However, indirect evidence indicates that there was an increase in mortality and breeding failure of little penguins {Eudyptula minor) at some Victorian sites following the 1995 mortality event [Dann, 2000 #937], and there were also reports of dead little penguins and Australasian gannets associated with pilchard mortalities in New Zealand (Smith et al., 1996; Taylor, 1997). Following the 1998 mortality event the proportion of pilchards in the diet of Australasian gannets declined considerably and remained low, compensated for by an increase in the amount of other prey species taken (Bunce & Norman, 2000; Bunce, 2000a).

This shift in the gannet diet, presumably due to an unavailability of pilchards following the mortality event, may have important implications for gannet breeding success and survival. Cairns (1987) predicted that a reduction in prey availability at a seabird colony would, depending on its severity, affect several distinct parameters of breeding performance, including adult activity budgets, chick growth, breeding success and adult mortality. Indeed, large-scale breeding failures and reductions in seabird populations have been associated with natural and fisheries-induced depletion of prey stocks worldwide (e.g. Crawford & Shelton, 1978; Fumess & Cooper, 1982; Duffy, 1989; Crawford & Jahncke, 1999). Furthermore, variability in the reproductive success of seabirds may also be influenced by other factors, including age-related improvements in reproductive ability, including increased foraging efficiency and breeding experience, or reproductive effort (e.g. Coulson, 1968; Blus & Keahey, 1978; Weimerskirch, 1990; Bradley et al., 1995). Indeed, Gibbs et al. (2000) reported that for Australasian gannets breeding in Port Phillip Bay, Victoria, a number of reproductive parameters, including breeding success and chick growth, improved with age of breeding adults and nesting position within the colony.

In this study, the breeding success of Australasian gannets in Port Phillip Bay, between 1997-2000 and during years of contrasting prey availability was investigated. Variations in the breeding success of gannets related to age and nest position were determined. In addition, the impact of the 1998 pilchard mortality event on this population and its potential differential effect on breeding performance in relation to breeder age and nest position is examined. Methods

Study site This study was conducted during the 1997-1998, 1998-1999 and 1999-2000 breeding periods at Pope's Eye Marine Reserve (36°16'42"S, 144°4r48"E), located off Queenscliff near the entrance to Port Phillip Bay, Victoria. This colony is the largest (194 pairs in 1999-2000) of eight breeding sites within Port Phillip Bay, all of which occur entirely on artificial structures. The history and development of this colony, and the local breeding biology and phenology of annual breeding events at this site, have been described previously (see Norman & Menkhorst, 1995; Norman et ah, 1998; Gibbs et al, 2000; Bunce et al., 2000). Briefly, the annual breeding period typically commences around August with pair-bond formation and nest construction, egg laying usually occurs by mid October, with most chicks hatching by late November and fledging generally takes place around February. There may, however, be considerable spread in the timing of breeding events, and relaying may occur following unsuccessful nesting attempts.

Data collection Visits to the colony, influenced by weather, were made at approximately 7 day intervals (ranging from 3 to 21 days) during each breeding period. The position of all nests present were mapped using photographs and sketches, and band numbers of previously marked birds (chicks in Port Phillip Bay have been banded since 1966, see Norman, in prep.) recorded. Nests were subsequently grouped (Fig. 1.1) according to position in the colony (1, peripheral; 2, intermediate; 3, central), reflecting both the distribution of known-aged (banded) adults and perceived nest quality (primarily related to degree of exposure), following Gibbs et al. (2000); although in this study nests were grouped into 3 categories (not 4 as used by Gibbs et al. (2000)) to better reflect the distinction between nest quality groups and for ease of comparison. On each visit to the colony the status of each nest (i.e. empty, or containing an egg or a chick) was recorded. Eggs were numbered using a permanent marker (and nest number recorded), and laying dates (estimated from date of first encounter and hatching © CM CO Q. a. Q. 3 0 0 0 m. » CD CD 0

m ©k© o 5^ © 00 ©^w © © ©^ ©; ©0

©I 00

o ©o ■ © ©, © l». GO © o o © © © a> 0 O^© © o o o o

Figure 1.1. Distribution of known-age breeding Australasian gannets (Moms serrator) gannets and nest groups at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, 1997-2000 (group 1, peripheral; group 2, intermediate; group 3, central). Number in circle is the age of known-age birds (average age if pair both known-age), '+' indicates if known-age is a minimum known-age. 10

date), egg mass (± 0.1 g, using an electronic balance) and egg volume index (length x width /2, ± 0.1 mm, using callipers) were determined (see Gibbs et al, 2000). Growth of chicks from selected nests (39 nests in 1997-1998 and 19 in 1998-1999) was monitored using chick body mass was recorded at intervals ranging from 3 to 21 days between hatching and fledging using Pesola spring balances (0-lkg ± 5 g and 0-5 kg ± 25 g).

Breeding success and chick growth The breeding performance of Australasian gannets breeding in Port Phillip Bay was determined by comparing the following parameters, hatching success (eggs hatched as proportion of eggs laid); fledging success (chicks fledged from eggs hatched); and breeding success (chicks fledged from eggs laid) between breeding periods and for different nesting positions using non-parametric analysis of variance (i.e. Kruskal- Wallis). Chick growth was also investigated, using body mass as an index. Gompertz curves were fitted to chick mass data using non-linear regression, following the methods of Ricklefs (1968) and Navarro (1991). The model fitted was:

M'= A * EXP(-EXP(K *(T - I)))

where M' is the predicted mass (g), A is the asymptote (g), K is the growth rate (day"'), I is the point of inflection (days), and T is chick age (days). The Gompertz model (rather than the logistic or von Bertalanffy models) was selected as it has previously been demonstrated to best approximate the average growth curve of Australasian gannet chicks (see Bunce, 2000c). To evaluate the growth performance of chicks, the following variables were considered: the parameters A, K, and I of the regression model, the time (days) taken to reach 99% of the asymptote (used as an indication of the chick rearing period) and chick mass (g) at 100 days of age (used as an indication of maximum chick mass pre-fledging). All variables were analysed for the effect of nest position and breeding period using a two-way analysis of variance(ANOVA). 11

Results

Breeding success The breeding performance of Australasian gannets in Port Phillip Bay showed considerable variation during each of the three breeding periods examined in this study (Table 1.1). Hatching success (55%) was significantly lower in the 1998-1999 breeding period compared with 1997-1998 (81%) and 1999-2000 (73%) breeding periods (H = 27.877, d.f. = 2, p < 0.05), although there was no significant difference in fledging success between breeding periods (H = 1.740, d.f. = 2, p > 0.05). Breeding success was also considerably lower in 1998-1999 (49%) compared with the 1997-1998 (74%) and 1999-2000(64%) breeding periods(H = 23.245, d.f. = 2, p < 0.05). The growth of chicks also varied between breeding periods. The asymptotic mass of chicks in the 1997-1998 breeding period was significantly (F = 9.205, d.f. = 1, p < 0.05) greater than in 1998-1999 (Fig. 1.2). However, there was no significant variation in growth rate (F = 0.269, d.f. = I, p > 0.05), or point of inflection (F = 3.458, d.f. = I, p > 0.05) between breeding periods. The time taken to reach 99% of asymptotic mass in 1997-1998 was significantly (F = 6.114, d.f. = 1, p < 0.05) less than that calculated for chicks in the 1998-1999 breeding period (Fig. 1.3). Similarly, chick mass at 100 days of age was significantly greater (F = 7.974, d.f. = I, p < 0.05) in 1997-1998 than in 1998- 1999 (Fig. 1.4).

The effect of nest position on breeding performance The distribution of known-age breeding adults varied significantly (F = 13.586, d.f. = 2, p < 0.05) with nest position (and assumed nest quality), i.e. birds nesting centrally within the colony (group 3) were older than intermediately nesting (group 2) and peripherally nesting (group 1) individuals (Fig. 1.5). Gannets nesting in groups 2 or 3 commenced breeding earlier than birds nesting in group 1; the median laying date of first-laid eggs for group 2 or 3 nests was 26 September, whereas for group 1 nests it was 9 October (F = 13.838, d.f. = 2, p < 0.05). Egg mass also varied between nesting positions and was significantly less(F = 6.521, d.f. = 2, p < 0.05) for group 1 nests 12

Table 1.1. The hatching (eggs hatched as proportion of eggs laid), fledging (chicks fledged from eggs hatched) and breeding success (chicks fledged from eggs laid) of Australasian gannets {Moms serrator) breeding at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, 1997-2000.

Hatching Fledging Breeding Breeding period success(%) success(%) success(%)

1997-1998 81 91 74

1998-1999 55 89 49

1999-2000 73 88 64

1997-2000 71 90 63 13

4000

3 3750 (0 V) (0 E

o a 3500 E > (0 ni 0) D) (0 L. 3250 <

3000 1997-1998 1998-1999 Breeding period

Figure 1.2. Average asymptotic mass of Australasian gannet {Moms serrator) chicks at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, during the 1997-1998 (n = 39) and 1998-1999 (n = 19) breeding periods. 14

125

Ui > ta

118 o o 4-> Q. E > 111 (0 (0 H- o

O) 104 O) £ u (0 (U 97

c 0)

ra *-> 90 o E I-

83 1997-1998 1998-1999

Breeding period

Figure 1.3. Time taken to reach 99% of asymptotic mass of Australasian gannet(Moms serrator) chicks at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, during the 1997-1998 (n = 39) and 1998-1999(n = 19) breeding periods. 15

4000

(D U> n:

>. (0 ■o 3750 o o

n S (t) (0 (0 E 3250 _o !E o

3000 1997-1998 1998-1999

Breeding period

Figure 1.4. Average mass of Australasian gannet {Morus serrator) chicks at 100 days at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, during the 1997-1998 (n = 39) and 1998-1999 (n = 19) breeding periods. 16

0) a

■o n O) c '■B 0) V

o u D) ra o cn m <

1 2

Nest position

Figure 1.5. Comparison of the age of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000, related to nesting position (group 1, peripheral; group 2, intermediate; group 3, central). 17

compared with group 2 or 3 nests (Fig. 1.6). However, there was no significant variation in egg volume index in relation to nest position (F = 0.495, d.f. = 2, p > 0.05). The breeding performance of adults also varied between nesting groups (Table 1.2). Hatching success of birds nesting on the edge of the colony and outlying rocks (group 1) was significantly lower than those nesting in intermediate (group 2) or central positions (group 3) within the colony (H = 38.529, d.f. = 2, p < 0.05). Fledging success was also considerably lower for group 1 nesting birds(H = 48.078, d.f. = 2, p < 0.05). Similarly, the breeding success of individuals nesting in central (79%) or intermediate (62%) positions within the colony was significantly greater (H = 75.625, d.f. = 2, p < 0.05) than for peripheral nests (38%). Chicks in peripheral nests also had lower growth parameters. The asymptotic mass of chicks from peripheral nests (group 1) was significantly (F = 12.670, d.f. = 2, p < 0.05) lower than that of chicks in intermediate (group 2) or central (group 3) nests (Fig. 1.7). However, the growth rate (F = 0.307, d.f. = 2, p > 0.05), point of inflection (F = 0.754, d.f. = 2, p > 0.05), or time taken to reach 99% of asymptotic mass did not vary significantly with nest position (F = 1.422, d.f. = 2, p > 0.05). Nevertheless, chick mass at 100 days of age increased significantly (F = 15.971, d.f. = 2, p < 0.05) with nest position, and was considerably greater for chicks from group 2 or 3 nests compared with chicks from group 1 nests (Fig. 1.8).

( I The relative impact of the 1998 pilchard mortality on the breeding performance of gannets In the 1998-1999 breeding period, whilst the average breeding performance of all gannets was reduced, the relative reduction in the breeding performance of individuals in different nest groups varied significantly. Group 1 birds, nesting peripherally, were greater affected than individuals nesting intermediately (group 2) or centrally (group 3) within the colony (Table 1.3). The hatching success of nests in group 1 was reduced by 43% in 1998-1999 (%^ = 4.1, d.f. 1, p < 0.05) compared with average hatching success in the previous breeding period (i.e. 1997-1998), whereas the hatching success of group 2 or 3 nesting birds in 1998-1999 was reduced by only 34% (%^ = 0.2, J./ = 1, p > 0.05) or J 27% (x = 0.6, d.f. = \,p> 0.05) respectively. The reduction in the fledging success of 18

110 r

105

3 (0 (0 n 100 E U) O) V 0 01 (0 V > <

1 2

Nest position

Figure 1.6. Variations in egg mass of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000, related to nesting position (group 1, peripheral; group 2, intermediate; group 3, central). 19

Table 1.2. The hatching (eggs hatched as proportion of eggs laid), fledging (chicks fledged from eggs hatched) and breeding success (chicks fledged from eggs laid) of Australasian gannets {Moms serrator) breeding at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, 1997-2000, related to nest position (group 1, peripheral; group 2, intermediate; group 3, central).

Hatching Fledging Breeding Nest position success(%) success(%) success(%)

Group I 56 67 38

Group 2 69 90 62

Group 3 82 97 79 20

4000 t-

s 3750 (0 (0 n E

o 4-f Q. 3500 E >. (0 (0 o D1 n 0) > 3000 <

2750 1 2

Nest position

Figure 1.7. Average asymptotic mass of Australasian gannet {Moms sermtor) chicks at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, during the 1997-1998 (n = 39) and 1998-1999 (n = 19) breeding periods, related to nesting position (group 1, peripheral; group 2, intermediate; group 3, central). 21

4000 |-

(1) O) 3750 - (0

(0 >. (0 ■o 3500 - o o

(0 3 3250 0) tn (0 £ _o !E 3000 o

2750 1 2

Nest position

Figure 1.8. Average mass of Australasian gannet (Morus sermtor) chicks at 100 days at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria, during the 1997-1998 (n = 39) and 1998-1999 (n = 19) breeding periods, related to nesting position (group 1, peripheral; group 2, intermediate; group 3, central). 22

Table 1.3. The relative decline in the hatching (eggs hatched as proportion of eggs laid), fledging (chicks fledged from eggs hatched) and breeding success (chicks fledged from eggs laid) of Australasian gannets {Morus serrator) breeding in Port Phillip Bay, Victoria, in 1998-1999 compared with the previous breeding period (1997-1998).

% change in % change in % change in Nest position hatching success fledging success breeding success

Group 1 43 9 48

Group 2 34 2 33

Group 3 27 4 27

All groups 32 3 33 23

nests in group 1 (9%) in 1998-1999 compared with average fledging success for the previous breeding period, was also considerably greater (x^ = 13, J./ = 1, p < 0.05) than it was for group 2(x^ = 0.2, d.f = 1, p > 0.05) or group 3 (x^ = 0.1, J./. = 1, p > 0.05) nests. Similarly, the relative reduction in the breeding success of group 1 nesting birds (48%)in 1998-1999 was significant (x^ = 6.4, d.f. = \,p< 0.05), but not for birds nesting in group 2 (x^ = 0.03, d.f. = I, p> 0.05) or group 3 (x^ = 1.3, J./ = 1, p > 0.05), which declined by only 33% and 27% respectively.

Discussion

In this study there was considerable variation in the breeding performance of gannets between breeding periods, with hatching, fledging and breeding success and chick growth of Australasian gannets breeding in Port Phillip Bay declining considerably during 1998-1999 compared with the previous breeding period. Although, the breeding performance of gannets improved in the subsequent breeding period (1999-2000) it was still less than that recorded in 1997-1998. The parameters of breeding performance reported here, whilst generally slightly better than those previously recorded at this site (Norman & Menkhorst, 1995; Gibbs et al., 2000) resemble that reported for other gannet populations elsewhere (Jarvis, 1970; Nelson, 1978; Wingham, 1984). This reduction in the breeding success and reproductive performance of gannets in Port Phillip Bay in 1998-1999 was presumably related to the large-scale mass mortality of pilchards in southem Australian waters at that time (Bunco & Norman, 2000; Gaughan et al., 2000). Pilchard deaths were first recorded in Victorian waters on 6 November 1998 at Discovery Bay, just east of the South Australia border. During the next five weeks deaths were recorded along the Victorian coastline and had reached eastern Victoria by 8 December 1998 (Bunco & Norman, 2000). This mortality event is thought to have affected up to 70% of the local pilchard biomass in some regions (Gaughan et al., 2000), and followed a similar mortality which occurred throughout southem Australian waters in 1995 (e.g. Fletcher et al, 1997). An apparent unavailability of pilchards following the mortality events is reflected in changes in the 24

gannet diet (Bunce & Norman, 2000). Traditionally, gannets breeding in Port Phillip Bay fed predominantly on pilchards, which comprised approximately 50% of the diet in gannets before these mortality events (Norman & Menkhorst, 1995). However, following these pilchard mortalities the proportion of pilchard in the gannet diet declined considerably, to less than 5% and has remained low (Bunce & Norman, 2000; Bunce, 2000a). In addition, the commercial catch of pilchards in Victorian waters also declined following the pilchard mortalities. Before these mortality events the pilchard fishery was considered to be Victoria's largest inshore commercial fishery, in terms of catch by weight, with a peak annual catch between 1992 and 1994 of over 2000 tonnes. Since the pilchard mortalities in 1995 and again in 1998 the commercial pilchard catch in Victorian waters has declined considerably, with less than 250 tonnes taken in 1998-1999 (Neira et al, 1999; P. Coutin, pers. comm.). Interestingly, this apparent extreme reduction in the availability of pilchards following these mortality events may be somewhat analogous to massive, rapid overfishing and the collapse of fish stocks, providing an indication of the impact of such events on seabirds. A reduction in reproductive performance of seabirds associated with declines in food availability has previously been indicated in a number of studies (e.g. Anderson et al., 1982; Uttley et al., 1994; Phillips et al., 1996), and ultimately may act as a density- dependent factor regulating population sizes (Lack, 1954a; Ashmole, 1963). Cairns (1987) predicted that a reduction in prey availability at a seabird colony would, depending on its severity, affect several distinct parameters of breeding performance. Changes in adult activity budgets and colony attendance, including the numbers of birds attempting to breed (considered the most sensitive to changes in food availability) would be apparent with only a slight reduction in food supply (Uttley et al., 1994; Phillips et al., 1996; Olsson, 1997). Under more restrictive conditions, chick growth rates may be reduced and breeding success affected (Ricklefs et al., 1984), similar to that observed in this study. An increase in adult mortality would only be anticipated when food was extremely scarce [Piatt, 1997 #939; Dann, 2000 #937]. Whilst there are reports of increased morality of little penguins and Australasian gannets following the 1995 pilchard mortality, there is little evidence of seabird morf^ities following the 1998 mortality event (Smith et al., 1996; Taylor, 1997; Dann et al., 2000). 25

It was thought that the impact of the 1998 pilchard mortality on gannets was minimised by foraging flexibility in gannets as evidenced both in size of prey consumed and range of species taken (Bunce & Norman, 2000). To a large extent, following the 1998 mortality event, pilchards were replaced in the gannet diet by barracouta (Thyrsites atun), garfish {Hyporhamphus melanochir) and anchovy {Engraulis australis) and a variety of other inshore pelagic schooling fish species (Bunce & Norman, 2000; Bunce, 2000a). Such a dietary change may have important implications (e.g. for breeding success) given differing nutritional qualities of prey species. Pilchards are a high-energy food source, of greater calorific value than other major prey species in the gannet diet (Bunce, 2000a), and have commonly been suggested as 'preferred' prey (Batchelor & Ross, 1984; Kirkham et ai, 1985; Berrati et al., 1993). The consequences of a low quality diet, following decreased availability of pilchards, may be that greater foraging effort and food consumption is required, which may ultimately affect the reproductive success and survival of gannets (Bunce, 2000a). For example, Batchelor and Ross (1982) reported that Cape gannet {Moms capensis) chicks raised on a diet of pilchard attained greater fledgling masses (up to 25% heavier) and had a greater chance of survival than chicks raised on a lower quality diet of stockfish {Merluccius capensis). Hence, when preferred prey is limited, older or more experienced breeders, with improved foraging efficiency and prey capture ability, may be less affected than younger or less experienced birds. The reproductive performance of Australasian gannets breeding in Port Phillip Bay improved with nest position or age of breeding adults (see also Gibbs et al., 2000). Older birds tended to nest more centrally within the colony, while younger individuals generally bred in intermediate areas and on outlying rocks, exposed to sea and weather conditions. Such stratification of age-classes according to nest location also occurs at other seabird colonies (e.g. Coulson, 1968; Ainley etal, 1983). Klages (1994), however, considered that peripheral nesting sites were 'not physically inferior' to central sites, since gannet colonies were established 'invariably at places that provide space for everyone'. In Port Phillip Bay, colonies have been established at sites with very limited nesting pace and little of prospect for expansion, and where peripheral nests are indeed inferior (being seriously affected by sea and weather conditions), consequently central nesting may 26

confer some enhanced breeding success (see Gibbs et ai, 2000; Norman, 2000). In this study, hatching success, fledging success, breeding success and chick growth of gannets breeding in peripheral nests was considerably lower compared with more centrally nesting, older birds. Other studies have reported similar age-related improvements in reproductive performance of Australasian gannets (Gibbs et al., 2000) and other seabirds (Coulson, 1968; Blus & Keahey, 1978; Weimerskirch, 1990; Bradley et al, 1995). Indeed, Coulson & Horobin (1976) showed that the breeding success of young Arctic terns (Sterna paradisaea) was less than half that of older breeding birds. It has often been suggested that an increase in age-related reproductive performance is related to factors associated with breeder quality, including enhanced breeding experience, nest-site quality and increased foraging efficiency, or by increased reproductive effort of older individuals (Coulson, 1968; Blus & Keahey, 1978; Curio, 1983; Weimerskirch, 1990; Bradley et al, 1995). Williams (1966) predicted that the amount of reproductive effort allocated to current breeding events should increase as the likelihood of survival to reproduce in future breeding events declines (i.e. with age). For example, Pugesek (1987) reported that increasing reproductive effort with age in Califomia gulls {Larus califomicus) was associated with reduced survival, with older gulls tending to fledge more offspring than younger individuals but having lower survival rates. This variation in age-related reproductive performance is also reflected by the differential effects of the 1998 pilchard mortality on Australasian gannets breeding in Port Phillip Bay. Whilst reproductive performance of gannets was reduced in the 1998- 1999 breeding period, younger breeding birds occupying peripheral nesting sites were affected to a much greater extent than older birds nesting in more central positions within the colony. Breeding success of these peripherally (i.e. younger) nesting birds declined by approximately 50%, whereas the breeding success of intermediate and centrally (i.e. older) nesting birds declined by only 33% and 27% respectively. Hence it appears that older, more centrally nesting, gannets were able to 'buffer' the effects of the 1998 pilchard mortality on reproductive performance to a much greater extent than younger, less experienced birds nesting on the periphery of the colony, presumably by increased reproductive ability or effort. Older birds are presumably better able to adjust foraging 27 effort to maintain constant levels of parental care and hence may act to decouple the effect of reduced food supply from reproductive or chick growth parameters. CHAPTER 2 28

Effects of supplementary feeding and artincial twinning on nestling growth and survival in Australasian gannets (Morus serrator)

Abstract. The effect of supplementary feeding and artificial twinning on nestling growth and survival in Australasian gannets {Moms serrator) breeding at Pope's Eye Marine Reserve in Port Phillip Bay, Victoria was investigated in 1997-1998 and 1998-1999. Australasian gannets were capable of raising additional young; however, twin young were generally of poorer quality, had slower development, attaining lower maximum mass, and had lower survival rates. Chicks given supplementary food attained maximum weights greater than both single and twin chicks. The reduction in breeding performance of gannets provided with additional offspring, when coupled with the greater fledgling masses of supplementary-fed chicks, suggests that parents are unable to provide sufficient food for two young, indicating that this population is potentially limited by food availability. That clutch size in this species is limited to a single egg is supported by their apparent inability to successfully incubate two eggs. Recent increases in the size of the Australasian gannet population in Victoria indicate that other factors, such as natural or anthropogenic perturbations in environmental conditions may also be important in mediating population changes. 29

Introduction

Seabird populations are generally considered to be moderately stable, a consequence of a set of shared life history traits that include delayed maturity, low reproductive rates, slow development of young and high adult survival. Such traits reflect adaptations to feeding on prey with sparse, patchy and unpredictable distributions (e.g. Lack, 1954b; Croxall, 1987; Furaess & Monaghan, 1987; Ricklefs, 1990). Large- scale mortality of adult seabirds is rare (Cairns, 1987) and is generally associated with reductions in food availability due to stochastic climatic or other events (Schrieber & Schrieber, 1984; LaCock, 1986; Norman et ah, 1992; Piatt & Vanpelt, 1997; Stenhouse & Montevecchi, 1999), or (less frequently) through human influence (Warham & Serventy, 1978; Barrett, 1979; Piatt et ah, 1990; Gales etal., 1998). Food availability is thought to act as a direct, density-dependent factor regulating seabird numbers, with reductions in food availability resulting in lower breeding success and higher adult mortality (Lack, 1954a; Ashmole, 1963; Cairns, 1987, 1992a). However, direct measurements of food availability at sea are often difficult to obtain, as the food resource can not be sampled in a reliable and representative way (Ricklefs et al., 1984; Montevecchi & Myers, 1995; Monaghan, 1996; Croxall et al, 1999). The effects of the experimental manipulation in the amount of food supplied to chicks, nevertheless, has been used provide an indirect measure of food availability (Harris, 1978; Navarro, 1991; Ward & Kennedy, 1996; Cook & Hamer, 1997). Similarly, twinning experiments, in which adults are given additional eggs or offspring to rear, have been used to provide information on optimal brood size and food limitation in seabirds (e.g. Nelson, 1964; Norman & Gottsch, 1968; Jarvis, 1974; Waghom, 1982; Wehle, 1983). For example. Nelson (1964) found that Northem gannets {Moms bassana) could successfully rear two young, whereas Jarvis (1974) found that Cape gannets (M. capensis) could not, and has been suggested to reflect local differences in prey availability at least at that time (e.g. Waghom, 1982; Navarro, 1991). This paper examines the significance of food availability as a factor mediating reproductive rates, breeding performance and ultimately in regulating numbers of Australasian gannets {M. serrator) breeding in Port Phillip Bay, Victoria. Artificial 30

twinning and the supplementary feeding of young was used to investigate the effects alterations in the level of food demand at the nest have on nestling growth and survival in Australasian gannets, providing an indirect indication of food availability.

Methods

Study site The study was conducted during the 1997-1998 and 1998-1999 breeding periods at Pope's Eye Marine Reserve (38°16'42"S, 144°4r48"E) and adjacent Wedge Light (38°16'32"S, 144°42'05"E), located off Queenscliff near the entrance to Port Phillip Bay, Victoria. Previous studies (see Norman & Menkhorst, 1995; Gibbs et al, 2000) have established the local breeding biology and phenology of annual events at these sites.

Artificial twinning Artificial 'twins' were created by placing either an additional egg or a newly- hatched chick taken from elsewhere in the colony in to selected nests (twin nests). Eggs or chicks of similar age and/or weight were used to minimize the potential effects of hatching asynchrony and/or sibling rivalry. Eggs were marked for identification and were checked regularly throughout incubation. Chicks were colour marked when newly hatched and then later colour banded and identified as either a (heavier) or [3 chick according to initial mass rank. This allowed the fate of each egg or chick to be accurately determined. In 1997-1998 artificial twins were established by providing adults at nine nests with an additional freshly-laid egg and a further four nests with an additional chick. This was repeated in 1998-1999, with four nests receiving an additional egg and another

four an additional chick.

Supplementary feeding Chicks from selected nests (four nests in 1997-1998 and 1998-1999) were hand- fed supplementary food consisting of freshly-thawed pilchards (Sardinops sagax), one of the main prey items in the Australasian gannet diet (Norman & Menkhorst, 1995), at 31 regular intervals ranging from 2-8 days. The amount of food fed was ca 5% of chick mass; small pieces of fish were fed to chicks from 5 to 40 days of age, and whole fish thereafter. Body mass of chicks was recorded, using Pesola spring balances (0-1 kg and 0-5 kg), at regular intervals of 3 to 21 days from hatching to fledging in both breeding periods.

Chick growth Sigmoid curves were fitted to growth data using non-linear regression, following Ricklefs (1968) and Navarro (1991). The models fitted were:

Logistic: M'= A /(1 + EXP(-K*(T - I))), Gompertz: M'= A * EXP(-EXP(K*(T-I))), and von Bertalanffy; M'= A * (1 - EXP(-K*(T-I)))^ where M' is the predicted mass (g), A is the asymptote (g), K is the growth rate (day"'), I is the point of inflection (days), and T is the time or chick age (days). The amount of variation explained (R^) by each model was used to select the best model (Zach et al. 1984). To evaluate growth performance of chicks, the following variables were considered: the parameters A, K and I of the best fit model, and the time taken to reach 95% of asymptote (used as an indication of the chick rearing period). All variables were analysed for the effect of breeding period (year) and treatment type (single, twin, and food addition) using a two-way analysis of variance (ANOVA). The average mass of twin chicks for each twin nest was used in the comparison between treatment groups, to reduce the influence of the large variation in growth rates between a and [3 chicks. Hatching and fledging success, and growth rates of chicks recorded from unmanipulated nests were used as 'controls' and sample sizes were kept small to minimize the level of disturbance caused to the colony. 32

Results

Breeding success Hatching success in experimental nests given an additional egg was substantially reduced (Table 2.1). In 1997-1998, of nine nests provided with two eggs, only two pairs successfully incubated and hatched both eggs (although only one of these chicks survived past the first few days post-hatching), adults at four nests hatched only one egg and the remaining three pairs failed to incubate and hatch any eggs. Similarly, in 1998-1999, of four nests provided with two eggs, only adults at one nest successfully hatched an egg, the remaining three pairs failed to incubate and hatch any eggs. Hatching success of twin eggs (35%) for the two breeding seasons combined was significantly less (x^ = 17.6, d.f. = 1, p < 0.05) than the average hatching success(70%) of nests containing a single egg. Survival of twin chicks was also lower than that of both single and supplementary-fed chicks (Table 2.1). In 1997-1998, of four nests provided with two chicks, adults at three nests successfully fledged both young and the remaining pair successfully fledged one young. However, in 1998-1999, only adults at one nest successfully fledged both young, another pair successfully fledged one young, while the remaining two pairs failed to successfully fledge any young. Chick mortality was highest during the early post-hatching period, with death presumably resulting from starvation. Fledging success of twin chicks (63%)for the two breeding seasons was significantly less (X^ = 8.6, c?./ = 1, p < 0.05) than the fledging success (90%) of single chicks. Twin nests fledged 1.3 chicks per nest during the two breeding seasons, which was not significantly different from the 0.9 chicks which were fledged in control nests {t = -1.579, d.f. = 26, p > 0.05), although possibly a constraint of small sample sizes. Chick mortality for supplementary-fed chicks (1.0 chicks fledged per nests) was similar to that reported for the rest of the colony (t = 0.01, i/./ = 1, p > 0.05).

Chick growth and parameters ofthe growth model The average mass (± s.d.) of chicks from unmanipulated nests at hatching was 70.4 ± 12 g (n = 39), about 72% of the fresh egg mass (98.6 ± 7.7 g, n = 154). Chick growth was slow during the first days post hatching, and then subsequently increased 33

Table 2.1. Summary of hatching and fledging success of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, 1997-1999, for each treatment group and for each breeding period. Twin egg indicates nests which were provided with two eggs (n = 13); similarly twin chick is a nest provided with two chicks (n = 8); food addition indicates nests where chicks were given supplementary food (n = 8); and unmanipulated nests are 'controls'(n = 8).

Breeding period Treatment group Hatching success(%) Fledging success(%)

1997-1998 Twin egg 44 -

Twin chick - 88

Food addition - ICQ

Control 81 91

1998-1999 Twin egg 13 -

Twin chick - 38

Food addition - 100

Control 55 89

1997-1999 Twin egg 35 -

Twin chick - 63

Food addition - 100

Control 70 90 34

quite rapidly until 60 days of age, after which growth slowed. The rate of mass gain was highest in chicks between 15 and 45 days of age, and reached a maximum of approximately 80 g day"' at 30 days. The mean maximum mass attained (3650 g) was at approximately 80 days of age (and approximately 130% of adult body mass) after which the weight of chicks declined prior to fledging (Wingham, 1984; Gibbs et al, 2000). In this study, all three models closely approximated the average growth curve of Australasian gannet chicks, with very little difference between each model (Gompertz model, = 94.1%; logistic model, = 93.8%; von Bertalanffy model, R^ = 94.0%). However, while the Gompertz model both under- and over-estimated chick mass at different stages of development, it did so to a lesser extent than the logistic or von Bertalanffy models, explaining slightly more of the variation, and providing the least- biased description of body mass growth for Australasian gannet chicks (Fig. 2.1). The growth of chicks in twinned nests varied considerably. Generally, growth of a chicks was similar to that or slightly less than that of single chicks from unmanipulated nests, although growth of the second (P) chick was considerably slower (Fig. 2.2). The average asymptotic mass of a chicks was considerably greater than that of P chicks (t = 3.568, d.f. = 3, p < 0.05), but there was no difference in growth rate (t = 1.303, d.f. = 3, p > 0.05), inflection time (t =1.462, d.f. = 3, p > 0.05) or time taken to reach 95% of asymptotic weight (t = 1.100, d.f. = 3, p > 0.05). Chick growth also differed between treatment groups and between breeding seasons. The asymptotic mass for supplementary-fed chicks was significantly greater than that of both single and twin chicks in 1997-1998 (F = 29.621, d.f. = 2, p < 0.05), with twin chicks also attaining much lower asymptotic mass than singles (Fig. 2.3). There was no significant difference in inflection time (F = 0.088, d.f. = 2, p > 0.05) or growth rate (F = 0.510, d.f. = 2, p > 0.05) between treatment groups. The time taken to reach 95% of asymptotic mass in 1997-1998 was longer for chicks from twin nests compared with both supplementary-fed chicks or single chicks from unmanipulated nests, although this difference was not significant (F = 0.112, d.f. = 2, p > 0.05). Similarly, in 1998-1999 supplementary-fed chicks attained greater asymptotic mass than both single and twin chicks, though these differences were not significant (F = 1.415, d.f. = 2, p > 0.05). Inflection time (F = 1.974, d.f. = 2, p > 0.05) and growth rate (F = 0.542, d.f. 2, p 35

4000 r-

3000

S / (0 V) to E > 2000 ■o o CQ //

— — logistic

1000 Gompert

von Bertalanffy

40 80 120 Age (days)

Figure 2.1. Comparison of the three growth models, i.e. logistic, Gompertz and von Bertalanffy, fitted to the average growth curve of Australasian gannet (Morus serrator) chicks in Port Phillip Bay, Victoria, 1997-1999, following Ricklefs (1968) and Navarro (1991). 36

4000

3000 / / / 3 Ui / v> m / E 2000 >. / y T3 O m / r" / / / / single chick

1000 // a chick / P chick

y y

30 60 90 120

Age (days)

Figure 2.2. Comparison of the growth of twin chicks(a chick and P chick) and single Australasian gannet(Morus serrator) chicks during the breeding periods of 1997-1998 and 1998-1999 in Port Phillip Bay, Victoria. 37

4250

(0 (0 (0 E .2 3500

a. E >« (0 (0 u D) 2 2750 Q> > <

2000 single food twin addition Treatment group

Figure 2.3. Average asymptotic mass of Australasian gannet {Moms serrator) chicks in Port Phillip Bay, Victoria, from each treatment group; single (n = 8), food addition (4) and twin (4), during the 1997-1998 and 1998-1999 breeding periods. 38

> 0.05) was also not significantly different between treatment groups. The time taken to reach 95% of asymptotic mass in 1998-1999 was again longer for chicks from twin nests compared with both supplementary-fed chicks and controls, but this difference was not significant(F = 0.112, d.f. = 2, p > 0.05).

Discussion

Breeding success and reproductive rates This study has shown that Australasian gannets breeding in Port Phillip Bay, Victoria are capable of raising additional offspring if provided with extra young. Such findings are consistent with previous studies on other gannets (Nelson, 1964; Jarvis, 1974; Robertson, in Nelson, 1978; Wanless, 1978; Waghom, 1982; Navarro, 1991). Nests provided with two chicks produced more young than those with single young or supplementary-fed young. However, there was considerable variation in the success of twinned nests between seasons. In 1997-1998, more young were produced in twin nests than in single nests, whereas in 1998-1999, twin nests produced fewer young per nest. Indeed, half of the nests given two young in 1998-1999 failed to successfully fledge any young. Given this variation in the ability of gannets to rear additional offspring in different seasons a clutch size of one may act as a 'cautionary' mechanism to ensure that there is not complete reproductive failure in a given breeding season (e.g. Lack, 1954a; Nelson, 1964; Jarvis, 1974). Clutch size in gannets may, however, be limited by an inability of adults to successfully incubate two eggs, a possible constraint of an increase in the amount of energy required for incubating two eggs, behavioural constraint or a physiological inability to effectively warm more than one egg (Smith, 1989; Monaghan & Nager, 1997; Thompson et al, 1998). Gannets and other Pelecaniformes, including boobies and pelicans, are unusual among birds in that they lack a vascularized brood patch and incubate their eggs under the webs of their totipalmate feet (Nelson, 1978; Evans, 1995). Hatching success in twin nests was substantially lower than for adults incubating a single egg. In addition, no pairs provided with two eggs successfully raised two young. Of the 39

two nests in which two eggs were successfully incubated, three of the four young appeared weak and died soon after hatching, suggesting that development of young may have been impaired. However, Nelson (1964) showed that northem gannets provided with an additional egg to incubate did equally as well as those incubating single eggs. In Nelson's (1964) twinning experiments, though, at least in some instances, the eggs used had been partially incubated prior to twinning. In addition, Evans (1995) found that the incubation temperature of Australasian gannets breeding in New Zlealand was reduced slightly (1.5°C) when provided with a second egg to incubate during the mid-incubation period. From this, he concluded that a reduction in incubation temperature may slow development, potentially prolonging the incubation period, however, he considered that it was unlikely that clutch size in Australasian gannets was limited by an inability to warm two eggs (Evans 1995). Although, Evans (1995) measured the incubation temperature of second eggs only over a single day immediately following placement in nests during the mid-incubation period. Hence, in both instances the constraints of incubating two eggs for the full incubation period has not been adequately addressed and consequently warrants further investigation.

Chick growth,food availability and population regulation In this study, chicks from twin nests attained lower asymptotic mass, took longer to reach maximum weight, and had lower survival rates than those of both supplementary-fed chicks and single chicks. In addition, within twin nests there was considerable variation in growth between chicks; a chicks attaining asymptotic mass similar to or slightly less than that of single chicks from unmanipulated nests and considerably greater than that of (3 chicks. Similar patterns of chick growth have been reported in twinning experiments on Australasian (Waghom, 1982) and Cape gannets (Jarvis, 1974), however, other studies have not (Nelson, 1964; Robertson, in Nelson, 1978; Wanless, 1978; Navarro, 1991). Chicks provided with supplementary food in this study attained greater asymptotic masses than those of both single and twin chicks, again similar patterns of chick growth of supplementary fed young have been reported in some studies (e.g. Harris, 1978; Hamer & Hill, 1994; Bukacinski et al, 1998), but not others (Navarro, 1991). These variations in the effect of artificial twinning and supplementary 40

feeding on nestling growth and survival reported between studies may be partly explained by differences in local fish assemblages and food availability (e.g. Waghom, 1982; Navarro, 1991). Alternatively, variations in parental quality, regulation of parental effort, behavioural or physiological constraints may also be important (Hamer et ai, 1994; Uttley et ai, 1994; Phillips et ai, 1996; Cook & Hamer, 1997) or potential methodological problems associated with studies as mentioned. Hence, caution should be applied when inferring levels of food availability from such studies since any possible effects are likely to be confounded by many potential factors. The pattern of nestling growth and survival of Australasian gannet chicks described in this study, i.e. lower asymptotic mass and greater mortality of twin chicks, and the greater asymptotic mass of supplementary-fed chicks suggests, that adults can not provide sufficient food for two young, which could potentially reflect, at least to some extent, limited prey availability (e.g. Harris, 1969; Jarvis, 1974; Ricklefs et al, 1984; Navarro, 1991). A reduction in the reproductive performance of seabirds associated with declines in food availability has been shown in many studies (e.g. Anderson et al, 1982; Uttley et al, 1994; Phillips et al, 1996), and ultimately may act as a direct density- dependent factor regulating population sizes (Lack, 1954b; Ashmole, 1963). Calms (1987) predicted that a reduction in prey availability at a seabird colony would, depending on its severity, affect several distinct parameters of breeding performance ranging from changes in adult activity budgets and colony attendance apparent only with a slight reduction in food supply (e.g. Uttley et al, 1994; Phillips et al, 1996; Olsson & Brodin, 1997) to reduced chick growth rates and breeding success (e.g. Ricklefs et al, 1984; Navarro, 1991) and an increase in adult mortality (e.g. Piatt & Vanpelt, 1997; Dann, et al 2000) under more restricting conditions. Unfortunately, there were no direct estimates of food availability during the study. There was, however, a large-scale mass mortality of pilchards in southern Australian waters in 1995 and again in 1998 (Fletcher et al, 1997; Jones et al, 1997; Bunce & Norman, 2000) which is likely to have affected local prey availability at that time. However, the impact of this mortality event on Australasian gannets was presumably minimised by flexibility in the gannet diet and an increase in the amount of other prey species taken (Bunce & Norman, 2000). 41

While food availability may be an important regulatory factor affecting breeding success and mortality, ultimately influencing population sizes of seabirds (e.g. Lack, 1954a; Ashmole, 1963), the growth of the Australasian gannet population in Port Phillip Bay does not appear to be limited. Over the past two decades (1980-2000) the Australasian gannet population in Victoria has increased substantially, both in terms of numbers of breeding gannets (with average annual increases in excess of 6%)and also in the number of sites used for breeding (Norman et al. 1998; Bunce et al., 2000). Hence, other factors such as availability of suitable nesting space (Ramos, et al. 1997; Blaber et al., 1998; Norman et al., 1998), anthropogenic influences (Warham & Serventy, 1978; Barrett, 1979; Gales et al., 1998) and changes in climatic conditions (Veit et al., 1996; Piatt & Vanpelt, 1997; Guinet et al., 1998; Montevecchi & Myers, 1997) are also likely to be important in mediating population changes (see Bunce et al., 2000). CHAPTER 3 42

Long-term trends in the Australasian gannet(Morus serrator) population in Australia: the effect of climate change and commercial fisheries

Abstract. The Australasian gannet {Moms serrator) population has expanded considerably this century, both in New Zealand and Australia. In 1980-1981, the world population of about 53,000 breeding pairs included 6,600 pairs nesting in Australian waters. Since 1980, the gannet population in Australia has increased threefold, to an estimated 20,000 breeding pairs, an increase of 6% per year, although the population may have been expanding somewhat earlier (i.e. before 1980). The reasons for the substantial increases in the Australasian gannet population are poorly understood, but here we consider the potential effects of recent fluctuations in climatic and oceanographic conditions, and changes in major local commercial fisheries. We note that there has been a significant trend towards more frequent, and stronger. El Nino Southern Oscillation events, and warmer summer sea surface temperatures. These climatic factors together with potential increased discarding of fisheries bycatch and changes in fisheries activities, may account for at least some of the observed increase in the local Australasian gannet population. 43

Introduction

Australasian gannets {Moms serrator) breed in the cool temperate coastal waters of southeastern Australia and also at several localities around New Ziealand. In Australian waters, colonies are located at Lawrence Rocks, Point Danger and Port Phillip Bay situated off the southern coast of Victoria and Black Pyramid, Pedra Branca and Eddystone Rock off the coast of Tasmania (Fig. 3.1). Locally, Australasian gannets are a major marine predator feeding predominantly on pilchards {Sardinops sagax) and other small inshore pelagic schooling fish and squid (Norman, 1992; Norman & Menkhorst, 1995; Bunce, 2000a; Bunce & Norman, 2000). In 1980-1981, the world population of about 53,000 breeding pairs included 6,600 pairs nesting in Australian waters (Wodzicki et ah, 1984). However, local populations, and those in New Zealand, have expanded since 1980, if not before (Wodzicki et al, 1984; Norman et al, 1998; Greene, 1999). Quite apart from more intrinsic regulatory factors (e.g. Lack, 1954b; Ashmole, 1963; Cairns, 1992b), seabird populations may also be influenced by oceanographic and climatic conditions. Indeed, local and widespread variation in oceanographic and other climatic conditions over interannual and longer time scales have been well documented (e.g. Quinn et al., 1978; Trenberth & Shea, 1987; Trenberth & Hoar, 1996) and have been shown to affect seabird populations (Duffy, 1989; Montevecchi & Myers, 1997; Boersma, 1998; Guinet et at., 1998; Guinard et at., 1999). For example, the Southern Oscillation has long been recognised as a major phenomena driving global-scale ocean climate variability and is closely associated with changes in atmospheric circulation, influencing the direction and strength of trade winds, upwelling activity and sea surface temperature (e.g. Quinn et al. 1992; Trenberth & Shea, 1987). Recent evidence suggests that synchronous worldwide fluctuations in small pelagic fish stocks, such as pilchard (sardine), which are important prey items for seabirds, including gannets, may be influenced by such global-scale, ocean-atmospheric conditions and have been termed 'small pelagic regimes'(Lluch-Belda et al, 1992; Lluch-Cota et al, 1997; Schwartzlose et al, 1999). Crawford and Jahncke (1999) demonstrated that seabird populations in different and widely separated systems were significantly correlated with prey abundance, and fluctuated according to shifts in these small pelagic regimes. Similarly, 44

Port Phillip Bay

Point Danger C3 Lawrence Rocks

G Black Pyramid•

Pedra Branca % Eddystone Rock

Figure 3.1. Distribution of Australasian gannet {Moms serrator) breeding colonies in

Australian waters. 45

Montevecchi and Myers (1997) demonstrated that wanner sea surface temperatures (SST) in the northwest Atlantic are strongly correlated with the expansion of the northern gannet {Morus bassana) population and increased availability of mackerel {Scomber scombrus) as a prey item. Fisheries-induced changes in the marine environment, either through direct or indirect effects of fisheries activities, may also influence seabird populations (e.g. Fumess & Cooper, 1982; Gales et al., 1998; Stenhouse & Montevecchi, 1999; Norman, 2000; Tasker et al., 2000). Competition between seabirds and fisheries for limited food resources has been associated with declines in some seabird populations (Fumess & Cooper, 1982; Wanless et al, 1998; Crawford & Jahncke, 1999). Conversely, the discarding of fisheries bycatch may also affect some seabird populations, artificially supporting large numbers of seabirds (Blaber et at., 1995; Garthe et al, 1996; Walter & Becker, 1997). In this paper we document the increase in the Australasian gannet population breeding in Australian waters, and summarise changes in both gannet numbers and use of breeding sites. These population changes are discussed in relation to changes in climatic and oceanographic conditions and the activity of selected major commercial fisheries operating in southeastern Australia. The potential effects of these factors on prey distribution and abundance and consequently on gannet numbers are examined.

Methods

Changes in Australasian gannet population Changes in the number of Australasian gannets breeding in Australian waters were determined from historical population counts as reported in the published literature, and supplemented with more recent direct counts (or observations) by the authors (and by H.M. Gibbs and A. Govanstone, pers. comm.). This information was then used to estimate the annual rate of population change at each breeding colony. The number of gannets at breeding colonies within Australian waters was first noted in 1873 (Nelson, 1978) with some anecdotal accounts prior to this (Grant, 1803; Bonwick, 1853). 46

Infrequent or occasional reports of gannet numbers at various breeding colonies are available between 1873 and 1980. Since 1980, gannet numbers at most Australian breeding sites have been surveyed more frequently. At Tasmanian colonies, the number of active nests present were determined from aerial photographs taken regularly on or about 1 November between 1980 and 1998 (approximately 10 breeding periods in total). In some contrast, direct counts (or observations) were made at Victorian gannetries during visits to breeding colonies between 1980 and 1999-2000; on each visit the number of active nests present were recorded. Undoubtedly there may be some problems associated with comparing numbers of gannets obtained from various sources and using different methods, consequently gannet numbers presented here, although generally thought to be reliable, should possibly be viewed with some caution. They do however, indicate the magnitude of changes in relative, if not absolute, terms.

Changes in climatic conditions Two large-scale variations in climatic and oceanographic conditions. Southern Oscillation and SST, are examined here and their potential impact on gannet numbers discussed. The Southern Oscillation Index (SOl) describes variability in the Southern Oscillation, a large-scale ocean-atmospheric phenomenon, and is derived from the standardised anomaly of the mean sea level pressure difference between Tahiti and

Darwin. El Nino refers to occasional anomalous warm SSTs, which results in abnormally heavy rainfall in the equatorial Pacific and extreme drought conditions in northern Australia, and is associated with negative or low SOl. The anti-El Nino (or La Nina) refers to the contrasting situation, represented by positive SOl, when a strengthening of the southeast trade winds prevail and there is anomalously low SST over the equatorial Pacific. Both the El Nino and Southern Oscillation form the complex global El Nino-Southern Oscillation (ENSO) phenomenon (e.g. Quinn et al. 1992; Trenberth & Shea, 1987). Monthly SOl for the period, January 1876-February 2000, were obtained from the Bureau of Meteorology (Australia) archives. The SOl was examined for possible recent anomalies by comparing mean monthly SOl from March 1977 to February 2000 (275 months) with mean monthly SOl for the period March 1876 to February 1977 (1215 months) using a two-tailed t-test, the time between independent 47 observations was taken as 6 months. Although this test is somewhat confounded by the persistence in the time series it provides a good general indication of the anomalous nature of recent fluctuations (see Trenberth & Hoar, 1996). The interval March 1977 to February 2000 was selected since there have been multiple ENSO events during this period and it also corresponds well with that of major changes in Australasian gannet numbers (see below). Monthly mean SST for Bass Strait (140°B 40°S to 150°E 40°S) from January 1854 to December 1997 were obtained from the National Centre for Atmospheric Research (NCAR) Comprehensive Oceanic-Atmospheric Data Set (COADS). The monthly data, derived from summaries of individual observations and provided at 2° resolution, were taken to reflect sea conditions for Bass Strait generally. To reduce seasonal and interannual variation in SST, summer (December-February) and decadal summer anomalies in SST were calculated for Bass Strait for each decade between 1860 and 1999. »

Changes in commercialfisheries While there are a number of commercial fisheries operating in southeastern Australia targeting major prey items of gannets, in this study we have examined recent changes in the activity of two commercial fisheries and discuss their potential impact on gannet numbers. The commercial pilchard fishery was established in 1935 and has traditionally been regarded as Victoria's largest inshore fishery in terms of landed catch (Neira et at., 1999). Temporal changes in this fishery were determined from annual total catches of pilchards in Port Phillip Bay from 1935 to 1998, and catch rates (catch per unit effort (CPUE) in kg day"') between 1978 and 1998, obtained from the Marine and

Freshwater Resources Institute in Victoria. The South East Fishery (SEF) was first established during the early 1900s. It is a complex multi-species fishery containing between 70-100 species of fmfish and invertebrates of commercial and is the largest commercial fishery in southeastern Australia, supplying approximately 60,000 tonnes year"' (Tilzey, 1994). Information on major changes in the SEF (from Smith 1991; Baulch & Pascoe, 1992; Tilzey, 1994; 48

Knuckey & Liggins, 1998) are discussed in relation to their potential impact on gannet populations.

Effect ofchanges in climatic conditions and commercialfisheries on gannet numbers The effect of these variables (i.e. SOI, SST, commercial fishery catches) on gannet numbers was analysed using multiple linear regression. Estimates of the relative importance of each variable were derived by determining the increase in the co-efficient of determination (r^) when each variable was added to a multiple regression model that included all other variables.

Results

Changes in Australasian gannet population Numbers of Australasian gannets breeding in Australian waters have increased considerably since earlier this century at all Australian colonies (Fig. 3.2), with the exception of Cat Island, which has now all but disappeared (Phillipps, 1993; N. Brothers, unpubl. data). The total breeding population of gannets in Australian waters has expanded from 6,600 pairs in 1980-1981 to approximately 20,000 breeding pairs in 1999- 2000; an increase of 6% per year. Expansion of gannet numbers at some breeding colonies has been relatively moderate due to limited nesting space (e.g. Pedra Branca), whilst at breeding sites where available nesting space has not apparently been limiting (e.g. Point Danger and Port Phillip Bay), population growth has been considerably greater.

Victorian breeding colonies

In 1873, the total Victorian gannet population consisted of 200 pairs breeding at Lawrence Rocks (Nelson, 1978), and presumably had not changed markedly since earlier that century (see Grant, 1803; Bonwick, 1853). The number of gannets breeding at Lawrence Rocks (38°25'S; 141°40'E) was first reported to be increasing in 1961 when the colony contained 631 pairs (McKean, 1966). Since then the population has continued to expand with 1,456 breeding pairs reported in 1978 (Harris & Norman, 1981), an increase 49

10000 Port Phillip Bay — □ ~ Lawrence Rocks Point Dancer d-d 1,000 4^ 0) _ □— o n D" c C (0 O)

s 100 A E 3 z

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

A Black Pvramid 10000 ® " Pedra Branca • Eddystone Rock

& 1000

O C c (0 O) ° 100 o n E 3 z

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Date (year)

Figure 3.2. Changes in numbers of Australasian gannets (Moms serrator) breeding in Australian waters, 1900 to 2000; (a) breeding colonies within Victorian waters, (b) Tasmanian breeding colonies. 50 of 2% per year between 1873 and 1978. By 1996-1997, the number of breeding gannets at Lawrence Rocks had increased to 6,200, an annual rate of increase of 5% per year since 1980-1981; however, all available nesting space at Lawrence Rocks is now fully occupied (Norman et al., 1998). As nesting space was fully occupied at Lawrence Rocks (about 1996), a new breeding colony was established on the nearby adjacent mainland at Point Danger. Australasian gannets were first observed roosting on Point Danger during 1995 and, following the erection of a fox-proof fence around the perimeter of the colony in 1996 (Pennington, 1996), gannets attempted to breed. In the 1997-1998 breeding period there were 350 active nests at Point Danger (Norman et al, 1998) and by 1999-2000, the number of gannets had increased to 660 pairs, an increase of approximately 24% per year since the colony was established. The number of Australasian gannets breeding in Port Phillip Bay (38°16"S; 144°41"E) has increased from just three pairs at Wedge Light in 1966 (Wheeler, 1968; Norman & Menkhorst, 1995) to 507 pairs of gannets in 1999-2000, breeding entirely on eight artificial structures, and representing an average annual increase of approximately

17% between 1966 and 1999-2000. Much of this increase has occurred since the mid 1980s, with six new breeding sites established within Port Phillip Bay since then, and an annual total population increase of approximately 24% between 1988 and 1999-2000.

Tasmanian breeding colonies The number of gannets recorded at breeding colonies in Tasmanian waters has also increased, with the exception of Cat Island. The fate of the Cat Island gannetry (39°57'S; 148°21'E) is well documented, declining from an estimated 5,000-10,000 gannets in 1908 to 1,000 birds in 1935; it has now all but disappeared, primarily as a result of predation and vandalism by humans and fire (e.g. Warham & Serventy, 1978; Phillipps, 1993 N. Brothers, unpubl. data). The Black Pyramid gannetry (43°5rS; 147°02'E) has grown steadily from 1961 when it contained an estimated 500 nests (Green & MacDonald, 1963) to 2,481 pairs in 1980, an annual increase of approximately 9%. The population has continued to expand and now (1998) contains some 12,300 breeding pairs, with much of this increase (12% 51 per year) occurring between 1989 (5,120 pairs) and 1997 (12,370 pairs). Similarly, at Eddystone Rock (43°51'S; 146°59'E), the number of gannets has increased from an estimated 20 adults in 1947 to 46 breeding gannets in 1978 (Brothers, 1979a). In 1998, there were 189 pairs of breeding gannets present; an increase of 11% per year since 1978. Available nesting space at Eddystone Rock is now fully occupied. The number of breeding pairs at Pedra Branca (43°5rS; 146°59'E) remained relatively stable between 1939 (1000 pairs) and 1978 when there were an estimated 500- 1000 breeding pairs (Brothers, 1979b). However, since then the colony has increased and in 1995 there were 3,317 pairs estimated to be breeding at Pedra Branca, an increase of 2% per year since 1939 and an increase of 7% per year since 1978. Since 1995, suitable nesting space at Pedra Branca has been limited and new birds attempting to breed are restricted to nesting on lower ledges and other unsuitable areas affected by sea and weather conditions.

Changes in climatic conditions Recent fluctuations (March 1977-February 2000) in mean monthly SOI have differed from the long-term average (March 1876-February 1977), with a significant trend towards negative SOI, or ENSO conditions, in recent years {t = 3.56, d.f. = 246, p < 0.05). This negative trend in SOI is consistent with an increase in both the frequency and strength of ENSO events over the past two decades (Fig. 3.3a). The SST data for Bass Strait from 1860-1999 reveal that locally, there has been a gradual long-term warming trend of summer SSTs in Bass Strait, with a significant decadal increase from 1950 onwards, increasing from 16.6°C earlier this century to 17.6°C more recently (Fig. 3.3b).

Changes in commercialfisheries The commercial pilchard fishery in Victorian waters has undergone major changes in terms of both fishing methods and total catch since its establishment (Neira et al, 1999). Annual pilchard catches have increased substantially from less than 10 tonnes prior to 1949, and from 220 tonnes in 1980, peaking at over 2,000 tonnes between 1992- 1993 and 1993-1994. However, despite these large increases in total catch of pilchards in 52

15 X 0) ■O C 10 -

c ■I 5H (0

u (li 0 — O C -5 0) SI g -10i 0) -15 o o o o o o o o o o o o o o to CO O) o CM CO in (O r^ CO O) o 00 CO CO CO O) O) O) O) o> o> CJ5 o> a> O) o T- CM

0 01 18.0

0) L. 3 (Q 17.5 L. V Q. E (D 17.0 I- 0) u CO 16.5 t: 3 w (0 o 16.0 (0 o o O o o o o o o o o o o o to h- CO O) o CM CO t in to to CO O) o CO CO CO CO o> a> a> o> O) o> a> c» a> o> o T- CM Date (decade)

Figure 3.3. (a) Mean monthly Southern Oscillation Index (SGI), March 1876 to February 2000; obtained from Bureau of Meteorology (Australia) archives (LOWESS filtered, see Trenberth and Hoar, 1996); (b) decadal summer Sea Surface Temperature (SST) anomalies for Bass Strait, 1860 to 1999 (from NCAR COADS). 53

Port Phillip Bay, CPUE during this period has also increased from 23 kg day"' in 1978 to approximately 280 kg day"' between 1992-1993 and 1993-1994 (Fig. 3.4). The SEF has also undergone considerable changes in terms of catch, fishing method and fishing grounds since its establishment earlier this century (Tilzey, 1994). The introduction of fishing vessels equipped with otter trawls in the 1970s and the expansion of the fishery into previously unfished waters in western Bass Strait and off Tasmania, and the corresponding substantial increase in catch (Fig. 3.5) are likely to have had a significant impact on the Australasian gannet population. Whilst the discarding of bycatch of both non-target quota and non-commercial species in the SEF is common, with up to 50% of the catch being discarded, little information exists on changes in bycatch discarding practices in this fishery (Knuckey & Liggins, 1998). However, since the introduction of Individual Transferable Quotas (ITQs) for the SEF in 1992, the unintended catch of quota species is likely to have lead to higher discarding rates (Baulch & Pascoe, 1992; I. Knuckey, pers. comm.). Unfortunately, there are no data currently available on the utilization of discards by gannets or any other seabirds in the SEF, although it is likely that large numbers of seabirds, including gannets, are supported by this fishery.

Ejfect ofchanges in climatic conditions and commercialfisheries on gannet numbers The relationship between gannet numbers and changes in the SOI, SST in Bass Strait, and annual commercial pilchard catches in Port Phillip Bay (changes in discarding of bycatch in the SEF were not included as accurate data are not available) was highly significant (F = 35.935, df. = 3, p < 0.05) and explained over 70% of the variation in gannet numbers (Table 3.1). Increases in the annual commercial catch of pilchards in Port Phillip Bay were highly correlated with the expansion of the gannet population in Australian waters, explaining 88% of the variation in the model. Changes in the SOI were negatively correlated with increases in the gannet population, i.e. gannet numbers increased in periods of low or negative SOI. Increases in SSTs in Bass Strait were also correlated with the expansion of the gannet population, explaining 21% of the variation in gannet numbers. 54

pilchard kill 2500

>. re ii CQ Q.

2000 350 Q. ■c o Annual pilchard catch Q. 300 C — Catch per unit effort (CPUE) "S 1500 re -250 £ >« o ■O 'qL O) -200 m W 1000 =) Q) DU C "150 c O o x: ■100

%re 500 o re ■50 3 C C < *

m o in o o CO in O) a> o O) o>

Date (year)

Figure 3.4. The annual catch (tonnes) of pilchards {Sardinops sagax) in Port Phillip Bay, Victoria, 1935-1999 (Neira et al, 1999) and catch rates (kg day"'), 1978-1994 (P. Coutin, pers. comm.). 55

>. L. 25000 V s: (0 ii. 4-> (0 n 20000 UJ £ 4-' 3 O (O 15000

(0 a) c c 2 10000

u +-> re u « 5000 O I-

1930 1940 1950 1960 1970 1980 1990

Date (decade)

Figure 3.5. Annual total catches in the South East Fishery from 1930-1990 (taken from Smith, 1991). 56

Table 3.1. The relationship between changes in the number of Australasian gannets {Moms serrator) at Australian breeding colonies and changes in the annual commercial catch of pilchards in Port Phillip Bay, Southern Oscillation Index, and Sea Surface Temperatures in Bass Strait. The relative contribution of each variable was assessed by the relative change in variation explained by the model when each variable was added (see methods).

Co-efficient of Relative contribution

Variable determination (r^) Probability (p) (% explained variation)

Pilchard catch 0.504 < 0.05* 88

Southern Oscillation Index 0.061 >0.05 1

Sea Surface Temperature 0.005 >0.05 21

Full model (r'^) 0.715 < 0.05* 72 57

Discussion

The Australasian gannet population has expanded considerably in recent years both in New Zealand (e.g. Wodzicki et al, 1984; Greene, 1999) and Australia (Marchant & Higgins, 1990; Norman et al., 1998). This study has shown that the gannet population in Australian waters continues to expand, now involving a breeding population of some 20,000 breeding pairs (Table 3.2) and an increasing use of new breeding sites. Elsewhere, and in the past, gannet populations have been seriously affected by human persecution and destruction (e.g. Nelson, 1978), but apart from Cat Island, there is little local evidence to suggest similar events occurred in Australian waters. Indeed, early records in Victoria (i.e. Lawrence Rocks) indicate that the gannet population has been low for some time (see Grant, 1803; Bonwick 1858), expanding only in recent years (McKean, 1966; Marchant & Higgins, 1990; Norman ef a/., 1998). At Lawrence Rocks, for example, numbers of breeding gannets were first reported to be increasing from 1952 (McKean, 1966). In Port Phillip Bay, gannets commenced breeding in 1966, with the local population increasing substantially since that time (Norman et al., 1998). Similarly, in Tasmanian waters, numbers of gannets at Black Pyramid began increasing from 1961 and at Eddystone Rock from 1947 (Green & MacDonald, 1963; Brothers, 1979b). However, much of the observed increase elsewhere has occurred since the early to mid- 1980s, with the total Australian gannet population expanding at approximately 6% per year between 1980-1981 and 1999-2000. Growth at some colonies has, however, been much greater during this period. In Port Phillip Bay and at Point Danger the breeding population has expanded at rates as high as 24% per year (see Fig. 3.1). As nesting space has become fully occupied at some colonies, new breeding sites have become established and these areas have been rapidly colonised, suggesting that there may be a large 'pool' of younger gannets waiting to breed (see also Norman & Menkhorst, 1995; Norman et al, 1998). This 'pool' may be quite large since numbers of gannets (almost all in adult plumage) in Port Phillip Bay, where they form a large part of the local avian biomass, often exceed the local breeding population (Norman, 1992). Norman et al. (1998) suggested that this 'pool' has increased substantially in recent years as a consequence of local productivity. 58

Table 3.2. Current status of populations of Australasian gannets {Morus serrator) at Australian breeding colonies. Size indicates number of breeding pairs and the average rate of increase for each colony is shown.

Colony Size (pairs) Season Comments References

Lawrence Rocks 3100 1996-97 Increase from 406 pairs in 1952 to 1474 adults McKean (1966); in 1980-81 (2%p.a.). Between 1980-81 and Norman etal.(1998) 1996-97 growth rate of 9% p.a.; all available nesting space now fully occupied.

Point Danger 660 1999-00 Established 1995-96(4 nests), increase of Norman etal. (1998); 24% p.a. between 1997-98 and 1999-00. This study

Port Phillip Bay 507 1999-00 Increase from 3 nests at Wedge Light in 1966 Norman et at.(1998); to 1014 adults at eight breeding sites in 1999-00 This study averaging 17% p.a., but 24% p.a. since 1988.

Black Pyramid 12339 1998-99 Increase from 500 nests in 1961 averaging Green & MacDonald (1963); 7% p.a,. but 9% since 1980-81. This study

Pedra Branca 3013 1998-99 Increase from 1000 nests in 1939 averaging Brothers (1979b); 2% p.a., but 4% p.a. since 1985. All available This study nesting space now fully occupied.

Eddystone Rock 189 1998-99 Increase from 20 adults in 1947 to 46 adults in Brothers (1979a); 1978(3% p.a.). Between 1978 and 1998-99 This study growth rate of 7% p.a., all available nesting space now fully occupied. 59

Observed rates of annual increase in gannet numbers at Australian breeding colonies are well above those reported for gannets elsewhere (Nelson, 1978; Barrett, 1979; Crawford et al., 1983; Waghorn, 1983; Wodzicki et al., 1984). Nelson (1978) considered that the annual rate of increase of northern gannet {Morus bassana) populations, calculated from census data, reproduction and mortality rates, to be 3% per year and suggested that population growth rates greater than this probably represent immigration of adult birds into the breeding population. However, it is unlikely that the substantial expansion of the local population in Australian waters reported here has resulted from immigration of breeding adults from other distant populations as relatively uniform population increases have been reported at all Australian colonies and numbers of gannets breeding in New Zealand have also been increasing for some time (Wodzicki et al., 1984; Greene, 1999; C.J. Robertson, pers. comm.). In addition, no New Zealand banded birds have ever been sighted at Australian breeding colonies whereat least for Victorian breeding sites, philopatry is high (see also Norman, 2000). Conversely, the observed increase in the Australasian gannet population in Australian waters in recent years may be generated by increases in the productivity of the local breeding population. Norman (2000) estimated annual adult mortality of gannets breeding in Port Phillip Bay, based on resight and recapture data between 1994 and 2000, as 6.4%, a rate similar to that reported for other gannet populations (Nelson, 1978; Waghom, 1983). Consequently, in a stable population the pre-breeding mortality of young would be approximately 77% (see Nelson, 1978). However, as the Australian population is currently expanding at a rate of 6% per year, pre-breeding mortality of young is probably considerably lower than this, suggesting improved local productivity or food availability.

Effects ofchanges in climatic conditions and commercialfisheries activities The reasons for the large and relatively recent expansion of the Australasian gannet population are poorly understood, but here we consider the potential effects of recent fluctuations in SOI and SST, and changes in major local commercial fisheries. Each of these variables have varied significantly over recent years and are correlated with changes in gannet numbers in Australian waters. 60

Although there is large intra- and interannual variation in the SOI, a sudden global 'shift' in climate mean state or regimes apparently occurred during the-mid to late 1970s (e.g. Graham, 1994; Beamish, 1995; Trenberth & Hoar, 1996). Indeed, recent (1977 onwards) fluctuations in the SOI, with a predominance of ENSO events over the past two decades, are highly anomalous and unexpected given the previous record, with a probability of occurrence of about once in every 2000 years (see Trenberth & Hoar, 1996). Given the major influence of ENSO events on ocean-atmospheric conditions, this shift in global climate may also be expected to have a significant influence on small pelagic fish stocks. Lluch-Cota et al. (1997) suggested that high sardine (i.e. pilchard) abundance was associated with periods of increased ENSO activity. In addition, Harris et al. (1988) reported that locally, increases in westerly winds over Tasmania and resultant upwellings of nutrient-rich cold subantarctic waters during ENSO events are positively correlated with increases in a number of commercially important fish stocks. However elsewhere ENSO events have also been associated with reductions in food resources, particularly in the eastern Pacific off Pern and Benguela Current in southern Africa, resulting in large-scale breeding failures and population changes of surface feeding seabirds (Schrieber & Schrieber, 1984; LaCock, 1986; Duffy, 1989; Boersma, 1998). Sustained or strong ENSO events may also be correlated with increases in SST (e.g. Quinn et a/.1992; Trenberth & Shea, 1987). Locally, there has been a gradual long- term warming trend of summer SSTs in Bass Strait and over southeastern Australia generally (see also Harris et at., 1992). SST anomalies are known to influence the movements of migratory and pelagic fish and consequently are potentially a major factor in determining the abundance and distribution of marine species, including seabirds (Lluch-Belda et at., 1989; Lluch-Cota et al., 1997; Montevecchi & Myers, 1997; Schwartzlose et al., 1999). For example, Lluch-Belda et al. (1992) reported that sustained warm water periods in the eastern Pacific were correlated with an extended distribution, greater abundance and increased spawning of sardine and globally, periods of high sardine abundance and increased fishery catches are associated with warm water periods (Lluch-Belda et al., 1989; Lluch-Cota et al., 1997; Schwartzlose et al., 1999). Similarly, pilchards are known to move into Port Phillip Bay during warm water periods when productivity is greater resulting in an increase in commercial catches during this 61

period (Neira et al., 1999). Indeed, annual landings in the commercial pilchard fishery in Victorian waters have increased substantially over recent years; however, these increases presumably reflect (at least in part) an increased demand for a low value fish for pet food, feed for tuna farms, commercial and recreational bait, and to a lesser extent, human consumption (Neira et al, 1999), but may also indicate increased availability (Bunce, 2000b). Consequently, it appears likely that warm water periods in Bass Strait (influenced by the frequency and strength of ENSO events) may affect both the distribution and movements, and abundance or local availability of pilchards (and presumably other prey items) and are positively correlated with the expansion of the gannet population in Australian waters. In 1995 and again in 1998, there was a large-scale mass mortality of pilchards in southem Australian waters, which is thought to have affected up to 10 to 15% and 70% of the local pilchard biomass, respectively (Fletcher et al, 1997; Gaughan et al, 2000). The impact of the 1998 mortality event (and presumably that in 1995) on Australasian gannets was reduced to some extent by flexibility in the gannet diet in response to the decreased availability of pilchards following the mortality (Bunce & Norman, 2000), although, the reproductive performance of gannets declined following this mortality (Bunce et al, 2000). Nevertheless, despite these pilchard mortality events the gannet populationdn Australian waters has continued to expand (see also Norman et al, 1998). In addition to climatic and oceanographic induced changes in local productivity and food availability there have also been substantial changes in the activities of major commercial fisheries in southeastern Australia over recent years, which are also likely to have affected food availability and consequently, impacted on gannet populations. In particular, the discarding of bycatch of both non-target quota and non-commercial species in the SEE, with up to 50% of the catch being discarded (Knuckey & Liggins, 1998), has potentially provided a valuable and substantial artificial increase in local food availability. Indeed, the utilization of discarded fishery bycatch by seabirds, including gannets, has been well documented and is known to artificially support large numbers of seabirds (Blaber et al, 1995; Garthe et al, 1996; Walter & Becker, 1997). However, little information exists on the utilization of the discarded bycatch in the SEE and therefore warrants further investigation. 62

Conclusions This study has shown that the population of Australasian gannets breeding in Australian waters has increased considerably since earlier this century in both number and in breeding sites occupied, with the observed rates of population increase almost double that previously recorded for other gannet populations. However, similar changes in other locally important seabird populations, such as little penguins {Eudyptula minor) have not been observed (P. Dann, pers. comm.). A significant trend towards more frequent, and stronger, ENSO events and warmer sea surface temperatures are likely to have resulted in increased local productivity and food availability and together with substantial discarding of bycatch in the SEE, presumably account for at least some of the observed increase in the local Australasian gannet population. As the population continues to expand, and local populations fully occupy all available nest sites, the annual rates of increase in the gannet population are likely to decline. Indeed, nesting space is already limited or fully occupied at a number of breeding sites, with expansion only likely to continue at Black Pyramid and Point Danger unless further new breeding sites are established. CHAPTER 4 63

Dietary changes of Australasian gannets(Morus serrator) reflect variability in pelagic Hsh stocks.

Abstract. Australasian gannets {Morus serrator) are a major local marine predator feeding predominantly on commercially exploited inshore pelagic schooling fish. The relationship between the prey taken by Australasian gannets breeding in Port Phillip Bay, Victoria, and commercial fisheries catches, between 1988 and 2000, and the availability of prey is examined. The relative proportion of pilchard {Sardinops sagax) in the gannet diet and annual commercial fishery landings was significantly correlated, presumably reflecting the importance of pilchard in the gannet diet and as a commercial fishery. There was, however, no significant correlation between other major prey items in the gannet diet, including anchovy (Engraulis australis), garfish {Hyporhamphus melanochir), barracouta {Thyrsites atun) and blue mackerel {Scomber australasicus) and commercial fisheries catches of these species. It is considered that the abundance of important prey items in the gannet diet, such as pilchards, and commercial landings are proportional to stock abundance. The implications of this relationship for fisheries management is examined. 64

Introduction

Increasing exploitation of pelagic fish populations worldwide has often resulted in overfishing and the collapse of a number of commercial fisheries and declines in many surface-feeding seabird populations, highlighting the competing demands for limited marine resources (Murphy, 1977; Crawford & Shelton, 1978; Nettleship et al., 1984). The lack of sustainability and uncertainty of traditional fisheries management results from a dependence on stock assessments, which are generally based on past catches and catch rates (e.g. Botsford et al., 1997). Whilst changes in commercial fisheries catches may reflect at least to some extent, changes in prey availability, they are also heavily influenced by environmental variability, technological improvements, market forces and political demands for increased harvests; they are also commonly plagued by wide-spread under-reporting of actual catches (Cairns, 1992a; Montevecchi, 1993; Botsford et al., 1997). Furthermore, the catch rates of densely-schooling pelagic fish are often not strongly correlated with stock abundance (Rice, 1992; Montevecchi, 1993). Fisheries- independent assessments of fish stocks, which include egg and larval production assessments and hydroacoustic surveys, may provide less biased indices of stock abundance, but again there may be problems associated with the validity and robustness of these methods (Cairns, 1992a; Montevecchi & Myers, 1995; Botsford et al., 1997). For example, even when it is possible to locate pelagic prey on surveys, the schools of many species occur in 'hydroacoustically invisible' surface waters and may also avoid ships (see Montevecchi & Myers, 1995). Seabirds are often highly visible, wide-ranging upper trophic level consumers that aggregate in areas of increased ocean productivity and may therefore be natural monitors of marine environmental conditions (Croxall, 1987; Fumess & Monaghan, 1987; Burger, 1988; Montevecchi, 1993). In addition, seabirds commonly feed on commercially exploited fish stocks (often taking prey of similar size) and consequently may be good indicators of commercially exploited pelagic fish stocks, providing reliable, simple and effective fisheries independent indications of pelagic conditions (e.g. Berruti & Colclough, 1987; Montevecchi & Berruti, 1991; Caims, 1992a; Montevecchi & Myers, 1996). 65

Australasian gannets {Moms serrator) breed in the cool temperate waters of

southeastern Australia and also at several localities around New Zealand. In southeastern Australia, gannets are a major local marine predator, feeding predominantly on inshore pelagic schooling fish and cephalopod species (Norman & Menkhorst, 1995; Bunce, 2000a; Bunce & Norman, 2000). Of the major prey items in the gannet diet pilchards (Sardinops sagax), anchovy {Engraulis australis), garfish {Hyporhamphus melanochir), barracouta (Thyrsites atun), and blue mackerel {Scomber australasicus), all are taken by commercial fisheries, and there is a high level of spatial and temporal overlap in the harvest of these species (and also often targeting similar size classes) by both gannets and the commercial fishery, suggesting that there may be considerable competition (Bunce, 2000a). In addition, the representation of major prey items in the gannet diet is known to fluctuate, presumably reflecting changes in the local availability of prey (Bunce & Norman, 2000). In this study, the relationship between the prey harvests of gannets and the availability of prey is examined by comparing changes in the relative proportions of major prey items in the gannet diet and the annual landings of commercial fisheries in this region. It is considered that the abundance of prey in the gannet diet and commercial fisheries landings are proportional to stock abundance and the implications of this relationship for fisheries management is examined.

Methods

Data on the relative proportions (contribution by mass) of these major prey items in the diet of gannets breeding in Port Phillip Bay between 1988-1989 and 1991-1992,

1994-1995, and 1997-1998 and 1999-2000 were taken from Norman & Menkhorst (1995), Gibbs (1995) and Bunce (2000a). Samples of foods taken by gannets were collected opportunistically as voluntary regurgitations from adults and chicks throughout the breeding period. Once collected, samples were placed in polyethelene bags and frozen until later identification and separation into individual taxa. The representation of 66

prey species in the diet was assessed as contribution by mass, frequency of occurrence and weighted relative occurrence (see Montague & Cullen, 1988). Annual commercial catches of these major prey items from Port Phillip Bay (pilchard, anchovy, garfish) and all Victorian waters (all species), during these periods were obtained from database held by the Catch and Effort Unit (Fisheries Victoria, Marine and Freshwater Resources Institute). The relative proportions of prey items in the gannet diet and annual commercial fishery landings were then compared using Spearman

rank-order correlations.

Results

The relative proportions of major prey items in the gannet diet varied considerably between 1988 and 2000 (Table 4.1). Before 1995, gannets fed predominantly on pilchard; however, following the spread of pilchard mortalities in southem Australian waters in 1995 and again in 1998 the proportion of pilchard in the gannet diet has declined considerably (Bunce & Norman, 2000). The relative proportion of other prey in the gannet diet has generally increased from 1995, presumably due to a decreased availability of pilchards (see Bunce, 2000a; Bunce & Norman, 2000). Annual commercial catches in the commercial pilchard fishery, both in Port Phillip Bay and all Victorian waters, have also declined substantially since 1995. However, commercial catches of other major prey items in the gannet diet have fluctuated between years (Table 4.2). There was a significant correlation between the proportion of pilchard in the gannet diet and annual commercial harvests for the period 1988-2000 (Fig. 4.1), both locally within Port Phillip Bay (rs = 0.981, p < 0.05, n = 8) and also for all Victorian waters (rs = 0.979, p < 0.05, n =8). However, there was no significant relationship between gannet and commercial fisheries harvests within Port Phillip Bay for anchovy (rs = -0.807, p > 0.05, n = 6 (Fig. 4.2a)), or garfish (rs = 0.595, p > 0.05, n = 6 (Fig. 4.3a)). Similarly, the portion of other prey items in the gannet diet was not strongly correlated with fisheries catches for all Victorian waters; anchovy (rs = -0.675, p > 0.05, n = 6 (Fig. 67

Table 4.1. Relative proportions (contribution by mass) of the major prey items taken by Australasian gannets(Moms serrator) breeding in Port Phillip Bay, Victoria, 1988-2000.

Relative proportion in the gannet diet(%)

Species 1988-1989 1989-1990 1990-1991 1991-1992 1994-1995 1997-1998 1998-1999 1999-2000

Pilchard 43 44 41 41 50 25 20 4

2 13 Anchovy 5 0 - 1 2 24

Garfish 3 1 3 2 - 1 0 30

Barracouta 31 4 5 18 - 49 51 13

Blue Mackerel 0 5 0 8 50" 8 6 20

may also include anchovy, garfish, barracouta and blue mackerel. Table 4.2. Annual commercial fishery catches (tonnes) in Port Phillip Bay (PPB) and for all Victorian waters(VIC) of the major prey items in items in the diet of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1988-2000 (obtained from the Catch and

Effort Unit, Marine and Freshwater Resources Institute, Fisheries Victoria).

Annual commercial fishery catches (tonnes)

1988-1989 1989-1990 1990-1991 1991-1992 1994-1995 1997-1998 1998-1999 1999-2000

Species PPB VIC PPB VIC PPB VIC PPB VIC PPB VIC PPB VIC PPB VIC PPB VIC

Pilchard 1443 2243 836 1555 1363 2319 1485 2441 1451 2535 453 791 251 277 30 33

Anchovy 33 61 16 34 44 44 45 45 86 457 182 326 n.a. 141 n.a. n.a.

Garfish 51 127 49 146 62 174 53 155 38 118 31 91 28 100 n.a. n.a.

Barracouta 0 3 0 3 0 4 0 1 0 11 0 31 0 20 n.a. n.a.

Blue mackerel 0 1 0 62 0 0 0 0 0 88 0 6 0 2 n.a. n.a.

0\ 00 69

100

.2 '"5 30 V c c nj O)

c o ■■E o Q. 10 O

I I I I t 10 30 100 300 1000 3000

Annual commercial fishery catch (tonnes)

Figure 4.1a. Relationship between the proportion of pilchard {Sardinops sagax) in the diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of pilchards in Port Phillip Bay, between 1988 and 2000. 70

100

• % .2 "■5 30

o c c (0 G)

c o ■■E o 10 Q. O Q.

10 100 1000 10000

Annual commercial fishery catch (tonnes)

Figure 4.1b. Relationship between the proportion of pilchard (Sardinops sagax) in the diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of pilchards for all Victorian waters, between 1988 and

2000. 71

100

30 0) c c (Q cn

c o '€ o Q. 10 O

30 100 300

Annual commercial fishery catch (tonnes)

Figure 4.2a. Relationship between the proportion of anchovy (Engraulis australis) in the diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of anchovy in Port Phillip Bay, between 1988 and 2000. 72

100

0) ■■5 - 30 0) c c ca u> c

c o '€ o a 10 o

ft 1 ft I 25 100 400

Annual commercial fishery catch (tonnes)

Figure 4.2b. Relationship between the proportion of anchovy {Engraulis australis) in the diet of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of anchovy for all Victorian waters, between 1988 and 2000. 73

100

0) ■■5 - 30

c _o +3 o a 10 o

25 40 80

Annual commercial fishery catch (tonnes)

Figure 4.3a. Relationship between the proportion of garfish {Hyporhamphus melanochir) in the diet of Australasian gannets {Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of garfish in Port Phillip Bay, between

1988 and 2000. 74

100

o :: 30 o c c ta cn

c o t o a 10 o

a I •__! I 80 100 125 160 200

Annual commercial fishery catch (tonnes)

Figure 4.3b. Relationship between the proportion of garfish {Hyporhamphus melanochir) in the diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of garfish for all Victorian waters,

between 1988 and 2000. 75

4.2b)), garfish (rs = 0.435, p > 0.05, n =5 (Fig. 4.3b)), barracouta (Fj = 0.563, p > 0.05, n = 6 (Fig. 4.4)), and blue mackerel (Fj = 0.131, p > 0.05, n = 6(Fig. 4.5)).

Discussion

The prey harvests of Australasian gannets breeding in Port Phillip Bay are strongly correlated with annual commercial pilchard fishery landings and are presumably proportional to actual prey abundance. The robustness of this relationship (at least for pilchards) is remarkable and supports the contention that dietary data from seabirds are useful in predicting fishery conditions in local meso-scale areas around breeding colonies, but perhaps also in larger macro-scale regions extending beyond seabird foraging zones around breeding colonies (see Berruti & Colclough, 1987; Montevecchi & Myers, 1995). Montevecchi and Myers (1995) also reported that dietary changes of northem gannets {Sula bassana), breeding off the coast of Newfoundland, were significantly correlated with commercial catches of pelagic fish and squid at multiple spatial and temporal scales, and suggested that seabird prey harvests could provide reliable indices of prey abundance. The strength of the relationship between the proportion of pilchard in the gannet diet and commercial catches for all Victorian waters (i.e. beyond gannet foraging ranges) may be partly influenced by the majority of the Victorian commercial catch being taken within Port Phillip Bay (Neira et al., 1999). Nevertheless, pilchards taken within Port Phillip Bay probably do not represent a separate stock, consequently changes in the gannet diet presumably reflects variation in stock size at that scale (Neira et al., 1999; P. Coutin, pers. comm.). The strong correlation between the relative proportion of pilchard in the gannet diet and commercial fisheries landings may be expected given the importance of pilchard as prey in the diet (Norman & Menkhorst, 1995; Bunce, 2000a). Australasian gannets breeding in Port Phillip Bay are a major local marine predator, known to consume large quantities of commercially exploited prey, including pilchards which may comprise up to 50% of the diet (Norman, 1992; Norman & Menkhorst, 1995; Bunce, 2000a). In addition, pilchards are an energy rieh food source generally of greater nutritional value 76

100

0) "•5 ^ 30 c c (0 O) _C (

c o ■■E o o 10

10 30 100

Annual commercial fishery catch (tonnes)

Figure 4.4. Relationship between the proportion of barracouta (Thyrsites atun) in the diet of Australasian gannets (Moms serrator) breeding in Port Phillip Bay, Victoria, and

the annual commercial harvests of barracouta for all Victorian waters, between 1988 and

2000. 77

100

■S 30 c c (0 O)

c o

o a 10 o

_L _L _L J 3 10 30 100

Annual commercial fishery catch (tonnes)

Figure 4.5. Relationship between the proportion of blue mackerel {Scomber australasicus) in the diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, and the annual commercial harvests of blue mackerel for all

Victorian waters, between 1988 and 2000. 78

than other prey items, and consequently are likely to be the preferred prey of gannets (Norman & Menkhorst, 1995; Bunce, 2000a), a view also supported in other studies (Batchelor & Ross, 1984; Berruti & Colclough, 1987; Klages et al., 1992; Adams & Klages, 1999). Given the apparent preference for pilchard, gannets may actively target or select pilchard (Adams & Klages, 1999), and therefore the relative proportion of pilchard in the gannet diet is less likely to be influenced by the availability of other prey (Berruti & Colclough, 1987). Similarly, the commercial pilchard fishery in Victorian waters is relatively unrestricted, actively targeting and exploiting large quantities of pilchards (in excess of 2,500 tonnes in 1994-1995) and with no limits on the total allowable catch (Neira et al., 1999). In addition, there is a high level of spatial and temporal overlap in the harvests of pilchards by both gannets and the Victorian commercial pilchard fishery (also often targeting similar size classes), suggesting that there may be considerable competition between gannets and the commercial fishery (Bunce, 2000a). Consequently, changes in both the harvests of pilchards by gannets and the commercial fishery are likely to be influenced by the availability of pilchard, and presumably reflect actual stock size

or abundance. In 1995 and again in 1998, there was a large-scale mass mortality of pilchards in southern Australian waters, which is thought to have affected up to 10 to 15% and 70% of the local pilchard biomass, respectively (Fletcher et al., 1997; Gaughan et al., 2000). This large decline in pilchard abundance was reflected by reductions in the harvests of pilchard by both gannets and the commercial fishery, indicating that they are reliable indicators of pelagic fish stocks (Norman et al, 1998; Bunce & Norman, 2000). For example, before the mortality events, pilchards comprised approximately 50% of the gannet diet, with the remainder consisting of a few species (Norman & Menkhorst, 1995). However, following the pilchard mortalities the proportion of pilchard in the gannet diet declined to less than 5% and has subsequently remained low (Bunce & Norman, 2000). Similarly, the annual commercial pilchard catch in Victorian waters peaked at roughly 2000 tonnes between 1992 and 1994; however, recent catches have been considerably lower with approximately 30 tonnes of pilchard landed eommercially in Victorian water in 1999-2000 (Neira et al., 1999; P. Coutin, pers. comm). Hence, it appears that 79 predator-derived prey abundance indices do reflect the magnitude and direction of changes in prey stocks in relative, if not absolute, terms (Montevecchi, 1993). The lack of a demonstrated correlation between gannet harvests and fisheries catches of other prey items in the gannet diet (i.e. anchovy, garfish, barracouta, and blue mackerel) may be expected as these prey are not as important in the gannet diet (Norman & Menkhorst, 1995; Bunce, 2000a). Consequently the relative proportion of each in the gannet diet is likely to be influenced by the availability of pilchard (Bunce, 2000a). In addition, the commercial fisheries for each of these prey are typically not as extensive as compared to the Victorian commercial pilchard fishery (Neira et al., 1999; P. Coutin, pers. comm). Whilst anchovy, garfish, barracouta and blue mackerel are taken by commercial fisheries operating in Victorian waters, they are generally not consistently targeted or taken in large quantities (see Table 2), hence, changes in the commercial catch, most likely reflects changes in market demand rather than changes in actual availability. In addition, recent increases (i.e. since 1995) in the commercial catch of these species presumably reflects (at least in part) a reduced availability of pilchards as an alternate exploitable fish stock, following the 1995 and 1998 pilchard mortalities. Consequently, although changes in the representation of these prey in the gannet diet may reflect to some extent changes in prey availability, the relationship between the prey harvests of gannets and that of commercial fisheries is hampered by fishery catches of these species not necessarily reflecting changes in the abundance of these fish stocks. However, it may also be important to note that the sample sizes used in these comparisons is constrained by a small data set, consequently the strength of these comparisons may be improved with the addition of subsequent data from future years. In particular, the relationship between the proportion of anchovy in the gannet diet and that taken by the commercial fishery for this species indicates a possible competitive

rt interaction that warrants future investigation. The relationship between the harvest of pilchard by gannets and the commercial fishery suggests that changes in the diet composition of gannets (i.e. the proportion of pilchard in the diet) may accurately reflect changes in the availability of prey, potentially providing a simple, reliable and effective estimator of stock size and an alternative to traditional fisheries independent estimates of stock size and indicator of changes in 80

pelagic conditions. Other studies have also reported the prey harvests of seabirds to reflect changes in pelagic fish stocks (e.g. Berruti & Colclough, 1987; Montevecchi et al., 1988; Montevecchi & Myers, 1995; Cherel & Weimerskirch, 1995). Montevecchi et al (1988) reported that extreme reductions of the dominant prey in the diet of Northern gannets {Moms bassana) were directly associated with subsequent local fisheries failures and suggested that deficits in the mean annual harvests of major prey items by gannets can be used to predict local commercial fishery collapses. The relationship between seabird dietary changes and annual commercial fishery catches can potentially provide a post hoc measure of the abundance of commercially exploited pelagic fish stocks at the end of a year or fishing season, that can be used to establish total allowable catches for the following season (Berruti & Colclough, 1987). The incorporation of such data into fisheries management models is likely to improve the validity and robustness of current commercial fisheries management practices and the effective management of marine ecosystems (e.g. Berruti & Colclough, 1987; Cairns, 1992a; Montevecchi & Myers, 1995). CHAPTER 5 81

Changes in the diet of the Australasian gannet {Moms serrator) in response to the 1998 mortality of pilchards (Sardinops sagax).

Abstract. The diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay was investigated before and after a mass mortality of pilchards {Sardinops sagax) in southern Australian waters in 1998. Gannets usually feed on inshore pelagic schooling fish, such as pilchards and barracouta {Thyrsites atun), and to a lesser extent on other species. Pilchards comprised approximately 50% of the gannet diet examined before the spread of pilchard deaths into Victorian waters, but this component declined to 5% following the mortality event. This reduction was compensated by a substantial increase in the amount of barracouta taken, supporting the view that the gannet is a flexible forager. However, the 1998 mortality of pilchards is likely to have wider implications since pilchards are an important prey for other piscivorous fish, seabirds and marine mammals. 82

Introduction

A large-scale mortality of pilchards (Sardinops sagax) occurred in southern Australian waters in 1998. Deaths were first reported on 2 October near Spencer Gulf, South Australia; the front of dead pilchards then progressed east and west, often against prevailing currents. Mortalities spanned almost the entire Australian range of the species, from near Rottnest Island in Westem Australia to Forster on the central coast of New South Wales. Masses of dead pilchards were found floating on the surface at sea, and washed up on beaches; in all cases gill lesions were found in affected fish and death appeared to have resulted from hypoxia (Anon., 1998; Murray, 1999). A similar mortality event, with an apparently identical epicentre, killed approximately 10-15% of pilchard stocks in 1995; that event also included New Zealand waters and both outbreaks are likely to have been caused by a novel (and exotic) Herpesvirus (Fletcher et ai, 1997; Griffin et al, 1997; Hyatt et ai, 1997; Whittington et ai, 1997). Prior to these large- scale pilchard mortalities in 1995 and 1998, there had been only one other localised mortality reported in Australian waters, at Hawley Beach in northern Tasmania (Copas, 1982). The mortality event in 1995 was widespread and severe, but no studies on the potential direct effects on avian piscivores were made. We are only aware of a post hoc review of mortalities and changes in abundance of little penguins {Eudyptula minor) at some Victorian sites (Dann et al, 2000). Australasian gannets (Morus serrator), a major predator of pilchards (Norman and Menkhorst, 1995), breed at several sites along the Victorian coastline including Port Phillip Bay where they form a large component of the local avian biomass (Norman, 1992; Norman et al, 1998). Previous studies have shown that gannets feed predominantly on pilchards, but may also take other inshore schooling pelagic fish and squid (e.g. Wingham, 1985; Norman and Menkhorst, 1995). Gannets nesting in Port Phillip Bay have been studied extensively (Norman, 1992; Norman and Menkhorst, 1995; Gibbs et ai 2000), and are easily accessible at some sites. Since the 1998 pilchard mortality appeared to follow the progression of that of 1995, and was sufficiently publicised to allow project development, this study was designed to determine whether 83 the diet of gannets breeding in Port Phillip Bay changed as the mortality front passed through Victorian waters.

Methods

The study was conducted at Pope's Eye Marine Reserve (38°16'42"S, 144°4r48"E), where roughly 200 pairs of Australasian gannets breed annually, off Queenscliff. This site is the largest of eight breeding sites in Port Phillip Bay. Previous studies at Pope's Eye (Norman, 1992; Norman and Menkhorst, 1995; Gibbs et ah, 2000 for location and details) have established the phenology of annual events and provide historical data for comparison with present studies. Samples of foods taken were collected opportunistically as 'voluntary' regurgitations made by adults and chicks upon approach by an observer. Once collected, samples were placed in polythene bags and frozen until later examination, when they were defrosted and weighed (to 0.1 g), before separation into individual taxa. Individual prey items were identified and weighed wherever possible, and fork lengths of fish were measured (to 0.1 mm) using callipers. Numbers of each prey species present were also determined. Collection of diet samples commenced on 2 October 1998, coinciding with initial records of pilchard mortalities in South Australian waters, but before any occurred off Victoria, and was continued until the end of the gannets' breeding period in March 1999. By this time the mortality front, present off Victoria in late November and early December, had passed. Diet composition was assessed as contribution by mass, frequency of occurrence (FOO), and weighted relative occurrence (WRO), following Hyslop (1980) and Montague and Cullen (1988). Comparison of the distribution of pilchard lengths between 1988-1992 and 1998-1999 was analysed using a two-sample Kolmogorov- Smimov test (Zar 1984). 84

Results

Pilchard deaths were first recorded in Victorian waters on 6 November 1998 at Discovery Bay, just east of the South Australian border. During the next five weeks other deaths were recorded along the coastline and had reached eastern Victoria by 8 December (Fig. 5.1). The mortality front passed Port Phillip Bay between 17 and 20 November 1998, a period when large numbers of pilchards were reported floating on the surface of the water about 1.5 nautical miles off Port Phillip Bay and in the Bay itself (Anon., 1998). Fifty-five regurgitates were collected between 2 October 1998 and 19 March 1999. These samples contained 291 individual prey items from 12 species of fish and squid. Although there were numerous prey species in samples, only pilchard and barracouta (Thyrsites atun) were consistently taken. Each regurgitation weighed on average 124 g (+ s.d. 74 , range 11-345, n = 55) and eontained 5.29 (± 5.8, 1-27, 55) individual prey items. Regurgitations consisted of a single species in 51 (92.7%) samples, with others containing two or three species. Individual prey items weighed on average 27.2 g(± 45.4, 3.2-276.1, 291) and were 129.2 mm (± 64.2, 43-414, 291) long. Before the 'die-off, pilchards comprised approximately 50% of the regurgitated mass (21 regurgitations) in October and November 1998; however, this representation declined to 5% in December 1998 and January 1999, immediately following the mortality event, and remained low in February and March 1999 (Fig. 5.2). The reduced proportion of pilchard in the diet was replaced by a substantial increase in other species' representation, particularly barracouta (Table 5.1). In addition, the proportion of pilchard in samples examined in 1998-1999 (29.1% FOO) was considerably less than in previous years (Fig. 5.3). Although, the proportion of pilchard in the gannet diet declined following the pilchard mortalities, the average mass of regurgitations did not change throughout the study period (t = 1.371, d.f. = 53, p > 0.05). The sizes of pilchards in samples obtained in 1998-1999 differed from those found between 1988 and 1992 (Fig. 5.4). Recent material indicated that gannets at Pope's Eye typically fed on pilchards 70-120 mm in length whereas the pilchards examined between 1988 and 1992, well before the recent pilchard mortalities, were slightly larger 85

Figure 5.1. Dates and locations of dead pilchards reported in Victorian waters 1998 (Anon. 1998). Arrow indicates location of study population. 86 October - November

60 (/) 50 W ra 40 E 30on 20 10

T3 ra Q) ra a> <0 ra O" Q. o w n « (o >. >. T3

QC CO

December - January

70 60 (0 (0 50 (0 40 E 30 20 10

c 4^ 4.* 0) 0) > ■g > n in = <0 o > L. 3 in o a "5 Q. E cr 3 3 (S > .s x: in L. 0) o o u (0 o CO E s o .•= <0 k. «3 CO c o 5 ■o 3 1- Q- != >. •o £ < V c «] CD (0 OQ o CC o ra 3 QC (0 m February - March

90 80 (0 70 W 60 50 40 30 20 10 0) > .-2 ti n ® w = <0 in a. n in " o I ^ ^ I ^ o w E 2 ^ > 2 n <0 TJ < O S in C m DC o n cc (0 Prey species

Figure 5.2. Seasonal variation in the diet of Australasian gannets breeding in Port Phillip Bay 1998-1999. October-November prior to pilchard deaths in Victorian waters, December-January and February-March the months following the pilchard mortality (Salmon = Australian salmon. Mullet = Yellow-eyed mullet). 87

Table 5.1. The percentage contribution of prey species to the diet of Australasian gannets breeding in Port Phillip Bay 1998-1999.

Prey species % Mass Frequency of Weighted Relative Occurrence(%FOO) Occurrence(%WRO)

Pilchard {Sardinops sagax) 20 29 20

Barracouta (Thrsites atun) 51 42 48

Blue mackerel {Scomber australasicus) 6 7 6

Australian salmon (Arripis trutta) 3 4 3

Anchovy {Engraulis australis) 2 4 2

Squid spp. 1 7 1

Red mullet {Upeneichthys porosus) 4 4 4

Blue sprat {Spratelloides robustus) 1 2 1

Yellow-eyed mullet {Aldrichetta forsteri) 1 2 1

Rosy wrasse {Pseudolabrus psittaculus) 9 7 9

Trevally {Pseudocaranx dentex) 1 2 1

Sandy sprat {Hyperlophus vittatus) <1 2 \D \o B. 00 o Barracouta Barracouta 3 ►—* o l-h M3 Blue mackerel MO Blue mackerel P 3 v; CL cn Salmon Salmon ►3 VO G) 00 O 00 c5' Anchovy Anchovy ,1.1 c« MO MO to 3" Squid Squid ^ ^ Ct (D •< p &* 5" (A Red mu et Red mullet 3 ■o o Q 3 O II II > 5" Blue sprat Blue sprat 3 0) > v> 3 Pf C/3 Mullet Mu et •-t 3 CO. § i Rosy wrasse Rosy wrasse C/3 OP P § 3 Trevally Trevally 3 CS o 3 a" >-1 Sandy sprat Sandy sprat k CD 3^ en o- g" 3* other spp. other spp. OP

OO 00 89

1998-1999

tn o 30 Q. E 25 n (0 20

10 5 1 il 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 171819 20

1988-1992

30 (0 O Q. 25 E (0 20 tn

0 --■Il 1 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 1819 20

Fork length mm (xlO)

Figure 5.4. Distribution of lengths of pilchards in the diet of Australasian gannets breeding in Port Phillip Bay, 1998-1999 and 1988-1992. 90

(90-150 mm); however, the difference in the distributions of pilchard lengths between the two periods were not statistically significant(D = 0.22, n = 20, p > 0.05).

Discussion

In this study gannets breeding within Port Phillip Bay fed predominantly on pilchards and barracouta and to a lesser extent on other species. Such results are consistent with previous local studies. Elsewhere, a range of species have been reported to be taken by gannets (see Norman and Menkhorst, 1995 for further details), presumably reflecting local,differences in fish populations and availability (Jarvis, 1970; Wingham, 1985; Klages, et al. 1992; Brothers et al, 1993; Crawford and Dyer, 1995; Montevecchi and Myers, 1997). Here, as pilchards died locally in 1998, their representation in the gannet diet declined and subsequently remained low. During 1998-1992, i.e. before the pilchard mortalities in 1995 and in 1998, pilchards comprised approximately 50% of the diet of gannets, with the balance consisting of a few other species (Norman and Menkhorst, 1995). However, following these mortalities the proportion of pilchards in all samples declined to 5%, with other species forming a major component. The decline in pilchards in food samples, reflecting unavailability, is unlikely to be due to seasonal factors. Indeed average monthly commercial catches of pilchards in Victorian waters are lowest during winter and spring and highest during late summer and autumn (Neira et al., 1997), a pattern not reflected in gannet foods during late 1998 and early 1999. Further, it seems likely that the earlier 1995 mortalities had some effect on age classes within local pilchard populations. The 1995 mortality, in addition to reducing the catch in general (Neira et al, 1999), was assumed to have affected older age classes of pilchards (Fletcher et al, 1997), and presumably subsequent productivity. The 1998 die-off is known, however, to have included younger fish (Anon., 1998). These differential age class effects may have important implications on the population structure of pilchards in future. The importance of this pilchard mortality on gannets was presumably minimised by foraging flexibility in gannets as evidenced both in size of prey consumed and range 91 of species taken. To a large extent, pilchards were replaced with a variety of species, particularly barracouta another inshore pelagic schooling species and important in the diet (Norman and Menkhorst, 1995). Such a change may have important implications (e.g. for breeding success) given potentially differing nutritional qualities of prey species. Pilchards are a high-energy food source, typically of greater calorific value than other prey and consequently are a 'preferred' prey item in gannet diet (Batchelor and Ross, 1982; Kirkham et al, 1985; Berruti et al., 1993). The consequences of a low quality diet, following decreased availability of pilchards, are that greater foraging effort and food consumption is required which may ultimately affect the reproductive success and survival of gannets. There are wider implications of the pilchard mortalities in both 1995 and 1998 that have yet to be considered, particularly the effects on other species depending on pilchards as food. It has been suggested that there were no significant changes observed in species (such as piscivorous fish, seabirds and marine mammals) which take pilchards as food, in response to the 1995 mass mortality (e.g. Fletcher et al, 1991\ Whittington et al, 1997). However, there was an increase in mortality and breeding failure of little penguins in Victoria in 1995 (Dann et al, 2000). In addition, there were reports of dead little penguins and Australasian gannets associated with pilchard mortalities in New Zealand (Smith et al, 1996). A significant wreck of Australasian gannets, the largest on record, occurred between August and November 1995 in New Zealand, immediately following the die-off of pilchards and a period of strong winds (Taylor, 1997). The extent to which this epizootic event has affected local pilchard stocks is still to be determined. However, the reduction in pilchard biomass along some parts of the coastline of Western Australia has been estimated as at least 20%, with the possibility that the reduction was nearer to 70% (Gaughan et al, 2000). While this study has provided some data for the Australasian gannet, it is unlikely that the full extent of these pilchard mortalities on the local marine biota will ever be understood. CHAPTER 6 92

Prey consumption of Australasian gannets(Moms serrator) breeding in Port Phillip Bay,south-east Australia and competition with commercial fisheries

Abstract. Prey consumption of Australasian gannets (Moms serrator) breeding in Port Phillip Bay, Victoria, was investigated between 1997 and 2000 and the competition between gannets and commercial fisheries in this region was assessed. Approximately 1000 gannets now breed in Port Phillip Bay annually and typically feed on inshore pelagic schooling fish species, such as pilchards (Sardinops sagax), barracouta (Thyrsites atun) and, to a lesser extent, anchovy (Engraulis australis), garfish (Hyporhamphus melanochir), mackerel (Scomber australasicus) and other species. Nutritional analyses of the major prey items in the gannet diet suggests that gannets may show some preference for particular prey types; however, variations in their representation in the gannet diet between breeding periods probably reflect changes in local availability. The daily energy requirement of individual gannets breeding in Port Phillip Bay, determined from activity patterns and activity-specific metabolic rates, was calculated as 4561 kJ day Using a bioenergetics model it was estimated that gannets breeding in Port Phillip Bay consume in total some 228.2 tonnes of prey during the breeding period including, 37.8 tonnes of pilchard, 80.5 tonnes of barracouta, 26.5 tonnes of anchovy and 27.5 tonnes of garfish. The potential foraging ranges of gannets during the breeding period, determined from the duration of foraging trips, was estimated at approximately 100km. Within this zone all of the major prey items in the gannet diet are taken by commercial fisheries; however, only pilchards are specifically targeted in large quantities. Limited information on the abundance or availability of these prey stocks inhibits quantification of the level of competition between gannets and the commercial pilchard fishery. Nevertheless, the potential for the fishery to negatively affect gannets is apparent. 93

Introduction

Increasing exploitation of pelagic fish populations worldwide has resulted often resulted in overfishing and the collapse of a number of commercial fisheries and declines in many surface-feeding seabird populations, highlighting the competing demands for limited marine resources (Crawford & Shelton, 1978; Duffy, 1983; Nettleship et al., 1984; Montevecchi et al., 1988). Such events have stimulated attempts to effectively manage commercial fisheries using knowledge of trophic interactions and ecosystem functioning (Cairns, 1992a). Estimates of the predation of pelagic fish stocks by seabirds have been essential in developing ecosystem-based fisheries management procedures and have also enabled the level of competition between some seabird populations and commercial fisheries to be assessed (Fumess, 1978; Fumess & Cooper, 1982; Wanless et al, 1998). Numerous studies have estimated the consumption of prey by seabirds in different marine environments, using a variety of methods (e.g. Weins & Scott, 1975; Furness, 1978; Furness & Cooper, 1982; Croxall et at., 1984; Guinet et al, 1996; Wanless et al, 1998). However, there is little information available on prey consumption of seabirds in Australian waters (see Gales & Green, 1990), despite the existence of large numbers of breeding seabirds in the cool temperate waters of southeastern Australia (Marchant & Higgins, 1990; Ross et al, 1995) and the presence of a number of major commercial fisheries in this region (Kailola et al, 1993). Australasian gannets {Moms serrator) breed at several localities in southeastern Australia and also around New Zealand (Marchant & Higgins, 1990; Bunce et al, 2000). In 1980-1981, the world population of about 53,000 breeding pairs included 6,600 pairs nesting in Australian waters, however, local populations, and those in New Zealand, have expanded considerably since 1980, if not before (Bunce et al, 2000). In southeastern Australia, gannets are a major local marine predator, feeding predominantly on inshore pelagic schooling fish and cephalopod species (Norman, 1992; Brothers, et al, 1993; Norman & Menkhorst, 1995). In this paper the prey consumption of Australasian gannets breeding in Port Phillip Bay, Victoria, southeastern Australia, is examined. Using a simple bioenergetics model, the prey consumption of gannets in this region 94 during the breeding period is determined, and incorporating data on estimated foraging ranges, the competition between gannets and commercial fisheries and other marine predators is assessed. This information is the first step towards developing an ecosystem- based model for the effective management of commercial fisheries in this region.

Methods

Study site and species In Port Phillip Bay, approximately 1000 gannets (507 pairs) breed entirely on artificial man-made structures, such as old navigational light beacons (Bunce et al., 2000). Previous studies have established the local breeding biology and phenology of annual events in this species at sites in Port Phillip Bay (Norman & Menkhorst, 1995; Gibbs et al., 2000). Briefly, the annual breeding period typically commences around August with pair-bond formation. Nest construction and egg laying usually occurs by mid October, with most chicks hatching by late November and fledging generally takes place in February. There may, however, be considerable spread in the timing of breeding events, and re-laying may occur following unsuccessful nesting attempts.

Diet analysis Samples of prey items taken by gannets were collected opportunistically as voluntary regurgitations from adults and chicks upon approach from an observer. Samples were collected throughout the annual breeding period, between August 1997 and March 2000. Once collected, samples were placed in polyethelene bags and frozen until later identification, when they were defrosted and weighed (to O.lg) before separation into taxa. Individual prey items were identified and weighed (to O.lg) wherever possible, and fork lengths (FL) of fish and mantle lengths of cephalopods were measured (to 0.1mm) using callipers. Numbers of each prey species present were determined. Diet composition, of both adults and chicks combined, was assessed as contribution by mass, frequency of occurrence (FOO), and weighted relative occurrence (WRO), following Hyslop (1980) and Montague & Cullen (1988). 95

Prey Consumption Model

Nutritional analysis Major prey items (as identified in this study, see below) taken by gannets, i.e. pilchard (Sardinops sagax), barracouta (Thyrsites atun), garfish (Hyporhamphus melanochir), and anchovy (Engraulis australis) were collected by commercial fishers from Port Phillip Bay between September 1999 and Febmary 2000. These samples were collected to analyse the nutritional quality of prey that gannets consume, and therefore were of a similar size to that taken by gannets and were collected during the breeding period and from within the foraging zone of gannets (see below). The nutritional quality of prey samples was then analysed following the methods of the Association of Official Analytical Chemists(AOAC, 1995). Energy content (kJ g"') was determined using bomb calorimetry, crude protein (%) was calculated using the mixed catalyst Kjeldahl method. The semimicro method (using CHCI3 and MeOH filtration) was used to assess crude fat (%), moisture(%) was calculated by drying at 135°C for 72 hours, and total minerals(% ash) were determined from the dry ash content (see AOAC, 1995 for details).

Activity patterns Gannets pairs incubating eggs (n = 4) and brooding chicks (n = 6) during the 1999-2000 breeding period were fitted with leg-mounted VHP transmitters (Sirtrack single stage transmitters) and the attendance patterns of these birds at the colony was monitored using a scanning/receiver datalogger unit (Telonics TS-1 scanner programmer, TR-2 receiver and TDP-2 advanced digital data processor, and Campbell Scientific Instruments CRIOXIM datalogger). This system was designed to record only the presence of adult gannets at the breeding colony attending nests, i.e. signals from the VHP transmitters were detectable only within a 50 m radius of the colony. Presence of the bird within the colony (i.e. a detectable signal) was taken to be the time the bird spent at the nest, as at this site there is little area available within the 50m radius for roosting. The absence of a signal from the VPnF transmitters was taken to be the time birds spent away from the colony (nest), foraging and resting at sea. 96

Bioenergetic model of prey consumption Daily energetic requirements of adult gannets were determined using attendance patterns of gannets breeding in Port Phillip Bay as determined in this study (i.e. time spent at the nest and time spent foraging at sea) and activity specific metabolic rates calculated for northern gannets {Moms bassana) as given by Birt-Friesen et al.(1989). A model of prey consumption of gannets breeding in Port Phillip Bay was developed, following the methods of Fumess (1978). This model requires knowledge of the following parameters, which were either determined in this study or derived from others: size of the breeding population, number of non-breeding birds, duration of the breeding period and average breeding success, daily energy requirements of gannets, the relative proportion of these requirements derived from different prey types, the energy content of this prey, and the birds' assimilation efficiency.

Competition with commercialfisheries The potential foraging range of Australasian gannets breeding in Port Phillip Bay was determined following Croxall et al. (1984). The parameters used to calculate foraging range were: the duration of foraging trips (i.e. time away from the colony, determined in this study); the proportion of time each foraging trip spent feeding, taken from Garthe et al. (1996); a flight speed of 14.9 m s"' (see Pennycuick, 1997); and an assumed correction factor for indirect flight of 1.33 (see Croxall et al., 1984). The foraging zone (i.e. the region within the potential foraging range of gannets) of Australasian gannets breeding in Port Phillip Bay was then estimated from these foraging ranges. The level of competition between Australasian gannets breeding in Port Phillip Bay and local commercial fisheries was investigated by comparing estimated foraging zones of gannets and the activities of commercial fisheries operating in this region and targeting major prey items taken by gannets. Annual landings in each of these fisheries, obtained from the database held by the Catch and Effort Unit (Fisheries Victoria, Marine and Freshwater Resources Institute), were compared with the theoretical intake of major prey items by gannets during the breeding period, determined using the bioenergetic prey consumption model. 97

Results

Diet analysis In the 1997-1998, 1998-1999 and 1999-2000 breeding periods, 131 regurgitations were collected from Australasian gannets (adults and chicks combined) at breeding sites in Port Phillip Bay. These samples consisted of 727 individual prey items from 12 species of fish and squid. While a number of prey species were represented in regurgitations, only pilchard and barracouta, and to a lesser extent, garfish and anchovy were consistently taken (Table 6.1). Each regurgitation weighed on average 151.6 g (+ s.d. 93.5, range 10.8-484.9, n = 131) and contained 5.6 (± 7.5, 1-51, 131) individual prey items. Regurgitations consisted of a single species in 121 (92.4%) samples, with other samples containing two (6.1%) or three species (1.5%). Individual prey items had a mean mass of 32.5 g (± 47.5, 2.1-276.1, 491) and were 157 mm (± 78.9, 78-414, 133) long. There was considerable variation in both the range and size of prey species taken between successive breeding periods. In 1997-1998 gannets fed predominantly on pilchards and barracouta, and to a lesser extent, on blue mackerel and squid. In 1998- 1999 pilchards, barracouta and a range of other minor species were taken, while in 1999- 2000 the proportion of pilchard and barracouta in the gannet diet declined, with gannets feeding predominantly on garfish and anchovy (Fig. 6.1). The mean mass of regurgitations differed significantly between breeding periods (F = 6.581, d.f. = 2, p < 0.05), being greatest in 1997-98, declining considerably in 1998-99 before increasing again in 1999-2000 (Fig. 6.2). The length of individual prey items also varied significantly (F = 5.289, d.f. = 2, p < 0.05) between breeding periods. However, there was no significant variation in the mass (F = 0.636, d.f. = 2, p > 0.05) or number (F = 0.204, d.f. = 2, p > 0.05) of individual prey items consumed. The mean mass of regurgitations increased as the breeding period progressed, but this difference was not significant (F = 0.457, d.f. = 3, p > 0.05). The mean mass (Fig. 6.3) of individual prey items increased significantly during the breeding period (F = 10.262, d.f. = 3, p < 0.05), as did the length of individual prey items(F = 4.113,

Table 6.1. Percentage contribution of prey species in the diet of Australasian gannets (Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000 (refer to methods for FOO and WRO).

Prey species % Mass Frequency of Weighted relative Occurrence(%) Occurrence(%)

Pilchard (Sardinops sagax) 16 22 19

Barracouta (Thyrsites atun) 38 31 34

Blue mackerel (Scomber australasicus) 11 12 11

Australian salmon (Arripis truttacae) 2 4 2

Anchovy (Engraulis australis) 9 11 8

Squid spp. 4 7 4

Red mullet(Upeneichthys vlamingii) 5 6 4

Blue sprat (Sprattus robustus) <1 1 <1

Yellow-eyed mullet(Aldrichetta forsteri) <1 3 <1

Garfish (Hyporhamphus melanochir) 10 14 13

Rosy wrasse (Pseudolabrus psittaculus) 3 <1 3

Trevally (Pseudocaranx dentex) <1 <1 <1

Sandy sprat (Hyperlophus vittatus) <1 0 <1 99

1997-1998

30 o o 20

Q. s: C

TJ >.

m cc 1998-1999 cc m 50

30 O O 20 u.

cr r a.

>. U) ■o Q. >.

m 1999-2000 cc 50"

30 O o 20

TJ 0 TJ — 0)

a. Q. JC o- r V) >. TJ >• cc Prey species

Figure 6.1. Seasonal variation in the diet of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000 (Salmon = Australian salmon. Mullet = Yello-w-eyed mullet; Frequency of Occurrence (FOG), see methods for details). 100

200

(0 n = 49 n = 27 .2 ISO 'ip (0 '5) 55 3 D) U ^ 100 O (0 (0 a E 0) ^ 50 i_ (1) > <

CVJ O) o> O)

Breeding period

Figure 6.2. Seasonal variation in the average mass of regurgitations of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000 (error bars indicate standard error). 101

100 3

>. 0)

« 75 - (0 3 ■o n = 52 ■> TJ C = 50 n =

(0 (0 n = 150 R] n = g6 E 25 - 0) O) (0 0) > <

Breeding stage (months)

Figure 6.3. Variation in the average mass of individual prey items in the diet of Australasian gannets {Moms serrator) breeding in Port Phillip, Victoria, 1997-2000 during the breeding period (error bars indicate standard error). 102

breeding period as the mean mass and length of individual prey items increased; however, this difference was not significant(F = 2.666, d.f. = 3, p > 0.05).

Nutritional analysis The total energy content, protein, cmde fat, and total minerals of the major prey items in the gannet diet are summarised in Table 6.2. The calorific value (kJ g"') of pilchards and barracouta was greater than that of garfish or anchovy (F = 9.620, d.f. = 4, p < 0.05); however, there was no significant difference in the amount of protein in prey species (F = 1.458, d.f. = 4, p > 0.05). The amount of crude fat also varied significantly between prey species (F = 37.534, d.f. = 4, p < 0.05), as did total mineral content (F = 32.893, d.f. = 4, p < 0.05), which was greatest in anchovy and pilchard.

Activity patterns The activity patterns of breeding gannets, determined by the proportion of time spent attending the nest and time spent at sea, did not vary significantly throughout the breeding period (t = 0.104, d.f. = 8, p < 0.05). During incubation gannets spent 41.6% of the time at sea and 58.7% at the nest, whereas during brooding 42.9% of time was spent at sea and 57.1% at the nest. In total, throughout the breeding period gannets spend 42.4% of the time at the nest site and 57.7% foraging at sea away from the colony.

Bioenergetics model ofprey consumption The daily energy requirements of gannets breeding in Port Phillip Bay, determined from attendance patterns and activity specific metabolic rates as given by Birt-Fiesen et al. (1989), is estimated to be 4561 kJ day '. The total energy requirements of this population (i.e. breeders, non-breeders, and chicks combined), derived from the estimated daily energy requirements of gannets and the parameters listed in Table 6.3, is 973.8 xlO^ kJ during the breeding period. The estimated total energy requirements for this colony were then converted to food consumption requirements, derived from the calorific value of prey and an assimilation efficiency of 75% (see Cooper, 1978), and integrated with diet information 103

Table 6.2. The mean nutritional composition of the major prey items in the diet of Australasian gannets(Moms serrator) breeding in Port Phillip Bay (n = 6 each taxon).

Energy Total

Moisture Protein content Crude fat minerals

Prey species (%) (%) (kJ g ') (%) (% ash)

Pilchard (Sardinops sagax) 66.6 21.4 8.6 9.8 7.5

Barracouta (Thryites atun) 68.8 19.6 7.1 13.6 3

Garfish {Hyporhamphus melanochir) 72.8 20.9 5.7 8 3.8

Anchovy (Engraulis australis) 74.9 19.8 5.2 4.4 5 Table 6.3. Parameters used to calculate the total energy requirements of Australasian gannets(Moms serrator) breeding in Port Phillip Bay, 1997-

2000.

Breeders and non-breeders Eggs Chicks Total population

Energy Total energy Total energy

Breeding Breeding Colony Daily energy Total energy Cost of egg Breeding requirements required requirements

Population period attendance requirements requirements production success per chick by chicks of population (no. of gannets)® (days)'' (bird days x 10^)" (kJ day"') (kJx 10®) (kJ xlO^)® (chicks/pair)® (kJ X 10^)'' (kJ X 10®) (kJx 10®)

1014 183 241.1 4561 1100.1 201.1 0.63 145 46.3 1146.6

From Bunce et al.(2000a) Based on length of breeding period (August-February), as given by Norman and Menkhorst(1995)

Includes correction for 30% non-breeding component of population (see Wanlesser a/., 1998) ^ Assuming cost of egg production (396.7 kJ), from Wingham (1989)

'From Bunce et al.(2000b)

From Montevecchi etal. (1984) 105

(relative proportion of major prey items in the diet) to estimate the total demand for major prey items taken by gannets (Table 6.4). In energetic terms, gannets breeding in Port Phillip Bay (i.e. breeders, non-breeders and chicks) consume an estimated 37.8 tonnes of pilchards, 80.5 tonnes of barracouta, 26.5 tonnes of anchovy and 27.5 tonnes of garfish during the breeding period. In total, assuming an average calorific value of 6.7 kJ g"' for all prey consumed (i.e. average energy content all foods is equivalent to that of major prey items), gannets in Port Phillip Bay consume approximately 228.2 tonnes of prey during the breeding period.

Foraging ranges and competition with commercial fisheries (and other marine predators) A total of 72 foraging trips were recorded during both the incubation (egg laying to hatching) and brooding (hatching to fledging) stages in the 1999-2000 breeding period. Duration of foraging trips ranged from 1 hour 8 minutes to 27 hours 6 minutes (mean = 9 hours 41 minutes ± s.d. 7 hours 25 minutes). However, foraging trips during the incubation period were significantly longer (t = 2.203, d.f. = 70, p < 0.05) than those during brooding (Fig. 6.4). Although gannets spent roughly the same amount of time at the nest and at sea throughout the breeding period, when incubating gannets made fewer but longer foraging trips, whilst when they were provisioning chicks foraging trips were generally more frequent and of shorter duration. The average maximum estimated foraging range of gannets therefore, was 149 km during the incubation period, and 88 km while brooding, with an average foraging range of approximately 94 km throughout the breeding period. The potential foraging ranges of Australasian gannets breeding in Port Phillip Bay are therefore within Port Phillip Bay and in the nearby coastal waters of Bass Strait (Fig. 6.5).

Discussion

Australasian gannets typically feed by plunge-diving for inshore pelagic schooling fish and cephalopod species. In this study gannets breeding within Port Phillip 106

Table 6.4. Estimate of the amount of prey consumed by Australasian gannets {Moms serrator) in Port Phillip Bay throughout the breeding period.

Total energy needed to be supplied by prey Amount of prey consumed (tonnes)

(kJ X 106)" Pilchard Barracouta Anchovy Garfish Total"

1528.8 37.8 80.5 26.5 27.5 228.3

'Assuming a digestive efficiency of 75% (Cooper 1978) 'Assuming an average energy content of 6.7 kJ g"' 107

1500

n = 7

C 1

c 1000 .2 a i— 3 ■o n = 65 a

O) 500 c 'o> (0 o 11.

incubation brooding

Breeding stage

Figure 6.4. Average duration of foraging trips during the incubation and brooding stages of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1999- 2000 (error bars indicate standard error). 108

* « HI

Average foraging range Foraging range (incubation) 100 km Foraging range (brooding)

Figure 6.5. The potential foraging zone of Australasian gannets {Moms serrator) breeding in Port Phillip Bay, Victoria, 1997-2000. Arrow indicates location of study population. 109

Bay fed predominantly on pilchards, barracouta and to a lesser extent garfish, anchovy and blue mackerel. These findings are consistent with previous local studies (Norman & Menkhorst, 1995; Bunce & Norman, 2000). Elsewhere, a range of prey species have been reported to be taken by gannets, presumably reflecting local differences in fish guilds and availability (see Jarvis, 1970; Wingham, 1985; Berruti & Colclough, 1987; Klages et al., 1992; Brothers et al., 1993; Montevecchi & Myers, 1997). The species composition of prey consumed by gannets breeding in Port Phillip Bay shows large intra- and inter-annual variation, presumably reflecting local changes in prey distributions and abundance. In 1995, and again in 1998, there was a large-scale mass mortality of pilchards in southern Australian waters, which is thought to have affected up to 10-15% and 70% of the local pilchard biomass, respectively (Retcher et al., 1997; Whittington et a:l., 1997; Gaughan et at., 2000). Before these mortality events, pilchards comprised approximately 50% of the gannet diet, with the balance consisting of a few other species (Norman & Menkhorst, 1995). However, following the pilchard mortalities in 1995 and 1998 the proportion of pilchard in the gannet diet declined considerably and has remained low for some time, presumably reflecting an unavailability of pilchards (Bunce & Norman, 2000). Whilst the mean mass of regurgitations did not vary significantly both before and after the pilchard mortality during the 1998-1999 breeding period (Bunce & Norman, 2000), mean total regurgitations weighed considerably less in this breeding period compared with the 1997- 1998 or 1999-2000 breeding period, suggesting a scarcity of prey following the spread of pilchard deaths into Victorian waters. In this study, gannets typically fed on fish ranging in size from 90 to 270 mm FL (80%), similar in size to prey consumed by Australasian gannets in Tasmania (Brothers et al, 1993) and New Zealand (Wingham, 1985), and Cape gannets {Moms capensis) in South Africa (Batchelor & Ross, 1984; Klages et at., 1992). Batchelor & Ross (1984) concluded that an optimum prey size for Cape gannets of 200 mm FL was controlled to some degree by the size of prey that dependent chicks could handle, and also presumably to some extent, the handling and capturing capability of adult gannets. The significant increase in both the size and mass of individual prey items in the Australasian gannet diet throughout the breeding period supports this, and suggests that the type of prey consumed 110 by adult gannets with young chicks may be constrained by the need to appropriately provision small offspring. In addition, when provisioning chicks, gannets generally made more frequent foraging trips that were of shorter duration, than during other stages of breeding, suggesting that the need to provision offspring may also influence foraging behaviour (e.g. Weimerskirch etal., 1993; Hamer & Hill, 1994; Bolton, 1995). Gannets may also show preference for particular prey types which are of greater nutritional quality. Batchelor & Ross (1984) suggested that the greater energy value of pilchards made them a preferred prey item in the Cape gannet diet. The implications for gannets, consuming a lower quality diet (in the absence of pilchards), are that greater foraging effort (and time) or prey consumption may be required to satisfy energy requirements (Batchelor & Ross, 1984; Bunce & Norman, 2000). Of the major prey items consumed by Australasian gannets breeding in Port Phillip Bay, pilchards typically had the greatest calorific value and are consequently also likely to be preferred prey (see also Batchelor & Ross, 1984). If gannets consume a diet consisting solely of anchovy they would need to consume 30% more prey (by mass), or roughly an additional six anchovy, to satisfy their daily energy requirements than gannets feeding solely on pilchard. Hence gannets should show a strong preference for prey, such as pilchards, which are of greater nutritional quality; however, the relative dietary proportions of major prey items in the gannet diet probably ultimately reflect local availability.

Prey consumption model The prey consumption model used in this study indicates that Australasian gannets breeding in Port Phillip Bay consume large amounts of prey, with roughly 230 tonnes of prey consumed by gannets (i.e. breeders, non-breeders and chicks) during the breeding period. Whilst these estimates of prey consumption are generally less than that reported in other systems, the size of the local breeding population is also less (Fumess & Cooper, 1982; Croxall et al, 1984; Guinet et at., 1996; Wanless et at., 1998), they are however, comparable given equivalent population sizes. These prey consumption estimates are undoubtedly subject to various sources of error, most of which are unquantifiable. While counts of the number of gannets breeding in Port Phillip Bay are known to be accurate, estimation of the non-breeding component Ill

of the population is more problematic. Here it has been assumed the non-breeding population of gannets in Port Phillip Bay is roughly 30% of the total population (see Wanless et ai, 1998). It is considered that this is a conservative estimate as in Port Phillip Bay, where gannets are a major component of the local avian biomass, gannet numbers often exceed that of the local breeding population (Norman, 1992), and newly established breeding sites are rapidly colonised (Norman & Menkhorst, 1995; Bunce et al., 2000). Wingham (1989) reported that the non-breeding proportion of the Australasian gannet population in New Zealand was as high as 17% of the total breeding population. Elsewhere the non-breeding component of gannet populations has been reported to be between 11% and 30% (Nelson, 1978; Tasker et ai, 1985; Wanless et ai, 1998). Another potential source of error is in estimating daily energy requirements. In this study, the daily energy requirements of Australasian gannets determined from activity patterns, using activity specific metabolic rates (Birt-Friesen et ai, 1989), was calculated as 4561 kJ day Similar daily energy requirements for Australasian gannets have also been reported (4315 kJ day T. Reid, unpub. data), although Wingham (1989) reported daily energy requirements of Australasian gannets breeding in New Zealand to be 2844 kJ day The latter estimate was modelled on standard metabolic rates and is therefore, presumably unrealistically low. Estimates of the daily energy requirements reported for other gannets vary but are generally similar to that reported here, ranging from 3696 kJ day"' to 4865 kJ day"' for northern gannets (Birt-Friesen et ai, 1989; Fumess, 1990; Wanless et ai, 1998) and 3065 kJ day"' to 3380 kJ day"' for Cape gannets (Cooper, 1978; Fumess & Cooper, 1982; Calms, 1987; Adams etai, 1991). Finally, estimates of prey consumption of gannets refer only to the breeding period (approximately six months), although gannets may occur in Port Phillip Bay throughout the year (Norman, 1992) consequently there will also be some additional predation during this period (i.e. non-breeding). In addition, the estimation for the total energy requirements of chicks, in order to simplify the model, does not include prey consumed by young that do not fledge successfully. Consequently, the estimation of prey consumption for Australasian gannets breeding in Port Phillip Bay is likely to underestimate the actual amount of prey taken both during the breeding period and annually. Nevertheless, this estimate of prey consumption provides the first indication of 112 the magnitude of the prey requirements of Australasian gannets in Port Phillip Bay, extending the limited knowledge of seabird prey consumption in southeastern Australia.

Competition with commercialfisheries The estimated foraging range for Australasian gannets breeding in Port Phillip Bay varied depending on the stage of breeding, ranging from 88 km during brooding to 149 km during incubation, but averaging about 100 km throughout the breeding period. Hence gannets breeding in Port Phillip Bay are likely to forage within Port Phillip Bay and the nearby coastal waters of Bass Strait. Although, based on a limited sample size and making assumptions based on studies of other gannets, these estimates are similar to that reported elsewhere. Using similar methods Wingham (1985) estimated the foraging range of Australasian gannets breeding in New Zealand to be 268 km (although using a faster flying speed and no correction factor for indirect flight), but considered most foraging occurred within a 61 km radius of the breeding colony (see Wingham, 1989). Other studies have reported foraging ranges for northern gannets ranging from mostly less than 40 km to greater than 500 km (Nelson, 1978; Tasker et at., 1985; Camphuysen et al., 1995; Garthe et at., 1999; Hamer et al., 2000) although most feeding areas appear to be within 150 km of the breeding colony (Wanless et at., 1998). All of the major prey species in the gannet diet are taken commercially; however, only pilchards are specifically targeted in large quantities as a commercial fishery (Kailola et at., 1993). The commercial pilchard fishery has traditionally been regarded as Victoria's largest inshore fishery in terms of catch by weight, with annual catches increasing considerably since its establishment and peaking at over 2000 tonnes between 1992-1993 and 1993-1994 (Neira et al., 1999). Recently commercial pilchard catches have declined considerably, with approximately 30 tonnes taken commercially in 1999- 2000, presumably due to an unavailability of pilchards following the widespread pilchard mortalities in southeastern Australia in 1995 and again in 1998 (P. Coutin, pers. comm.). Competition between the Victorian commercial pilchard fishery and gannets breeding in Port Phillip Bay is likely as both extensively exploit prey of similar size (Fig. 6.6) in this region (cf. Norman & Menkhorst, 1995 and Neira et al., 1999). In addition, most 113

Pilchards In gannet diet

30 g 25 ^ 20 c o 3 15 D- 0) LU 10 5 ■ ■ill 1 0 1 2 3 J ^5 fi 7 fl 9 10 11 12 13 14 15 16 1718 19 20 21 22

Commercial catch of pilchards

u c (1> 15 3 CT 0) L. 10 u.

5 -■III 0 1 2345 6789 10 11 12 13 14 15 16 17 18 19 20 21 22 Fork length (cm)

Figure 6.6. Comparison of the length of pilchards consumed by Australasian gannets (Morns serrator) breeding in Port Phillip Bay, Victoria 1988-1992 (from Norman & Menkhorst, 1995) and that taken by the commercial pilchard fishery in Victorian waters 1994-1997 (fromNeira et al., 1999). 114 pilchards caught commercially in Victoria are taken from within Port Phillip Bay between November and May (Neira et al., 1999), overlapping considerably with both the gannet breeding period and their estimated foraging range. Unfortunately, without accurate information on the abundance or availability of these prey stocks further quantification of the level of competition between gannets and the commercial pilchard fishery is not possible. Nevertheless, the potential for the fishery to negatively affect gannets is much greater than the converse. Given that pilchard availability currently appears to be extremely low, and that both the Victorian commercial pilchard fishery and Australasian gannets breeding in Port Phillip Bay are targeting similar quantities of prey, this level of competition is presumably now at its most intense. Consequently, this extreme predatory pressure on pilchard stocks, by gannets and the commercial fishery, may further depress the recovery of this species. It is important to note that in southeastern Australia there are also a large number of other breeding and locally occurring seabirds (and piscivorous mammals, such as seals and dolphins), which also take pilchard as food. For many of these species there are few recent or detailed data available on dietary composition or population sizes, hence any estimation of prey consumption for these species is difficult, although it may be substantial. Consequently, these species may also compete with gannets and the commercial fishery, and adding further to the demand for pilchards locally.

Conclusions This study has demonstrated that Australasian gannets breeding in Port Phillip Bay, Victoria, are a major marine predator consuming large quantities of inshore pelagic schooling fish species. Although direct assessment of the level of competition between gannets and commercial fisheries in this region is difficult without accurate information on the abundance or availability of local fish stocks, the impact of commercial fisheries on gannets may be considerable. Indeed, both gannets and the commercial pilchard fishery target and actively exploit prey of similar size and with considerable spatial and temporal overlap. At present there are no management restrictions on the total allowable catch in any commercial fishery targeting major prey items of gannets (Anon., 1996). An unrestricted commercial catch such as this represents a relatively short-sighted 115 management policy presumably only to be addressed when commercial extinction is a likely possibility, as is standard practice on a global basis and is ultimately a major threat to the marine ecosystem (Ludwig et al., 1993). Given the probable increased competition between gannets and the commercial fishery following the 1995 and 1998 pilchard mortalities in southern Australian waters, the estimates of prey consumption determined in this study are the first step towards developing an ecosystem-based model for the effective management of this fishery. GENERAL DISCUSSION 116

Ecosystem-based fisheries management and the use of seahirds in managing marine resources.

Australasian gannets {Moms serrator) breed at several localities in southeastern Australia and also around New Zealand, where they form a major part of the local avian biomass, with local populations (and also those in New Zealand) expanding considerably in recent years (see Chapter 3, Appendix 1). In southeastern Australia, gannets are a major marine predator and consume large quantities of inshore pelagic schooling fish and cephalopod species (Chapters 4 to 6). In this study, changes in prey availability affected several distinct reproductive and population parameters of Australasian gannets (Chapters 1 to 6), suggesting that gannets may be excellent indicators of pelagic food conditions. The potential application of this knowledge for developing ecosystem-based fisheries management models can be broadly divided into three categories, namely (i) bird-derived prey abundance indices, (ii) natural mortality rates in fish stocks calculated from seabird predation estimates, and (iii) bird-derived stock recruitment indices (see also Cairns, 1992a).

Bird-derived prey abundance index Current fisheries management models typically use abundance indices derived from indirect estimates calculated from past catches and catch rates, although such practices are known to be problematic (Cairns, 1992a; Montevecchi, 1993; Botsford et al, 1997). Whilst changes in commercial fisheries catches may reflect, to some extent, changes in prey availability, they are also heavily influenced by technological improvements, and market forces and political demands for increased harvests; they are also commonly plagued by wide-spread under-reporting of actual catches by commercial fishery operators (e.g. Anderson et al., 1980; Botsford et al., 1997). Furthermore, the catch rates of densely-schooling pelagic fish may not be well correlated with stock abundance (e.g. Rice, 1992; Montevecchi, 1993). Consequently, fisheries-independent estimates of stock abundance may be more reliable and effective for fisheries management (e.g. Botsford et al., 1997). These include hydroacoustic surveys, research surveys, or measures of egg and larval production; however, again there are problems 117

associated with the validity and robustness of these methods (Caims, 1992a; Montevecchi & Myers, 1995; Botsford et al., 1997). For example, even when it is possible to locate pelagic prey on surveys, the schools of many species occur in 'hydroacoustically invisible' surface waters and may also avoid ships (see Montevecchi & Myers, 1995). In addition, these methods are often highly labour intensive and costly. Alternatively, reproductive and population parameters of seabirds, which typically breed in large, and often highly visible colonies, possibly represent a simple and inexpensive alternative indicator of the abundance of pelagic fish stocks (Montevecchi & Berruti, 1991; Cairns, 1992a; Montevecchi, 1993). For example, Australasian gannets breeding in Port Phillip Bay, Victoria, had lower hatching and breeding success, and poor chick growth, in years of reduced prey availability (Chapter 1). In addition, chicks at nests in which the level of food demand was artificially increased (i.e. twins) also had poor growth and reduced fledgling survival, conversely chicks that were provided with additional food, attained greater fledgling masses and had increased fledgling survival (Chapter 2). Similarly, Uttley et al. (1994) reported that guillemots {Uria aalge) had reduced chick masses and fledging success in years of low food availability, as has also been demonstrated for other seabirds (Hamer et at., 1993; Crawford & Dyer, 1995; Phillips et at., 1996; Olsson, 1997). Whilst reproductive success and chick growth may reflect changes in prey availability, they may only indicate food supply within a certain range, i.e. food supply is above the minimum for survival of some chicks but below the level where adults can readily exceed chick needs (Cairas, 1987). Adults may 'buffer' the effects of varying food supplies by adjusting foraging effort to maintain constant levels of parental care and hence may act to decouple food supply from reproductive or chick growth parameters (Uttley et al., 1994; Phillips et al., 1996). However, the use of adult activity or time budgets as an indicator of pelagic prey abundance may also be hampered by the 'buffering' of increased foraging requirements when prey is limited, by a reduction in adult body condition (Chastel et al., 1995; Erikstad et al., 1997; Weimerskirch et al., 2000). In addition, variability in reproductive parameters may be also associated with age-related improvements in reproductive ability (Chapter 1), breeding experience and fluctuations in environmental conditions (Coulson, 1968; Blus & Keahey, 1978; Weimerskirch 1990; Erikstad et al., 1998; Gibbs et al., 2000). 118

Recent and substantial increases in the Australasian gannet population in Australian waters (Chapter 3, see also Appendix 1), although likely to have resulted from increased local productivity, may be a poor indicator of changes in local prey availability. Indeed, seabird population sizes may be influenced by various factors including immigration or emigration, adult mortality and changes in breeding success (Lack, 1954a; Ashmole, 1963; Cairns, 1992b). Typically, reductions in prey availability might be expected to first affect adult attendance pattems or activity budgets and reproductive parameters, an increase in adult mortality is only expected when food is extremely scarce (Cairns, 1987). The survival of young post-fledging rather than adult mortality may, however, be more sensitive to changes in food availability, as after fledging many seabird young (including gannets) are independent and must forage by themselves and master often complex foraging techniques. Consequently mortality of young post-fledging is generally high (Jarvis, 1974; Olsson, 1997). For example, Olsson (1997) reported that the post-fledging survival of king penguins {Aptenodytes patagonicus) was greater in years when prey availability was not limited. In addition, seabird population changes in relation to variations in food supply are likely to have a delayed effect due to the longevity and delayed maturity of many seabirds (e.g. Fumess & Monaghan, 1987; Cairns, 1992b), and therefore are likely to be not responsive to immediate changes in prey availability. The zones of variation of reproductive and population parameters of seabirds in relation to changes in food availability tend to occupy the lower and middle ranges of food supply, with little or no variation when food conditions are favourable (Cairns, 1987; 1992a). Thus, the simultaneous use of several reproductive and population parameters may qualitatively indicate trends when food conditions are limited. However, it appears unlikely that population or reproductive parameters can provide a quantitative indicator that operates at all levels of food supply (Cairns, 1992a). Furthermore, to detect changes in food availability from variation in seabird reproductive or population parameters, a detrimental impact must occur, i.e. a reduction in breeding success or an increase in adult mortality. Hence the effective application of such seabird population and reproductive parameters in fisheries management models is negated by the necessity 119

of detrimental impacts on seabird populations to detect changes in prey stocks, which contradicts the objective of ecologically sustainable fisheries management plans. Changes in the relative proportions of different prey items in seabird diets may provide an effective indication of stock abundance with little or no impact to seabird populations (Hislop & Harris, 1985; Montevecchi et al., 1988; Montevecchi & Myers, 1995). Prey harvests of Australasian gannets in Port Phillip Bay, Victoria, are strongly correlated with commercial fisheries catches (at least for pilchards) and variability in the diet composition of gannets presumably reflects changes in the local availability of fish stocks (Chapter 4). For example, the proportion of pilchard in the gannet diet declined considerably following the spread of a large-scale mass-mortality of pilchards in southern Australian waters in 1998 (Chapter 5). Montevecchi et al. (1988) also reported that extreme reductions of the dominant prey in the diet of Northern gannets {Moms bassana) were directly associated with subsequent local pelagic fishery failures. Similarly, Montevecchi (1993) suggested that changes in the diet composition of seabirds may provide a reliable indicator of pelagic conditions and changes in prey stocks, responsive immediately to changes in prey availability (e.g. Bemiti & Colclough, 1987; Montevecchi & Berruti, 1991; Montevecchi & Myers, 1995). However, some seabirds may select or actively target 'preferred' prey, which are of greater nutritional quality (Chapter 6), or require less effort for prey capture and handling (Batchelor & Ross, 1984; Berruti & Colclough, 1987; Montevecchi & Barrett, 1987). Consequently, changes in the diet composition of selective foragers may not be as responsive when prey are at very low levels, hence flexible foragers (including gannets) may provide a better indication of prey availability across a broader spectrum of prey abundance (e.g. Cairns, 1992a; Montevecchi & Myers, 1995). Prey abundance (population numbers or biomass) and availability, as determined by distribution and density, may represent separate factors for foraging predators. Whilst prey availability is a function of abundance, the two are independent to the extent that oceanographic conditions and behaviour affect the horizontal and vertical distribution and density of prey relative to the foraging capabilities of predators (e.g. Montevecchi, 1993). Consequently, changes in prey availability to a predator may not necessarily reflect changes in actual prey abundance; however, predator indices of prey abundance may still 120 be useful in reflecting the direction of changes in prey stocks. Furthermore, as both seabirds and commercial fisheries typically target and exploit pelagic fish stocks in near surface inshore waters, predator-derived prey indices may reliably reflect availability of prey to commercial fisheries. Hence, the inclusion of seabird-derived indices of prey availability into contemporary fisheries management models is likely to greatly improve the ecologically-sustainable management of commercial fisheries.

Birdpredation estimates used in calculating mortality Mortality due to specific predators can be estimated by the use of dietary information and bioenergetics models of food needs in the breeding season (or beyond). When incorporated with data on seabird foraging ranges, prey exploitation in specific geographic regions can be established and seabird-caused mortality to fish stocks can be estimated (e.g. Fumess, 1978; Fumess & Cooper, 1982; Wanless et al., 1998). Australasian gannets breeding in Port Phillip Bay, Victoria, feed extensively on inshore pelagic schooling prey both within Port Phillip Bay and the nearby coastal waters of Bass Strait (Chapter 6). The incorporation of such data into fisheries stock assessment methods, which typically currently use a constant (estimated) natural mortality rate (20%) is likely to improve estimates of stock projections in fisheries management models (Cairns, 1992a; Montevecchi, 1993). This may be further improved with the use of specialised population analysis models that incorporate predation rates for multiple species (including gannets) and can be used to refine natural mortality estimates (e.g. Cairns, 1992a; Botsford et al., 1997). However, the use of such multi-species models is currently limited by the lack of sufficient data for many marine predator species (e.g.

Botsford et al., 1997; Hollowed et al., 2000). The estimation of seabird predation rates also allows assessment of the level of competition between seabirds and commercial fisheries and can be used to predict the effects of changes in fisheries activities on seabird populations (Fumess, 1978; Fumess & Cooper, 1982; Wanless et at., 1998; Tasker et al, 2000). For example, all of the major prey items consumed by Australasian gannets breeding in Port Phillip Bay, Victoria, are also exploited by commercial fisheries. In particular, the potential for the commercial pilchard fishery in Victorian waters to negatively affect gannets is apparent (Chapter 6). 121

Similarly, Wanless et al. (1998) reported that based on estimates of prey consumption by seabirds, potential increased harvests of the commercial sandeel (Ammodytes marinus) fishery were likely to negatively affect breeding seabird populations in the Firth of Forth,

south-east Scotland.

Bird-derived stock recruitment index The predictive power of any fisheries management model will be greatly improved by inclusion of information on the recruitment of younger cohorts into the exploited population. Currently, most fisheries assessments contain no recruitment predictors other than the estimated long-term mean, typically calculated by examining changes in the proportion of particular age-classes in the commercial catch (see Cairns, 1992a). Consequently, even the inclusion of qualitative, bird-derived, estimates of pre- recruit abundance will undoubtedly improve the accuracy of stock projections in contemporary fisheries management models (Barrett et al., 1990; Caims, 1992a; Bertram & Kaiser, 1993; Laugksch & Adams, 1993; Reid etal., 1999). Changes in the size range of prey consumed by Australasian gannets (Chapter 5), and presumably other seabirds, may reflect local availability of particular age-size classes of prey and may provide a good indication of the strength of particular cohorts and stock recruitment. In addition, seabirds prey on fish below commercial size (generally smaller seabirds) and consequently may make excellent predictors of subsequent recruitment to commercial stocks, and also provide a quantitative indicator of pre-recmit cohort size (Montevecchi & Berruti, 1991; Caims, 1992a; Montevecchi, 1993; Monaghan, 1996). For example, Laugksch and Adams (1993) suggested that a general decrease in the size of anchovy (Engraulis capensis) consumed by African penguins (Spheniscus demersus) between 1980 and 1990 presumably reflected poor recmitment of anchovy. Reid et al. (1999) also reported that an increase in the proportion of large krill {Euphausia superba) in the diet of macaroni penguins (Eudyptes chrysolophus), gentoo penguins {Pygoscelis papua), and Antarctic fur seals {Arctocephalus gazella) at South Georgia, reflected a failure of small krill to recmit, resulting in a reduction of krill biomass in the following year. Similarly, Bertram and Kaiser (1990) considered that changes in the diet of rhinoceros anklet (Cerorhinca monocerata) nestlings may provide timely and 122

inexpensive information on the recruitment of Pacific sand lance {Ammodytes hexapterus) stocks in Canadian waters.

Scopefor improvedfisheries management This study has demonstrated that seabirds provide useful indicators of pelagic conditions, reflecting changes in stock abundance and consequently may provide detailed and unbiased knowledge of trophic interactions and ecosystem functioning, which will invariably be essential in developing effective ecosystem-based fisheries management models. Whilst variability in reproductive and population parameters of Australasian gannets reflect availability of pelagic fish stocks, their usefulness in fisheries management models is probably limited largely by their qualitative nature, restrictive range and poor predictive power. Alternatively, changes in the diet composition of gannets, which are strongly correlated with commercial fisheries landings of certain species (in particular pilchards), are a good indicator of prey abundance and, together with estimates of seabird predation rates, can be used to refine natural mortality estimates, which will improve the effectiveness of ecosystem-based fisheries management models. Further, Information from marine birds may also include assessments of prey condition (size, sex, reproductive state) and temporal and spatial (involving many colonies) distributions. At present, however, the application of such models is limited, being dependent on knowledge of the importance of other biotic and abiotic factors influencing pelagic conditions and fish populations (e.g. Shelton, 1992;

Botsford et al, 1997; Bax, 1998; Hollowed, etal., 2000). In addition, knowledge of the potential impact of changes in prey availability on seabird reproductive and population parameters, is essential and valuable for determining the likely effects of changes in commercial fisheries on seabirds and other locally important marine predator populations. REFERENCES 123

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Recent changes in the status of Australasian gannets Morus serrator in Victoria.

The Australasian gannet Morus serrator breeds around New Zealand, southern Tasmania and at colonies off the Victorian coast. In 1980-1981, the world population of about 53,000 breeding pairs included 6,600 pairs nesting in Australian waters (Wodzicki et al., 1984). When reviewed in 1990, the probable status of the population was considered stable (Marchant & Higgins, 1990). At that time, the main Victorian colony (2463 pairs in 1986-1987) was at Lawrence Rocks, off Portland, in western Victoria and a small number bred in Port Phillip Bay. However, local populations and those in New Zealand (Waghom, 1983; Hawkins, 1988), have expanded since 1990, if not before. In this note we indicate recent changes at Victorian colonies, based on our unpublished observations and the scant literature available.

Colony details

Lawrence Rocks McKean (1966) considered that there were 406 breeding pairs in October 1952 and (using totals of nests with eggs or chicks, and juveniles) 605 in the 1960-1961 breeding season; 1456 occupied nests were counted in late 1978 (Harris & Norman, 1981) and 1474 breeding birds in 1980-1981 (Fisher & Cooper, 1995). In November 1986, 2463 pairs were counted (from aerial black and white, and colour stereographic photographs and slides; file notes, see also E.J. Wingham, in Marchant & Higgins, 1990). While Norman & Menkhorst (1995) thought that numbers of breeding pairs were probably in excess of 3000 in 1993, Fisher & Cooper (1995) counted 5490 ± 150 adult birds (at nests) from aerial photographs taken in December 1993 and noted that the population had increased by about 11% annually since 1986. Some 6200 birds were counted in December 1996. At this site. Cape gannets {Morus capensis) were first reported in 1993 (four birds) and there were seven (with two chicks) in 1994 (Fisher & Cooper, 1995) 141

Point Danger In spring 1995, Australasian gannets (c. 500-1000) began roosting at the highest, most exposed part of Point Danger (Pennington, 1996), on the Victorian mainland near Portland, approximately 6 km from Lawrence Rocks. The elongated, gently sloping site, in coastal grassland (mainly Poa poiformis with low wind-pmned Leucopogon parviflora, Tetragonia implexicoma, Senecio lautus, Carpobrotus rossi and Leptospermum continentale), was about 1 m from the cliff edge, and some 15 m above the sea. Numbers declined, perhaps as a result of human disturbance and fox (Vulpes vulpes) activities, to about 300 birds in February 1996; occasionally banded birds were reported in this season. While four attempts at nest building were made, no egg laying was recorded. Since some Australasian gannets were killed by foxes, erection of a vermin-proof fence was commenced in winter, as was an integrated fox-baiting program throughout the south Portland area. Birds left the area in February 1996. In the 1996- 1997 breeding period, about 150 birds were present in early September (Pennington, 1996) and weekly observations from mid-November 1996 early January 1997 suggested similar numbers in late November and late December, with others on the sea below (R. Ressom, pers. comm.). Two or three nests were formed in the 1996-1997 breeding period and six broken eggs found (some up to 300 m from the colony; removed by ravens Corvus spp.). The headland was abandoned in early January 1997. Some 15-20 Australasian gannets were present in late July and early August 1997, about 50 on 7 August and 100 on 9 August, when pairing and seaweed collection was also observed. By late August 1997, about 450 birds roosted in the area; some birds were courting, and nest building and copulation was noted.

Wedge Light Norman & Menkhorst (1995) summarised details for this colony (on an artificial structure in Port Phillip Bay) from its inception in 1966 (at least three nests) to 1988 (45 active nests); there were up to 50 nests in the 1988-1992 period. Removal of the central hut between 1992 and 1994 increased the potential breeding area. In the 1996-1997 breeding period, 39 chicks were banded on the platform that then held 58 nests, including 142 two on structural cross beams under the platform. Some 44 nests had been started by 20 August 1997. An adult Cape gannet was first noted at Wedge Light in January 1981, paired with an Australasian gannet (Venn, 1982). Since then, breeding has occurred in most years (c/ Marchant & Higgins, 1990) and viable offspring have paired with other birds on the platform. In 1996-1997, an apparently full-plumaged Cape gannet bred at approximately the same position as a bird with similar plumage in previous years. This bird has now been banded.

Pope's Eye This colony (also primarily on an artificial stracture) was established in 1985 or slightly earlier (25 nests), in part at least by birds from Wedge Light. In September 1988, there were 46 active nests but between 1999 and 1992 there were up to 120, following construction of a new platform and walkway in 1989 (Norman & Menkhorst, 1995). In 1994, about 140 were reported (Gibbs, 1995) and in the 1996-1997 period 115 chicks were banded and up to 162 breeding pairs recorded. About 300 birds were roosting at Pope's Eye on 20 August 1997, and about 140 nests had already been formed. The nesting area has expanded from the original platform (1985) to newer structures, and onto adjacent rocks (1988-1992). In 1996-1997, when 24 (14.9%) nests were on rocks, 36 banded adults were caught. Of these, 29 had been marked as chicks at Pope's Bye, five at Wedge Light and two at Lawrence Rocks. No marked bird at a nest was less than three years old.

Other Port Phillip Bay colonies In 1993, Australasian gannets bred on an navigational pile (no. 6) in the eastern channel off Sorrento, and eight nests were being formed there on 23 July 1995 (P.W. Menkhorst, pers. comm.). On 5 February 1997, 26 chicks were present and an estimated 35 nests had been used; another pile (no. 10, off Rosebud) had 15 chicks and an estimated 20 breeding pairs. This site was in use in 1993 and, like no. 6, was apparently first colonised a few years previously. Banded adults were on nests,at both piles; one, at no. 10, had been banded as a chick at Wedge Light four years previously. Nests (24) were 143 recorded on the pile off Point Wilson on 20 November 1996, and other adult-plumaged birds were roosting on the long jetty nearby. Roosting there had been reported for some years, but this was considered to be the first instance of breeding. Regular observation (from the jetty about 40 m away) indicated that only five chicks fledged. Eight adults, all on nests, had been banded at Pope's Eye (5) or Wedge Light as chicks in the 1991-1992 or 1992-1993 breeding periods. A report of possible nesting on the southem Cole Channel marker, north-east of Swan Island, was not conformed on 5 February 1997, and no nests were found at other Cole Channel markers, at West Channel pile, or at South Channel or Hovell piles on the eastern side of the Bay.

Discussion and conclusions

Although the colony of Australasian gannets at Cat Island, in the Fumeaux Group, Bass Strait, has now disappeared (last known egg laid in the 1989-1990 period; I. Skira, pers. comm.), primarily as a result of predation and vandalism by humans (e.g. Warham & Serventy, 1978), other colonies have increased. Such increases have been reported in New Zealand (Waghorn, 1983; Wodzicki et al, 1984) and Australia (Marchant & Higgins, 1990). This study has shown that the Victorian population continues to expand, now involving a breeding population of some 6500 pairs (Table Al.l) which has increasingly used artificial sites and occupied a mainland area. At Lawrence Rocks, the increase has been occurring for some time(McKean, 1966; Harris & Norman, 1981; Fisher & Cooper, 1995) and has resulted in the establishment of a nearby mainland colony, as occurred in New Zealand (Hawkins, 1988; Marchant & Higgins, 1990). In Port Phillip Bay, too, it appears that the Wedge Light colony occupied all the nesting area then available about the time that another colony was established at Pope's Eye (Norman & Menkhorst, 1995). This colony has continued to grow and, although all flat (artificial) surfaces are now occupied, there is increasing use of outlying rocks, generally less successfully than elsewhere at Pope's Eye (Gibbs, 1995; Norman & Menkhorst, 1995). 144

Table Al.l. Current status of populations of Australasian gannets at Victorian colonies. Selected estimates of total adults, breeding pairs or active or occupied nests are used to indicate recent changes in numbers (as %,rounded).

Colony Size Season Comments References Lawrence Rocks 6200 (adults) 1996-97 Increase from 605 pairs in 1960-61 to 5490 McKean (1965); in December 1993; mean rate of7% annually, Fisher & Cooper (1995) but 12% between 1986 and 1993. Using 1996

data, increase of 7% since 1980 Point Danger 4 (nests formed) 1995-96 Other adults around (up to 150) This study Wedge Light 58 (nests) 1996-97 Increase from 3 nests in 1996 averaging Norman & Menkhorst(1995); 10% annually this study

Pope's Eye 162 (nests) 1996-97 Increase from 25 nests in 1985 at average Norman & Menkhorst(1995); of 18% p.a. this study

Point Wilson 24 (nests) 1996-97 First recorded breeding This study

Pile no. 6 35 (nests) 1996-97 Probably started early 1990s This study

Pile no. 10 20 (nests) 1996-97 Probably started early 1990s This study 145

Since the early 1990s, three new, small colonies have been established in Port Phillip Bay and the five colonies there involved about 300 breeding pairs in 1997. The annual rate of increase in breeding pairs at Wedge Light was about 27% between 1966 and 1981, but this has declined (using 1996 data; Table Al.l) since expansion there has almost stopped. The colony expanded little between 1967 and 1974, but doubled in size between 1974 and 1980 (Noiman & Menkhorst, 1995). Colony growth at Pope's Eye was very rapid between 1985 (18.5%) from 1985 to 1996 (162 nests) was achieved by return of chicks supplemented by immigration from Wedge Light and Lawrence Rocks (Gibbs, 1995; Norman & Menkhorst, 1995). Although the rate of increase at Lawrence Rocks was over 11% for the period 1986 to 1993 (Fisher & Cooper, 1995), using earlier data for 1960 (McKean, 1966) this longer term rate is reduced (to 6.9% up to 1993 and 6.7% to 1996, Table Al.l). However, the annual increase in the 1986-1993 period was almost twice (12.1%) that recorded from 1978 to 1986 (6.8%). Rates of increase at Victorian colonies are greater than the average of 2.3% given by Wodzicki et al. (1984) for New Zealand colonies (which range from -4.5 to 10.4%) and, for the Lawrence Rocks colony, the increase has been underway since 1952 when it contained 406 breeding pairs (McKean, 1966; Pescott, 1980; Marchant & Higgins, 1990; Fisher & Cooper, 1995 gave 40 pairs in error). However, as local populations fully occupy available nest sites, the annual rates of increase are likely to decline. Further, colony growth has not been simultaneous; expansion at Lawrence Rocks was rapid from 1986 onwards, at Wedge Light the increase was most extensive from 1974 to 1980 and that at Pope's Eye from

1985 onwards. Norman & Menkhorst (1995 and references included) discussed reasons for such increases, suggesting that immigration, reduce human depredation and changes in commercial fisheries might be involved. Certainly some pioneer birds at Lawrence Rocks, and in turn some banded as chicks at Wedge Light now breed on Pope's Eye (Norman & Menkhorst, 1995), and elsewhere in the Bay; such pioneers may be younger (rather than older) birds breeding for the first time. In addition, fledged Australasian gannet chicks return to breed at natal sites (e.g. Norman & Menkhorst, 1995; this study), and the 'pool' of younger adults capable of breeding has increased substantially in recent years as a consequence of local productivity (whether in Port Phillip Bay or at Lawrence 146

Rocks). This pool may be quite large since numbers of gannets (almost all in adult plumage) in Port Phillip Bay, where they form a major part of the local avian biomass, often exceed the local breeding population (Norman, 1992). Provision of additional platform area at both Pope's Eye and Wedge Light led to increased nesting; at both sites this was rapidly filled (suggesting future management options) but at Pope's Eye nesting by younger birds (Gibbs, 1995) has expanded onto surrounding rocks. Elsewhere in the Bay, new sites were occupied before 1993, though these too have limited space available. In contrast, occupation of available space at Lawrence Rocks between 1993 and 1995 has resulted in colonisation of a neighbouring headland where ample potential nesting area is available but where they are exposed to fox predation, and human disturbance. This expansion has continued in the absence of major changes in fishing activity or direction. Indeed for one major prey species, the pilchard Sardinops sagax, annual harvests have increased substantially in Port Phillip Bay (and in Victorian waters generally) from about 1980 onwards (Neira et al, 1995), a period which overlaps with the expansion of breeding gannets in the area. That the sizes of pilchards taken by gannets are very similar to those harvested by commercial fishers (cf. Norman & Menkhorst, 1995 and Neira et al., 1995) would suggest that any present competition has negligible effect. Certainly, in recent years, almost 70% of the pilchard catch was from Port Phillip Bay (this proportion was depressed by operations in 1983 and 1984; Neira et al., 1995). However, gannets may forage widely (Wingham, 1985), perhaps access schools unharvested or unavailable to fishers, or take a range of alternative food species (Norman & Menkhorst, 1995). This survey has shown that the population of Australasian gannets breeding in Victoria has increased both in number and in occupied sites since the last status review (Marchant & Higgins, 1990). While local Australasian gannet populations have increased, the Cape gannet has become established in two colonies in Victoria and hybrid chicks have been raised which breed with Australasian gannets. The Cape gannet presents and interesting management paradox; on one hand it is the rarest of Victoria's avifauna and on the, by interbreeding, it presents a threat to the genotype of the local species. It is of note that both species have recently been recorded (1987 onwards) at Saint Paul Island, in the southern Indian Ocean, where breeding attempts may also include a mixed pair (Lequette et al., 1995). 147

Acknowledgements

We are indebted to R. Ressom for the series of observations made at the Point Danger colony in 1996 and 1997, and to Heather Gibbs for approval to use her recent data from Pope's Eye. We would also like to record our gratitude for the assistance given by T. Putt throughout the 1996-1997 field season in Port Phillip Bay. Transport to channel markers in 1997 was provided by staff of the Department of Natural Resources and Environment (Geelong). We are grateful to R.H. Lyon for comments on a draft, and to the Australian Bird and Bat banding Scheme, Environment Australia, for the provision of bands and banding information as requested.

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232-261.

Minerva Access is the Institutional Repository of The University of Melbourne

Author/s: BUNCE, ASHLEY

Title: Population dynamics of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria; competition with fisheries and the potential use of seabirds in managing marine resources

Date: 2000

Citation: Bunce, A. (2000). Population dynamics of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria; competition with fisheries and the potential use of seabirds in managing marine resources. PhD thesis, Faculty of Science, Zoology, University of Melbourne.

Publication Status: Unpublished

Persistent Link: http://hdl.handle.net/11343/39514

File Description: Final thesis file

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