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. by 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: