Determining ecological effects of longline fishing in ...

Determining ecological effects of longline fishing

in the Eastern Tuna and Billfish Fishery

Determining ecological effects of longline fishing in the Eastern Tuna and Billfish Fishery FRDC 2004/063 March 2009 • • • •

Jock W. Young • Matthew J. Lansdell Alistair J. Hobday • Jeffrey M. Dambacher Shane P. Griffiths • Scott P. Cooper Rudy J. Kloser • Peter D. Nichols • Andrew T. Revill

Determining ecological effects of longline fishing in the Eastern Tuna and Billfish Fishery Jock W. Young, Matthew J. Lansdell, Alistair J. Hobday, Jeffrey M. Dambacher, Shane P. Griffiths, Scott P. Cooper, Rudy J. Kloser, Peter D. Nichols and Andrew T. Revill CSIRO Marine and Atmospheric Research Hobart, 7001 Tasmania, Australia

January 2009

Project No. 2004/063

FRDC 2004/063 Final Report

Copyright and disclaimer This work is copyright. Except as permitted under the Copyright Act 1968 (Cth), no part of this publication may be reproduced by any process, electronic or otherwise, without the specific written permission of the copyright owners. Neither may information be stored electronically in any form whatsoever without such permission. The authors do not warrant that the information in this book is free from errors or omissions. The authors do not accept any form of liability, be it contractual, tortious or otherwise, for the contents of this book or for any consequences arising from its use or any reliance placed upon it. The information, opinions and advice contained in this book may not relate to, or be relevant to, a reader's particular circumstances. Opinions expressed by the authors are the individual opinions of those persons and are not necessarily those of the publisher, research provider or the FRDC. The Fisheries Research and Development Corporation plans, invests in and manages fisheries research and development throughout Australia. It is a statutory authority within the portfolio of the federal Minister for Agriculture, Fisheries and Forestry, jointly funded by the Australian Government and the fishing industry. Author: Young, Jock W., 1955– Other Authors/Contributors: Matthew J. Lansdell, Alistair J. Hobday, Jeffrey M. Dambacher, Scott P. Cooper, Shane P. Griffiths, Rudy J. Kloser, Peter D. Nichols and Andy T. Revill Title: Determining ecological effects of longline fishing in the eastern tuna and billfish fishery ISBN: 9781921424489 (pbk.) Includes: contents, bibliography, appendices. Subjects: Tuna fisheries, Billfish fisheries, Marine ecology, Ecosystem management– Australia. Dewey Number: 333.9567814 Cover designed by Louise Bell and cover photography by Thor Carter, CMAR. Printed by [Print Applied Technology, Hobart] Published by CSIRO Marine and Atmospheric Research © Fisheries Research and Development Corporation and CSIRO Marine and Atmospheric Research [ Cite as: Young, J.W., Lansdell, M.J., Hobday, A.J., Dambacher, J.M., Griffiths, S.P., Cooper, S.P., Kloser, R.J., Nichols, P.D. and Revill, A.T. (2009). Determining ecological effects of longline fishing in the Eastern Tuna and Billfish Fishery. FRDC Final Report 2004/063. 325 pp. ]

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Table of contents 1.

Non-technical summary............................................................................................ 1 Outcomes achieved ....................................................................................................... 1 2. Acknowledgments .................................................................................................... 4 3. Background to project............................................................................................... 5 Consultation .................................................................................................................. 6 Research background.................................................................................................... 6 Eastern Australian pelagic ecosystems ......................................................................... 7 Pacific-wide comparisons ............................................................................................. 8 Literature cited.............................................................................................................. 8 4. Need .......................................................................................................................... 9 5. Objectives ............................................................................................................... 10 6. Physical and biological description of the ETBF ................................................... 11 6.1 Biological oceanography of the fishing grounds of the Eastern Tuna and Billfish Fishery off eastern Australia.......................................................................... 11 Abstract....................................................................................................................... 11 Introduction................................................................................................................. 13 Methods ...................................................................................................................... 14 Physical oceanography ........................................................................................... 14 Phytoplankton composition, biomass and productivity.......................................... 14 Macrozooplankton and micronekton net sampling................................................. 15 Acoustic sampling of intermediate trophic levels................................................... 16 Results......................................................................................................................... 17 Physical oceanography ........................................................................................... 17 Broad description................................................................................................ 17 Mesoscale features.................................................................................................. 19 Phytoplankton distribution and primary productivity............................................. 20 Macrozooplankton .................................................................................................. 25 Micronekton............................................................................................................ 26 Species composition ........................................................................................... 26 Depth distribution by taxa................................................................................... 29 Acoustic sampling................................................................................................... 30 Discussion................................................................................................................... 32 Physical oceanography ........................................................................................... 32 Primary productivity ............................................................................................... 32 Macrozooplankton .................................................................................................. 33 Micronekton biomass.............................................................................................. 33 Biomass estimation ................................................................................................. 34 Relationship between large predators and sound scattering layers......................... 35 Conclusions................................................................................................................. 35 Acknowledgments ...................................................................................................... 36 Literature cited............................................................................................................ 36 i


6.2 Ocean basin scale acoustic observations of mid-trophic fishes: potential and challenges ....................................................................................................................41 Abstract .......................................................................................................................41 Introduction .................................................................................................................42 Methods.......................................................................................................................43 Results .........................................................................................................................47 Fine scale net and acoustic observations.................................................................47 Basin Scale observations.........................................................................................49 Discussion ...................................................................................................................51 Comparison of net biomass with acoustics .............................................................51 Comparison of acoustic biomass with ecological models.......................................52 Potential and challenges of basin scale estimates of mid-trophic fish biomass ......53 Acknowledgements .....................................................................................................54 Literature cited ............................................................................................................54 7. Trophic ecology of the Eastern Tuna and Billfish Fishery from stomach content analysis ............................................................................................................................57 7.1 Single species study: Feeding ecology of broadbill swordfish, Xiphias gladius (Linnaeus, 1758), off eastern Australia in relation to physical and environmental variables ......................................................................................................................57 Abstract .......................................................................................................................57 Introduction .................................................................................................................58 Methods.......................................................................................................................59 Data collection.........................................................................................................59 Laboratory analysis .................................................................................................60 Data analysis ...........................................................................................................62 Results .........................................................................................................................63 Prey composition.....................................................................................................63 Feeding in relation to environmental and biological variables ...............................66 Stable Isotope analysis ............................................................................................71 Discussion ...................................................................................................................72 Feeding in relation to depth.....................................................................................73 Feeding patterns in relation to environmental and biological variables..................73 Swordfish trophic position ......................................................................................75 Acknowledgements .....................................................................................................75 Literature cited ............................................................................................................75 7.2. Pelagic cephalopods from eastern Australia: species composition, horizontal and vertical distribution determined from the diets of pelagic fishes. ........................80 Abstract .......................................................................................................................80 Introduction .................................................................................................................81 Methods.......................................................................................................................82 Selection of samplers ..............................................................................................82 Data collection.........................................................................................................83 Laboratory analysis .................................................................................................84 ii


Data analysis ........................................................................................................... 84 Species diversity ..................................................................................................... 85 Results......................................................................................................................... 85 Physical environment.............................................................................................. 85 Species composition ............................................................................................... 85 Horizontal distribution............................................................................................ 89 Regression analysis............................................................................................. 89 Swordfish-sampled cephalopods ............................................................................ 90 Yellowfin-sampled cephalopods............................................................................. 92 Cephalopod composition in relation to depth ......................................................... 92 Discussion................................................................................................................... 95 Predators as biological samplers............................................................................. 95 Species composition ............................................................................................... 95 Spatial and temporal relationships.......................................................................... 95 Horizontal distribution........................................................................................ 95 Vertical distribution ............................................................................................ 96 Acknowledgements..................................................................................................... 97 Literature cited............................................................................................................ 98 7.3 Feeding ecology of top pelagic fish predators off eastern Australia ............ 101 Abstract..................................................................................................................... 101 Introduction............................................................................................................... 102 Methods .................................................................................................................... 103 Data collection ...................................................................................................... 103 Laboratory analysis............................................................................................... 106 Data analysis ......................................................................................................... 106 Results....................................................................................................................... 107 Prey composition .................................................................................................. 107 Diet overlap........................................................................................................... 117 Prey-to-predator length relationships.................................................................... 118 Vertical distribution and feeding times................................................................. 123 Prey consumption and daily ration ....................................................................... 125 Latitudinal differences .......................................................................................... 126 Discussion................................................................................................................. 129 Diet, overlap and resource partitioning................................................................. 129 Predator-prey length relationships ........................................................................ 130 Feeding niche........................................................................................................ 131 Daily ration ........................................................................................................... 131 Latitudinal differences .......................................................................................... 132 Conclusions............................................................................................................... 133 Acknowledgements................................................................................................... 133 Literature cited.......................................................................................................... 133 8. Trophic ecology from biochemical analyses ........................................................ 138

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8.1 Discrimination of prey species of juvenile swordfish Xiphias gladius (Linnaeus 1758) using signature fatty acid analyses.................................................138 Abstract .....................................................................................................................138 Introduction ...............................................................................................................139 Methods.....................................................................................................................141 Sampling................................................................................................................141 Sample preparation trial ........................................................................................144 Laboratory analysis ...............................................................................................144 Lipid extraction .....................................................................................................144 Lipid classes ..........................................................................................................144 Fatty acids (FA).....................................................................................................145 Stomach contents analysis (SCA) .........................................................................145 Data analysis .........................................................................................................146 Results .......................................................................................................................146 Lipid content and composition..............................................................................146 Fatty acids .............................................................................................................148 Swordfish stomach content ...................................................................................158 Discussion .................................................................................................................158 Signature lipid profiles ..........................................................................................159 Diet of swordfish from FA profiles.......................................................................161 Signature lipid profiles as a proxy for stomach content analysis..........................161 Caveats and future research.......................................................................................162 Acknowledgements ...................................................................................................163 Literature cited ..........................................................................................................163 8.2 Stable Isotopic evidence for trophic groupings and bio-regionalization of predators and their prey in oceanic waters off eastern Australia ..............................167 Abstract .....................................................................................................................167 Introduction ...............................................................................................................168 Methods.....................................................................................................................169 Data analysis .........................................................................................................174 Results .......................................................................................................................174 Broad scale food web groupings ...........................................................................175 Size relationships...................................................................................................177 Grouping variables ................................................................................................177 Discussion .................................................................................................................182 Trophic groupings .................................................................................................182 Variability with size ..............................................................................................183 Evidence for bio-regionalization from SIA...........................................................183 Future research ..........................................................................................................185 Acknowledgements ...................................................................................................185 Literature cited ..........................................................................................................186 9. Defining pelagic habitats in the Eastern Tuna and Billfish Fishery......................189 Abstract .....................................................................................................................189 iv


Introduction............................................................................................................... 190 Methods .................................................................................................................... 193 Results....................................................................................................................... 195 Habitat persistence over time................................................................................ 200 Seasonal patterns in habitat size ........................................................................... 201 Discussion................................................................................................................. 203 Cluster-based identification of ocean habitats ...................................................... 204 How many clusters, sensitivity, and what conclusion .......................................... 204 Use of clusters for ecosystem monitoring in the ETBF........................................ 204 Acknowledgements................................................................................................... 205 Literature cited.......................................................................................................... 205 10. Ecosystem models of the Eastern Tuna and Billfish fishery ............................ 207 10.1 Analysing pelagic food webs leading to top predators in the Pacific Ocean: a graph-theoretic approach .......................................................................................... 207 Introduction............................................................................................................... 208 Methods .................................................................................................................... 211 Diet data................................................................................................................ 211 Food web graphs................................................................................................... 213 Key players ........................................................................................................... 213 Network aggregation............................................................................................. 214 Qualitative models and predictions of perturbation response............................... 215 Results....................................................................................................................... 215 Diet studies ........................................................................................................... 215 Food web structure and link strength.................................................................... 220 Aggregation .......................................................................................................... 220 Key players ........................................................................................................... 223 Qualitative predictions.......................................................................................... 223 Discussion................................................................................................................. 224 Comparisons with other systems .......................................................................... 225 Utility and limitations of the approach ................................................................. 226 Future research.......................................................................................................... 227 Acknowledgements................................................................................................... 227 Literature cited.......................................................................................................... 228 10.2 Simulated ecological effects of longlining and climate change on the pelagic ecosystem in eastern Australia.................................................................................. 233 Abstract..................................................................................................................... 233 Introduction............................................................................................................... 234 Methods .................................................................................................................... 236 The Ecopath with Ecosim approach ..................................................................... 236 Model construction procedure .............................................................................. 239 Spatial and temporal extent of the model ............................................................. 239 Model structure ..................................................................................................... 241 Sources of basic biological parameters and fishery data ...................................... 241 v


Fitting the Ecosim model to CPUE data ...............................................................242 Ecopath model results ...............................................................................................265 Ecosim model fitting .............................................................................................270 Ecosim model scenario results ..............................................................................273 Discussion .................................................................................................................281 Caveats and further work ..........................................................................................284 Acknowledgements ...................................................................................................286 Literature cited ..........................................................................................................286 11. Benefits and adoption........................................................................................299 12. Further development .........................................................................................299 13. Conclusion.........................................................................................................299 14. List of appendices..............................................................................................302 Appendix 1 ................................................................................................................302 Intellectual property ..............................................................................................302 Appendix 2 ................................................................................................................302 Staff .......................................................................................................................302 External contributors .............................................................................................302 Addresses for correspondence...............................................................................302 Appendix 3 ................................................................................................................303 Reports, seminars and peer-review publications...................................................303 Reports ..............................................................................................................303 Peer-reviewed publications ...............................................................................304 Oral presentations and posters...........................................................................304 Appendix 4 ................................................................................................................307 PESCI database: design and capabilities...............................................................307 Summary ...........................................................................................................307 Introduction .......................................................................................................307 Methods.............................................................................................................307 Conclusions .......................................................................................................312 Acknowledgements ...........................................................................................312 Literature cited ..................................................................................................312 Appendix 5 (from Hobday et al., this volume) .........................................................313 Appendix 6 (from Dambacher et al., this volume)....................................................320

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1.

Non-technical summary

2004/063

Determining ecological effects of longline fishing in the Eastern Tuna and Billfish Fishery

PRINCIPAL INVESTIGATOR: ADDRESS:

Dr J. W. Young CSIRO Marine and Atmospheric Research GPO Box 1538 Hobart TAS 7001 Telephone: 03 62325360 Fax: 03 62325000

OBJECTIVES: 1. Identify the spatial extent and the temporal stability of the main ecosystems of the eastern tuna and billfish fishery based on their species composition and physical environment. 2. Define the trophic structure within these ecosystems with emphasis on the relationship between target, bycatch and threatened and protected species. 3. Develop an ecosystem model for the ETBF fishery incorporating data on the relative abundance of the species, trophic linkages and the physical environment from which the impacts of longline fishing on the ecosystem can be investigated and from which alternative future management strategies can be evaluated.

Outcomes achieved This study examined the environment and ecosystem of the Eastern Tuna and Billfish fishery from direct observation and from qualitative and quantitative models. Detailed observations were made from the research vessel FRV Southern Surveyor, biological samples collected from the fishery, and from a range of laboratory-based analyses. We described the physical and biological processes in three areas where the fishery operates, and provide details of primary productivity and prey biomass and species composition in the region. We described the main food web pathways leading to top predators from analyses of stomach contents (SCA) and from biochemistry (stable isotope (SIA) and signature fatty acid analyses). Spatial analysis of biological oceanography of the region identified two main bioregions 1 or ecosystems 2 (hereafter referred to as ecosystems), separated by the Tasman front. This separation was consistent with the results from both SCA and SIA. These observations and analyses 1

Bioregion defined as an area constituting a natural ecological community with characteristic flora, fauna, and

environmental conditions and bounded by natural rather than artificial borders 2

The system of animals and plants and the environment they inhabit 1


were used to inform and ground-truth qualitative and quantitative models of potential scenarios of fishing impacts and climate change on the target and bycatch species of the fishery. The resulting models indicated that fishing pressure on individual predator species is unlikely to propagate widely through the fishery as other predators fill the gaps left by the impacted predators. The climate change scenarios modelled indicated that a warming ocean was likely to lead to more favourable conditions for a number of top predators in the region through increases in prey, particularly squid. These outcomes provide an empirical and model-based framework for the design and implementation of focussed studies to better inform predictions of impacts of climate change and fisheries on the ETBF and the pelagic ecosystems off eastern Australia.

Our main goal for this project was to develop qualitative and quantitative models from which managers could examine the potential impacts of fishing and climate change on the Eastern Tuna and Billfish Fishery (ETBF). We used the Ecopath with Ecosim software (www.ecopath.org) which can be constructed relatively quickly using mainly literature values. However, to our knowledge there have been no quantitative models developed for the southern hemisphere open ocean so values obtained from the northern hemisphere were likely to differ from actual conditions in the south western Pacific Ocean. The trophic ecology and many of the major inputs needed for such a model of the ETBF were largely unknown. To provide accurate information which would lead to a useful model a wide ranging research study was carried out, which included a research voyage, AFMA observer and fisher-collected biological samples, desk top studies of ocean data sets and a series of laboratory studies. To provide a home for all this data a database (PESCI) was also developed. The main outcomes from these studies are summarized below. •

A research voyage was made to the main region of the Eastern Tuna and Billfish Fishery east of Brisbane. On that voyage we completed a series of measurements to quantify and identify the primary (plant) and secondary (prey) production in the region. We also made detailed observations of the vertical structure around a seamount within the Tasmantid Seamount chain and in two other areas exploited by the fishery. We identified twenty five species of phytoplankton, their size distribution and biomass from inshore, seamount and offshore fishing areas. Later analysis provided an estimate of primary productivity for the region. We identified ~100 species of micronekton prey, and estimated their overall biomass in the three areas and to a depth of 500 m from a combination of opening closing nets and acoustics.

•

Detailed acoustic data collected on that voyage were analysed to provide an estimate of prey biomass for the region which was up to 15x higher than previously estimated.

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A desk top analysis of the spatial variability of the region was developed which distinguished seven locations where the mix of physical and biological variables indicated different environments in the ETBF, with a major separation point at approximately 32°S.

•

A series of studies of the trophic ecology of target and prey species taken by the ETBF was completed, before a more focussed analysis of the trophic ecology of the ten major predator species. These studies were all carried out from direct identifications of stomach contents (SCA), the data from which was entered into a purpose-built database. The data base contains all data from feeding studies carried out in eastern Australian pelagic waters since 1992 and ~4000 samples from 25 species with associated environmental data. A further 7,000 samples have now been added from other regions in the Pacific Ocean, via international collaborations, to enable wider spatial and temporal comparisons. These data were used to provide predator-specific analyses of prey composition, analysis of prey distributions and for predator prey matrices necessary for qualitative model development.

•

As SCA is labour intensive and requires specialist taxonomic knowledge we also investigated two different biochemical techniques – stable isotope analysis (SIA) and signature fatty acid analysis (FA) - for their potential to augment and possibly replace direct taxonomic identification in future studies of this kind. Using SIA it was possible to differentiate trophic level status for a wide range of predators and their prey, and was also able to discriminate spatial differences within predator populations of target species. Also, we were able to discriminate the most likely prey types for a given predator using signature FA.

•

SCA and SIA were completed on all target species and the results were consistent with an hypothesis generated from the spatial analyses that the ETBF fishes in at least two separate ecosystems off eastern Australia.

•

Predator prey matrices developed from the trophic analyses were then used to compare the trophic pathways of the ETBF with other fisheries in the Pacific Ocean using a qualitative model. This study was the first of its kind and showed the differing effects of climate change on top predators across the Pacific.

•

A quantitative model was then developed for the ETBF using EwE. Forty four separate groupings were created and balanced with data gathered from the field and laboratory analyses. Hind-casts of modelled biomass estimates of target species, particularly bigeye tuna, striped marlin and swordfish were closely correlated with standardised Catch per Unit effort data for the years 1950 through

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to 2007. A series of scenarios was then run to examine the impacts of fishing and climate change over the next twenty years. •

The model indicated that increased fishing effort on individual target species had a relatively minor effect on the abundance of other components of the ecosystem. The reasoning being that as most predator species in the region were operating at very similar trophic levels, when the abundance of a predator species was reduced by fishing, other predators would take their place in the system. Climate change scenarios indicated that predator abundance would increase under a warming ocean, through increases in the biomass of squids which are the prey of many predators in the region.

•

This research has provided a much needed empirical and process understanding of the ecosystem of the ETBF. In doing so, it has also identified a number of issues that need further research to improve our understanding of the potential effects of fishing and climate change on the east coast pelagic ecosystems. In particular, as the life histories of many target species have an inshore component, the crossshelf linkages and the potential importance of coastal and recreational fisheries, coastal development and coastal impacts of climate change have not been considered.

•

Priorities for further research should include o a comprehensive study of the inshore ecosystem adjacent to the ETBF so that impacts of commercial and recreational fishing in inshore waters can be evaluated against that of the offshore longline fishery o establishment of a mechanism for future monitoring of key biological indicators (e.g. trophodynamics, age, growth and reproduction) of target and non-target species within the ETBF so that early detection of ecosystem change, from fishing or climate change, is possible.

Keywords: Eastern Tuna and Billfish fishery, East Australian Current, pelagic ecosystem research, oceanography, ecology, monitoring, biochemistry, acoustics, fishery impacts, climate change

2.

Acknowledgments

This study was funded by the Fisheries Research and Development Corporation (FRDC) and CSIRO Marine and Atmospheric Research. Separate grants, which supported particular aspects of the project, were also provided by CSIRO through the Ernst Frolich Scholarship, the Australian Fisheries Management Authority (AFMA) and the National Facility Steering Committee via research time on FRV Southern Surveyor. The study benefited from wider associations; particularly with CLIOTOP (Climate Impacts on Ocean Top Predators), which is a sub-program of GLOBEC (Global Ocean 4


Ecosystem Dynamics), and with personnel from SPC (Secretariat for the Pacific Ocean), IATTC (Inter-American Tropical Tuna Commission), University of Hawaii (UH) and the Pelagic Fisheries Research Program (PFRP). A grant from the PFRP allowed us to examine wider aspects of Pacific Ocean trophic ecology. A Travel Grant Award from the organizers of the 4th International Billfish Symposium (2005) enabled Dr. J.W. Young to present data resulting from aspects of this study to an international audience from which a number of collaborations arose. We are particularly grateful for the input of Drs. Robert Olson (IATTC), Valerie Allain (SPC), Brittany Graham (UH) and Prof. Charles F. (Rick) Phleger (SDSU, CSIRO Frohlich Fellow). Support was also provided by Keller Kopf (Charles Sturt University). Sample collection was made possible by the willingness of the skippers of the Eastern Tuna and Billfish fishery. CSIRO field scientists Dr Karen Evans and Thor Carter collected samples. This report benefited from the comments made by Dr Campbell Davies and those of a number of anonymous reviewers to the individual chapters.

3.

Background to project

The Australian Government Fisheries and EPBC Acts (2001) require that Commonwealth fisheries are managed consistent with the principles of ecological sustainability. This implies that fisheries management should manage not only the harvest of target species but also the impacts of fishing on the ecosystem in which the fisheries operate. The Eastern Tuna and Billfish Fishery (ETBF) impacts on pelagic ecosystems by taking a range of apex predators including tunas, billfishes and sharks, mid order predators including dolphin fish and, occasionally as bycatch, high conservation-value species, such as turtles and seabirds. Over the short history of the domestic fishery there have been a number of impacts detected on individual species (e.g. swordfish depletion: Campbell and Hobday, 2003) or protected species group (e.g. turtles: Robins et al., 2007). These single incidents have the potential to accumulate into a larger ecosystem impact and therefore should be considered as a separate but related issue to single species management. Up to 2004 a number of projects and focus groups reviewed the status of knowledge on by-catch, ecosystem impacts and ecological risk of longlining in the ETBF and Southwest Tuna and Billfish Fishery (SWTBF). An Ecological Risk Assessment Project conducted by CSIRO, AFMA, BRS and MAFRI found that for many of the species caught by the fishery there was little information on which to evaluate that impacts of fishing (Hobday et al., 2004). Given this uncertainty, the project considered the risk of adverse effects of fishing on many of these species and ecosystems to be moderate to high. Furthermore, the Strategic Assessment of the ETBF highlighted the lack of understanding (and data) of the ecosystem impacts of the fishery. The ETBF FAG also recognized the inadequacies of the current logbook data for assessment of impacts of fishing on by-product, bycatch and ecosystems of the east coast of Australia. In response to these many drivers, AFMA undertook an observer program within the 5


ETBF. This program was designed to provide robust data on catches of all species, and provide a platform for collection of biological samples necessary to determine biological parameters needed to estimate potential fishing impacts. Recognising the importance of understanding the nature and complexity of communities and ecosystems impacted by the ETBF (and regional fisheries), and driven by the fact that AFMA require a basis for truly "ecosystem-based fishery management", we developed an integrated study designed at describing ecosystem structure and heterogeneity, and modelling the potential impacts of fishing and climate change under alternative future scenarios.

Consultation Consultation with industry, research and management was lengthy and resulted in a number of revisions to the original proposal before the proposal was accepted. Prior to the proposal’s acceptance CSIRO supported a number of smaller scoping studies and meetings to develop a project that would not only support environmentally based fishery management (EBFM in the fishery), but also took into account the growing need for an understanding of the potential impacts of climate change.

Research background The potential for longline fishing to influence the biomass of target species in pelagic ecosystems is well known, the extent of the impact is presently being debated (e.g. Cox et al., 2002; Myers and Worm, 2003; Sibert et al., 2006; Polacheck, 2006). There is less understanding, however, about the direct effects of substantial removals of target species (often top predators) on the other components of the ecosystem, such as prey species (often bycatch species), or the indirect effects that may lead to changes in lower trophic levels (Stevens et al., 2000; Essington et al., 2002). The impact of longline fishing on other trophic levels will also be influenced by the type of ecosystem in which the fishery operates. Recent results from modelling studies indicate that large-scale ecosystem differences might exist between neighbouring water masses in the central Pacific Ocean (Lehody, 2001). These differences may include the distribution and abundance of the target species, such that fishing effects in one region may not be the same in adjacent regions (Lehody, 2001). If these modelling results are representative of the real ocean, impacts of fishing pressure can only be understood by comparisons within and between the different regions. The difficulty with a regional ecosystembased approach, however, is the fundamental lack of knowledge regarding the biology of the constituent species and their trophic links (Cox et al., 2002). Rather than evaluating risk just to target species, there has been a shift in emphasis in the management of most state and commonwealth fisheries to an ecosystem approach (e.g. Fulton et al., 2005). An ecological risk assessment (ERA) developed by Hobday et 6


al. (2004) established a methodology to identify risk to all components of an exploited ecosystem, including species (target, bycatch, by-product, threatened and endangered), communities (assemblages) and habitats (habitat types). The ERA project team used the Eastern Tuna and Billfish Fishery (ETBF) to develop and evaluate the methodology. Preliminary results from the qualitative level-two risk assessment of the ETBF showed that some species and assemblages might be adversely impacted by longline fishing activities. Identification of these species and assemblages is required to evaluate impact of fishing at the second semi-quantitative level of the risk assessment. This analysis will require information on the ecology and biology of individual species and on their respective positions within the community. However, there is insufficient information on many of the species caught by the fishery (particularly bycatch and protected species), to allow the essential second-level assessment of the risk due to fishing activities. This leads to a likely overestimation of the real risk, as lack of information results in, according to the ERA methods, higher risk-scores being assigned to species or assemblages (Hobday et al., 2004). In fisheries such as the ETBF, which are managed under legislative arrangements that require adherence to the precautionary principle, risk overestimation due to lack of information has the potential to lead to unnecessary restrictions or modifications on fishing activities.

Eastern Australian pelagic ecosystems The Eastern Australian domestic longline fishery operates year round. It ranges from tropical Coral Sea waters to the southern limits of the East Australia Current and eastward past the limits of the AFZ and thus encompasses a range of potential ecosystems with a different physical environment, food web and community composition. There are at least two regions where significant differences have already been observed– the southeast shelf/slope region of New South Wales (Young et al., 2001) and the shelf/slope region off eastern Tasmania (Young et al., 1996). The fishing impacts on these and other ecosystems are unknown. The ETBF fishery targets several apex predator groups – tunas, swordfish and marlins, and other non-target apex species groups such as sharks are caught in large numbers. Although we have qualitative information on the main trophic connections between these groups, with little quantitative data it is not possible to determine how closely these species groups are linked. Spatial and temporal variations will further complicate these interactions. For example, the species-mix of the developing winter fishery for southern bluefin tuna in more southerly waters contrasts with the species-mix taken off southern Queensland, particularly in the summer months when swordfish dominate catches. Bycatch species also vary between regions, although knowledge of their distribution within the fishery is limited. Thus, management decisions applied to the whole fishery may be appropriate for one area/time combination but inappropriate or unnecessary for another.

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Pacific-wide comparisons The impact of pelagic longline fishing on oceanic ecosystems is receiving considerable attention (reviewed in Sibert et al., 2006) but is hampered by the limited “empirical information” presently available. A large scale ecosystem study of the cold and warm “pools” of the Central Pacific Ocean (Allain et al., 2001) presented at the Standing Committee for Tuna and Billfish (SCTB 16) held in Australia, emphasized the need for similar cooperative studies to fill the gaps presently limiting resolution of the ecosystem model they are developing. In particular, their work was hampered by the very large spatial scales over which they are sampling. The relatively confined area off east Australia that we sampled has allowed us to address fine-scale spatial and temporal variation through inclusion of seasonal sampling, which is missing from other studies. We used observer collections, a dedicated research cruise by the National Facility vessel, FRV Southern Surveyor, a series of laboratory studies and computer simulations, and collaborations with regional agencies such as the Secretariat for the Pacific Community Oceanic Fisheries Program and the Inter American Tropical Tuna Commission to construct a realistic understanding of the ecosystem/s of the fishery. We will identify gaps in knowledge for individual species, and for the community/ies in which the fishery operates. We will determine the linkages between these species, and provide, through modelled scenarios, how they would be impacted by variations in fishing pressure and climate change.

Literature cited Allain, V., Olson, R.J., Galvan-Magana, F. and Popp, B. (2001). Trophic structure and tuna movement in the equatorial Pacific pelagic ecosystem. PFRP Project 659559. Campbell, R.A. and Hobday, A.J. (2003). Swordfish – Seamount – Environment – Fishery interactions off eastern Australia. Working Paper BBRG-3 presented to the 16th meeting of the Standing Committee on Tunas and Billfish, held July 9– 17, Mooloolaba, Australia. Cox, S.P., Essington, T.E., Kitchell, J.F., Martell, S.J.D., Walters, C.J., Boggs, C. and Kaplan, I. (2002). Reconstructing ecosystem dynamics in the central Pacific Ocean, 1952–1998. II. A preliminary assessment of the trophic impacts of fishing and effects on tuna dynamics. Canadian Journal of Fisheries and Aquatic Sciences, 59, 1736–1747. Essington, T.E., Schindler, D.E., Olson, R.J., Kitchell, J.F., Boggs, C. and Hilborn, R. (2002). Alternative fisheries and the predation rate of yellowfin tuna in the eastern Pacific Ocean. Ecological Applications, 12, 724–734. Fulton, E., Smith, A., and Punt, A. (2005). Which ecological indicators can robustly detect effects of fishing. ICES Journal of Marine Science, 62, 540–51.

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Hobday, A.J., Smith, A.D.M. and Stobutzki, I. (2004). Ecological risk assessment for Australian Commonwealth fisheries, Final Report ‘Stage 1, hazard identification and preliminary risk assessment’, CSIRO Marine Research, Hobart. 174 pp. Lehody, P. (2001). The pelagic ecosystem of the tropical Pacific Ocean: dynamic spatial modelling and spatial consequences of ENSO. Progress in Oceanography, 49, 439–468. Myers, R. and Worm, B. (2003). Rapid worldwide depletion of predatory fish communities. Nature, 423, 280–282. Robins, C.M., Bradshaw, E.J. and Kreutz, D.C. (2007). Marine turtle mitigation in Australia's pelagic longline fishery / Canberra. Fisheries Research and Development Corporation, 2007. FRDC project; no. 2003/013. Sibert, J., Hampton, J., Kleiber, P. and Maunder, M. (2006). Biomass, size and trophic status of top predators in the Pacific Ocean. Science, 314, 1773–1776. Stevens, J.D., Bonfil, R., Dulvy, N.K. and Walker, P.A. (2000). The effects of fishing on sharks, rays, and chimaeras (chondrichthyes), and the implications for marine ecosystems. ICES Journal of Marine Science, 57, 476–494. Young, J.W., Bradford, R.W., Lamb, T.D. and Lyne, V.D. (1996). Biomass of zooplankton and micronekton in the southern bluefin tuna fishing grounds off eastern Tasmania, Australia. Marine Ecology Progress Series, 138, 1–14. Young, J.W., Lamb, T.D., Bradford, R.W., Clementson, L.A., Kloser, R.J. and Galea, H. (2001). Yellowfin tuna (Thunnus albacares) aggregations along the shelf break of southeastern Australia: links between inshore and offshore processes. Marine and Freshwater Research, 52, 463–474.

4.

Need

The ETBF is presently dealing separately with a number of ecological issues. For example, the impacts of longliners on seabirds and turtles are current high profile problems. However, other issues such as depletion of swordfish, SBT bycatch, availability of tunas in relation to oceanographic features, bycatch of sharks have all taken centre stage at different times in the short history of the domestic fishery. This approach is often reactionary, rather than strategic, and may not be the most effective use of research resources in the long-term. If the ETBF is to move substantially toward ecosystem-based fishery management, as it is required to do under the EPBC act, greater understanding and consideration of the broader ecosystem effects of fishing are required. If, for example, managers opt for time area closures as part of their management strategy to protect key species, developing these on a species-by-species basis has the potential to impact the whole fishery. To develop and evaluate time area closures that allow for minimizing risk of adverse impacts to the suite of key species, while allowing for optimal efficiency of fishing, understanding the associations, linkages and interactions between species is essential. This is the ecosystem approach. Developing an understanding of how ecosystem-associations relate to oceanographic features is also essential in open ocean systems where the dominant influence on 9


distribution and local abundance is oceanography. The approach we are taking will provide the information needed to support an ecosystem-based management framework. Through the analyses we are proposing we will identify regional “hot spots”, detail their linkages and provide detailed scenarios as to how we think different management strategies and fishing practices will or will not impact pelagic food chains, ecologically related non-target species, competitors (e.g. sharks, marlins etc.) and their associated ecosystems. The need for ecosystem-based fishery management for the Western Pacific region has also been supported by the equivalent of the present day Western Central Pacific Fishery Council (SCTBF Working Paper 9, 2002).

5.

Objectives

1.

Identify the spatial extent and the temporal stability of the main ecosystems of the eastern tuna and billfish fishery based on their species composition and physical environment. Define the trophic structure within these ecosystems with emphasis on the relationship between target, bycatch and threatened and protected species. Develop an ecosystem model for the ETBF fishery incorporating data on the relative abundance of the species, trophic linkages and the physical environment from which the impacts of longline fishing on the ecosystem can be investigated and from which alternative future management strategies can be evaluated.

2. 3.

10


6.

Physical and biological description of the ETBF

6.1 Biological oceanography of the fishing grounds of the Eastern Tuna and Billfish Fishery off eastern Australia J.W. Young^1, A.J. Hobday1, T.E. Ryan1, R.J. Kloser1, P.I. Bonham1, L.A. Clementson1 and M.J. Lansdell1 ^

Author to whom correspondence should be addressed. Tel. +61 3 6232 5360; fax: +61 3 6232 5012; email: [email protected] 1 Refer to Appendix 2 for author addresses.

Abstract Pelagic species, such as tuna and billfish, aggregate in particular oceanic areas where they are targeted in commercial fisheries. An understanding of the oceanographic environment in these aggregations is rare – but crucial to interpret exploitation patterns and fisheries sustainability. The biological oceanography of three major tuna fishing areas off eastern Australia – one inshore (depth 200–500 m), one over a major topographic feature (the Britannia Seamount, depth ~600–1000 m) and one offshore (depth >3,000 m) – was examined on a research voyage in September 2004. The inshore region was characterized by East Australia Current (EAC) waters which were noticeably warmer, less saline and lower in nutrients than waters to the east. These inshore waters were dominated by large diatoms, the biomass of which was significantly higher than in the seamount and offshore areas, apparently the result of a cold core eddy beneath the EAC surface filament. Net-based and acoustic biomass estimates were relatively higher at the surface than at the two other areas. Over the seamount and offshore more typical Tasman Sea waters prevailed, although the presence of a relatively deeper oxygen minimum layer over the seamount suggested topographically induced mixing in the area. However, nutrient data from CTD casts showed no evidence of seamount-induced upwelling suggesting only downward mixing of water (oxygen enrichment and relatively warmer water at depth), and not upward mixing of nutrient-rich water (seamount barrier). Notably, subsurface zooplankton biomass, dominated by salps and chaetognaths, was significantly higher around the seamount than in the two other areas, suggesting aggregation. Micronekton biomass was significantly higher along the flanks of the seamount than at equivalent depths at other sites. The offshore region was characterised by frontal activity associated with the Tasman front. Micronekton biomass was generally highest in surface waters in this region. Differences in micronekton species composition were also noted between areas. In particular, the myctophid Ceratoscopelus warmingii was significantly more abundant in offshore waters than elsewhere. These three areas, separated spatially by hundreds of kilometres, showed distinct oceanographic and biological characteristics, including

11


overall biomass and species differences. The processes underlying the productivity of these areas, although fundamentally different, provide the basis for distinct food webs, which all lead to aggregations of tunas and other top predators that are subsequently exploited by fishers.

“Our study underlined the importance of mesoscale features, particularly seamounts and eddies, in providing the productivity needed to support top predators”

12


Introduction The Eastern Tuna and Billfish Fishery (ETBF) is a wide-ranging fishery that at different times operates along the length of eastern Australia and seaward beyond the limits of the Exclusive Economic Zone (EEZ; Figure 1). However, the majority of the present catch is taken between 25°S–30°S, between the continental shelf and the Lord Howe Rise. It targets a range of tuna and billfish species but also a wide range of other species in the region. The rapid development of this fishery has meant that much of the research needed to adequately manage the fishery is either underway or yet to be done. There is a strong need for this research, not only to understand the impacts of fishing on the ecosystem but also to understand the highly variable spatial and temporal fluctuation in the catch (e.g. Campbell, 1999). Much of the understanding of these fluctuations has been derived from spatial analyses of catch effort but with less attention to oceanographic influences. More recently, efforts have been made using satellite data to relate fish catches to the surface oceanography (Campbell and Hobday, 2003). Without a greater understanding of the intermediate links in the relationship between surface physics and the top predators, however, explanations for the observed catch fluctuations are likely to be limited. The importance of the subsurface environment has not been adequately addressed, particularly in relation to the processes leading to concentrations of the larger predators. Although there is a significant body of literature on the vertical physical structure of the offshore waters of the Coral and Tasman Sea (e.g. Cresswell and Legeckis, 1986; Ridgway and Dunn, 2003) there have been relatively few biological studies in the region (e.g. Brandt, 1981; Tranter et al., 1986), particularly in recent times (Baird et al., 2008). The patterns of pelagic fish aggregation, visualised as captures in the fishery, suggest these fishes are responding to their environment in potentially predictable ways. In the northern hemisphere studies of large pelagic fish distributions identified associations with a number of physical variables, particularly seabed topography, fronts and sea surface temperature (e.g. Fiedler and Barnard, 1987; Podesta et al., 1993; Bigelow et al., 1999; Sedberry and Loeffer, 2001). We found similar associations off eastern Australia and were able to also identify sea surface salinity and fluorescence as factors associated with captures of swordfish and tropical tuna (Young et al., 2000; Young et al., 2001). However, there have been relatively few studies linking subsurface structure with the distribution of these fishes (although see Evans et al., 2005). The development of satellite altimetry and its potential to distinguish subsurface structure has such a potential (Polovina et al., 2001), but its interpretation will be greatly assisted by in situ studies. Our objectives, firstly, were to characterise the physical ocean habitat and biological community structure of the region to the level of potential prey species for predators targeted by the fishery. Three areas targeted by the fishery, the inshore fishery just south of Fraser Island, the Tasmantid Seamounts and the Lord Howe Rise, were studied 13


(Figure 1). We were interested in determining whether the physical and biological environment of the three areas was similar or if there were differences in physical pelagic habitat and community structure that could be traced to differences in physical and biological oceanography, seabed topography (seamounts) or some other factor. Second, we aimed to provide quantitative estimates of the biomass of these lower trophic levels for use in quantitative models of the ETBF ecosystem.

Figure 1: Study area showing sampling positions occupied during a research voyage to the main fishing area of the Eastern Tuna and Billfish Fishery (1–16 September, 2004). Symbols indicate the sampling methods.

Methods Physical oceanography In September 2004 we compared the biological and physical oceanography of three regions frequented by longliners within the Australia EEZ using the Marine National Facility FRV Southern Surveyor (Figure 1). The vertical structure of the regional oceanography was described from a series of hydrographic transects to 500 m in each area (Figure 1). On each transect, casts of CTDs (General Oceanics Mark IIIC CTD with a General Oceanics 12 bottle rosette and SeaTech fluorometer mounted on the frame) were made at ~10 nmi intervals to record temperature, salinity and fluorescence to a depth of 500 m.

Phytoplankton composition, biomass and productivity The composition and concentration of phytoplankton and their pigments were determined from 10-litre Niskin bottles taken during each CTD cast. Samples were also

14


incubated at sea following the methods of Waite et al. (2007) to determine primary productivity. In the laboratory, phytoplankton species composition was determined from samples that were concentrated to a 1 ml aliquot following Hötzel and Croome (1998), and examined microscopically with a phase contrast facility, and imaging software. Counts were made to genus or species level in the microplankton (i.e. above 20 μm diameter) and to class level in the nanoplankton (2–20 μm). Additionally, totals of diatoms, dinoflagellates, small flagellates and microzooplankton grazers were enumerated. The biomass (volumes) of counted cells was estimated from standard geometries detailed in Hillebrand et al. (1999). An estimate of total counts and biomass of taxa >10 μm, and between 2–10 μm was also made. Pigment composition was obtained by HPLC following a modified version of the Van Heukelem and Thomas (2001) method. Pigments were identified by retention time and absorption spectrum from a photo-diode array (PDA) detector and concentrations of pigments were determined from commercial and international standards (Sigma, USA; DHI, Denmark). Chlorophyll biomass (Cchl) was determined by trapezoidal integration (Waite et al., 2007). Standing phytoplankton biomass (Bphy ; t.km-2) was calculated following Bulman et al. (2006) as: Bphy = 40 x 20 x 10-3 x h x Cchl , where h is the mixed layer depth (m), Cchl is the integrated chlorophyll concentration (mg.m-2), and the factor of 10-3 converts mg.m-2 to t.km-2. To estimate primary production, carbon uptake was calculated using the method of Harris et al. (1987).

Macrozooplankton and micronekton net sampling Sampling for these intermediate trophic levels followed protocols described in Young et al. (1996). Briefly, the macrozooplankton was sampled during the day with bongo nets (mesh size 500 μm) fished obliquely to 200 m depth and were complimented by simultaneous surface tows. Flowmeters were used to determine water flow through the nets. Micronekton was sampled at night with a midwater trawl fitted with an opening/closing codend (MiDOC net). The codend used an electronic timer to fire nets at pre-set times. Depth, mouth opening, headline height and board spread of the trawl were monitored acoustically. Sampling consisted of a 40 minute oblique tow to 600 m, followed by 15 minute depth stratified tows at 600–400, 400–300, 300–200, 200–100 and 100–0 metres (Figure 2). Micronekton samples were sorted on board into major taxa (Myctophidae, other fishes, crustacea, squid and gelatinous zooplankton), photographed, weighed (± 0.05 g) and frozen. In the laboratory, micronekton samples were sorted to the lowest possible taxon and enumerated. Data was expressed as number per 10-5.m-3. The volume filtered by the MiDOC nets was calculated by the equation V = S•d•A, where S = ship's speed (m.s-1), d = duration of tow (in seconds) and A = net mouth opening (m-2). For all statistical 15


analyses the data were transformed (loge(standardised total wet weight)) to reduce the variance between the residuals. One- and two-way analysis of variance (ANOVA) was used to test for differences between areas, depths and their interaction. If significant, (P 1% of the abundance of the Eastern Tuna and Billfish Fishery (ETBF) catch. Collectively, they constitute ~90% of the pelagic fish biomass found in the region (Dambacher, 2005). In general terms these species occupy the same pelagic habitat. All are found mainly within the upper 500 m of the water column and often overlap spatially, as evidenced by multispecies longline capture records (Campbell and Young, 2008). Each, however, has their own environmental preferences with individual species associated with a range of environmental conditions (Bigelow et al., 1999). For example, yellowfin tuna are usually associated with warmer East Australia Current waters (Young et al. 2001), southern bluefin tuna with colder waters (Farley and Davis, 1997) and swordfish with seamounts (Sedberry and Loeffer, 2001; Campbell and Hobday, 2003). Dolphin 102


fish are found above the thermocline (Lansdell and Young, 2007) and adult albacore in the region spend most of their time below the thermocline (S. Hall, AFMA Observer program, pers. comm.). Nevertheless, these pelagic fishes generally feed on available prey which is mostly concentrated in the scattering layer (Bertrand et al., 2002; Kloser et al., this volume). Generally, open ocean ecosystems have limited productivity (Pauly and Christensen, 1995). The region of the ETBF is particularly limited as it is dominated by oligotrophic Coral Sea water (Condie and Dunn, 2006) and as such is likely to provide limited prey resources for predators. With these limitations how do the predators divide the available prey? Is it through differences in prey type or size, or at the depth or time at which they feed, or some other factor (Pusineri et al., 2008). In this study we aimed to distinguish whether distinct trophic assemblages of species or species groupings existed using a variety of trophic and ecological measures to understand how these predators share the pelagic ecosystem in the ETBF. We also examined the vertical distributions and feeding cycles of a subset of these species in the region to see whether there were spatial or temporal differences in their feeding behaviour which could distinguish possible feeding strategies. Finally, we estimated the daily ration for these species as such information is an important requirement for quantitative models.

Methods Data collection Stomachs of fish species of target and bycatch fishes were collected at sea on tuna longliners operating off eastern Australia between latitudes 25°S and 42 ºS between 1992 and 2006 (Figure 1, Table 1). Details of fish length, position and date of capture were included with each sample. All samples were frozen at sea and then transported to the laboratory where they were thawed for analysis. To examine diel feeding cycles and to support the estimation of daily ration, hook timers were set on longlines on 43 separate commercial longliner trips between 2004 and 2006 (Campbell and Young, 2008) (Table 2). Hook timers were set along the longline to depths of 300 m and recorded the depth and time when a fish struck a bait.

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Figure 1: Positions of sample collection from longline vessels operating within the Eastern Tuna and Billfish Fishery (ETBF) off eastern Australia. Light shaded dots represent individual fishing days between 1992–2006 rather than specific fish taxa or numbers captured. Dark shaded diamonds represent locations where at least one fish was caught on a hook-timer.

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Table 1: Fish species examined off eastern Australia. No. = total number of specimens analysed; mean length ± standard error (S.E.); length range; number of foreguts with food, number empty.

Species

Common Name

Code

No.

Thunnus alalunga Alepisaurus spp. Thunnus obesus Prionace glauca Coryphaena hippurus Tetrapturus audax Thunnus maccoyii Isurus oxyrinchus Xiphias gladius Thunnus albacares Total

Albacore Lancetfish Bigeye tuna Blue shark Dolphinfish Striped marlin Southern Bluefin tuna Shortfin Mako shark Swordfish Yellowfin tuna

ALB ALX BET BSH DOL MLS SBF SMA SWO YFT

71 149 184 147 118 99 1364 26 642 763 3563

Mean length (mm)

S.E. (mm)

905 934 1256 1374 1176 1878 1226 1504 1476 1283

12 15 16 37 15 22 8 108 14 7

Length range (mm) Min. Max. 630 390 680 450 500 1077 400 660 600 710

1130 1480 1820 3000 1780 2538 2840 2570 2700 2140

Non-empty stomachs

Empty Stomachs

46 114 151 108 95 83 1143 17 440 651

25 35 33 39 23 16 221 9 202 112

Length measurements: billfish (OFL, orbital fork length); others (LCF, length to caudal fork) Table 2: Species and number for which hook timer data were recorded off eastern Australia between 2004 and 2006 Species Swordfish Albacore Striped marlin Dolphin fish Yellowfin tuna Bigeye tuna Blue shark Lancet fish Total

N 51 138 14 24 68 20 9 4 325

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Laboratory analysis The diets of all fish that contributed >1% of the total estimated catch reported by independent observers were examined (Dambacher, 2005). Each stomach was given an estimate of fullness ranging from empty (1) to full (5) (Young et al., 1997), weighed (g) and photographed. Individual prey items were then identified to the lowest possible taxon, weighed, and counted. For each prey, depending on digestion stage, we recorded standard length for fish and mantle length for cephalopods (estimated length for digested individuals). Fish were identified from the taxonomic keys detailed in Young et al. (2006). Hard parts (beaks and otoliths), once identified were enumerated and their reconstituted mass included in later analyses. The addition of the reconstituted weights from hard parts follows a growing body of evidence that hard parts are part of the same daily feeding period as flesh (e.g. Jobling and Breiby, 1986; Christensen et al., 2005).

Data analysis We initially investigated feeding across the ten predator species in terms of (logtransformed) prey biomass in relation to predator species, predator length, season (summer, October to March; winter, April to September) and position of capture, SSF (µg l-1), SST (°C), using linear regression models from the statistical package S-Plus (see Young et al., 2006). We found that predator type (DF = 9, F = 14.4, P 32°S). Significant differences were tested using Wilcoxon signed rank tests (Siegel, 1956). The relationship between the lengths of predator and their prey were initially compared using Komolgorov Smirnov tests on cumulative percent frequencies of prey lengths.

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To assess predator-length prey-length relationships we used quantile regression techniques (see Scharf et al., 1998, 2000; Juanes et al., 2002). We first quantified lower bound, median and upper bound relationships of prey length-predator length scatters for each species. The choice of which quantiles to use to represent upper and lower bounds is based on sample size (Scharf et al., 1998). We compared slopes using a modified t-test (Scharf et al., 2000; Juanes, 2003). We then quantified ontogenetic shifts in relative prey sizes (prey length/predator length) by estimating the lower and upper bounds of relative prey length versus predator length scatters. Differences in the slopes of upper and lower edges indicate ontogenetic increases or decreases (depending on whether significant upper and lower bounds were diverging or converging) in trophic niche breadth (Scharf et al., 2000; Juanes, 2003; Bethea et al., 2004). To compare across species we calculated average trophic niche breadths as the average difference between upper and lower edges for each predator size and regressed average trophic niche breadth versus average predator length (Scharf et al., 2000). Diel feeding cycles were examined using methodology in Campbell and Young (2008). Actual time spent feeding is an essential component in the estimation of daily ration. However, for longline-caught fish in particular, this is a difficult parameter to estimate as time of fish landing will often bare no relationship to fish capture so the use of digestion rates or capture time is likely to be misleading. Hook timer data (Boggs, 1992) offer the most direct record of when a fish is likely to feed in longline-caught fish. Observed feeding times were adjusted according to effort and haul time using methodology outlined in Campbell and Young (2008). Daily ration was then estimated following procedures outlined in Olson and Galvan-Magana (2002) and Griffiths et al. (2007).

Results 3,563 stomachs from the ten most abundant large pelagic fish species off eastern Australia were examined from fish sampled between 1992 and 2006 (Table 1). Hook timer data from a further 325 fish were collected between 2004 and 2006 to establish feeding times and depths for the predator species over the diel period (Table 2).

Prey composition The fish predators examined consumed a diverse range of micronekton prey composed of fish, squid and occasionally crustacea (Figure 2, Tables 3 and 4). Although not restricted to one or other prey grouping there were two broad groupings of mainly fish or mainly squid predators. Surprisingly, the bulk of the predators were mainly piscivorous. In predators 100 cm albacore and swordfish were the only two predators with a mainly squid diet. Although not comprising a large proportion by weight, crustacean prey consistently occurred in a number of predator species indicating they were also an important dietary component, particularly for albacore and yellowfin tuna.

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100

Teleost Scombridae Carangidae Tetraodontidae Myctophidae Bramidae Alepisauridae Scomberesocidae Nomeidae Exocoetidae Other Teleostei

Prey Proportion (% Weight)

80

60

40

Mollusc Other Cephalopoda Carinariidae Argonautidae Histioteuthidae Ommastrephidae

20

0 ALO/X

DOL

SMA

YFT

MLS

SBF

BET

BSH

ALB

SWO

Predator Species

Figure 2: Relative proportions of prey families contributing >1% wet weight to prey biomass for fish predators off eastern Australia.

Table 3: Relative proportions of the major prey taxa by weight (% Wt) and occurrence (%O) for the major predators off eastern Australia. A) Predators < 100 cm; B) Predators 100+ cm. A) Crustacean Mollusca Teleostei Predator species %Wt %O %Wt %O %Wt %O ALB 11.68 63.89 52.32 63.89 26.61 66.67 ALX 7.40 69.51 20.21 75.61 69.75 75.61 BET 4.61 21.05 39.68 52.63 55.49 73.68 BSH 0.04 12.50 75.19 84.38 24.27 50.00 DOL 5.04 11.11 24.26 55.56 70.70 66.67 SBF 0.67 23.49 22.39 33.81 76.82 82.21 SWO 1.88 19.44 11.08 36.11 85.61 77.78 YFT 1.76 38.00 11.00 56.00 87.18 86.00 * MLS and SMA