Project No. 2002/028 June 2006 Fisheries Research and Development Corporation • CSIRO Marine and Atmospheric Research Trophic dynamics of the eastern shelf and slope of the south east fishery: impacts of and on the fishery • C. Bulman • S. Condie • D. Furlani • M. Cahill • N. Klaer • S. Goldsworthy • I. Knuckey Trophic dynamics of the eastern shelf and slope of the South East Fishery: impacts of and on the fishery C. Bulman, S. Condie, D. Furlani, M. Cahill, N. Klaer, S. Goldsworthy and I. Knuckey Project No. 2002/028 June 2006 Final Report for Fisheries Research & Development Corporation ii TROPHIC DYNAMICS OF THE EASTERN SHELF AND SLOPE FISHERY Enquiries should be addressed to: Dr Catherine Bulman CSIRO Marine and Atmospheric Research GPO Box 1538 Hobart Tasmania 7001 Copyright Fisheries Research and Development Corporation and Commonwealth Scientific and Industrial Research Organisation (‘CSIRO’) Australia 2005. 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 results and analyses contained in this Report are based on a number of technical, circumstantial or otherwise specified assumptions and parameters. The user must make its own assessment of the suitability for its use of the information or material contained in or generated from the Report. To the extent permitted by law, CSIRO excludes all liability to any party for expenses, losses, damages and costs arising directly or indirectly from using this Report. The Report must not be used as a means of endorsement without the prior written consent of CSIRO. The name, trade mark or logo of CSIRO must not be used without the prior written consent of CSIRO. 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. National Library of Australia Cataloguing-in-Publication Trophic dynamics of the eastern shelf and slope of the South East Fishery: impacts of and on the fishery. ISBN 1 921061 14 6 (pbk.) ISBN 1 921061 15 4 (pdf.) 1. Fishes - Ecology - Australia, Southeastern. 2. Fisheries - Australia, Southeastern. 3. Fishery resources - Australia, Southeastern. 4. Fishery management - Australia, Southeastern. I. Bulman, Cathy. II. CSIRO. Marine and Atmospheric Research. 333.9560994 FRDC Final Report 2002/028 NON TECHNICAL SUMMARY i NON TECHNICAL SUMMARY 2002/028 Trophic dynamics of the eastern shelf and slope of the South East Fishery: impacts of and on the fishery PRINCIPAL INVESTIGATOR: Dr Catherine Bulman ADDRESS: CSIRO Marine and Atmospheric Research GPO Box 1538 Hobart TAS 7001 p: 02 6232 5222 f: 02 6232 5000 OBJECTIVES: 1. Develop circulation and trophic models to describe the past and present structure and dynamics of the food web on the eastern shelf and slope of the South East Fishery, the impacts of variability in primary production on catches, and to predict future changes in response to recovery of marine mammals and major reductions in discarding. 2. Provide a quantitative assessment of food web related risks, in support of strategic assessment of the fishery under the Environmental Protection and Biodiversity Conservation Act. 3. Contribute to a regional ecosystem model for use in the National Oceans Office’s Regional Marine Plan for the South East, including detailed scoping and preliminary trophodynamic models for the Eastern Bass Strait Shelf. NON TECHNICAL SUMMARY: OUTCOMES ACHIEVED 1. A synthesis of existing knowledge about the trophic dynamics of the SEF ecosystem which was achieved through the collation of data from (i) targetted trophic studies and other physical, biological and oceanographic studies within the study area and broader South East Fishery zone; (ii) studies on the same or closely related species within other regions; and (iii) other ecosystem models with similar characteristics. 2. Identification of potential ecosystem responses to (i) changes in environmental conditions particularly through their impact on primary production; (ii) current trends in the recovery of seal populations; (iii) reductions in fishery bycatch; and (iv) changes in fishing effort. 3. Key tools and information to support the SEF in establishing an ecosystem-based approach to management of the fishery and satisfying EPBC requirements. FRDC Final Report 2002/028 ii TROPHIC DYNAMICS OF THE EASTERN SHELF AND SLOPE FISHERY Until recently, trophic modelling and ecological risk assessment have not been utilised routinely in fisheries management. Two factors have limited the application of such methods: the first has been the paucity of data, understanding, and tools to undertake such assessments, and the second has been a lack of perceived importance of assessing the trophic impacts of fishing. Now the data and tools have improved, and there has been a general recognition of the need to extend assessment of impacts of fishing beyond the immediate impacts on target species. Of all the fisheries in Australia, the South East Fishery (SEF) is best placed to undertake such assessments, thanks mainly to a long history of ecological research that has gone beyond a focus on target species. Much of this work was summarized recently in a special edition of Marine and Freshwater Research (2001 Vol 52). The overall objective of this study was to develop trophic models that describe the past and present structure and dynamics of the food web of the south-east Australian shelf around Eastern Bass Strait (EBS). Early results were provided to the National Oceans Office’s Regional Marine Plan for the South East Marine Region. The models were used to better understand the complex trophic interactions operating on the shelf and upper slope. While not yet capable of predicting precise magnitudes of change, they provide a means of identifying potential ecosystem responses to changes in environmental conditions or human activities. At the foundation of any food web are the primary producers, which place an upper limit on the energy available to other trophic groups. We used satellite ocean colour data to estimate phytoplankton biomass and primary productivity in the EBS model area. These estimates were used to constrain the primary production parameters and impose patterns of seasonal and interannual variability onto the dynamics of the trophic model. Primary productivity in the area was found to support the modelled ecosystem without the need to rely on importation of organisms such as phytoplankton and zooplankton by ocean currents. The small mesopelagic fishes that support the slope and shelf break fishes were imported into the area largely through the process of diel vertical migration causing them to be washed up onto the shelf. While the model demonstrates the linkages between primary production and commercial fish species, the trophic pathways are complex and no strong direct correlation could be detected between satellite-estimated primary production and fishery catches. The trophic model was developed primarily around the fishes of the commercial fisheries and the major vertebrate groups in the ecosystem including marine mammals and birds. Studies of the Australian seal populations suggest that they are doubling every 10 years, causing some fishers to express concern. We used the models to predict broad community responses to variations in the rate of seal population growth. Although seals are voracious feeders and highly visible, they are found to consume only a small proportion of the total consumption of fish in the system. Simulation results suggest that while higher seal numbers coincide with declines of their prey fish species, some target commercial species actually increase when seals increase. We also investigated how the biological community might respond to increased or decreased fishing pressures. Nearly all fisheries were predicted to have lower catches in the future, even if the fishing rate was increased. This was because fishing rate, F, is the ratio of catch to biomass, therefore, if F is constant, catches will decline as biomass declines. If biomass continues to decline, then larger F rates might not result necessarily in larger catches. Effort has declined over the past 10 years in most fisheries except the Commonwealth trawl fishery where it has risen. This has released fishing pressure on many species allowing some recovery. However this recovery was not necessarily sufficient to result in bigger predicted catches in the future compared to the current catches even if effort was increased. This result suggests that some stocks are currently over-exploited. FRDC Final Report 2002/028 NON TECHNICAL SUMMARY iii Eliminating discarding by retaining all bycatch appeared to have little effect on the fish populations since the model assumed that bycatch whether discarded or landed was no longer part of the living system. We were not able to determine effects on the detrital sub-system. Early last century, intensive sealing and whaling operations severely depleted these populations, while fish populations were largely unexploited. When the trophic model was recalibrated for these historical conditions with estimated pre-fishing abundances