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Visualizing the Food-Web Effects of Fishing for Tunas in the Pacific Ocean

Jefferson T Hinke, Pacific Fisheries Environmental Laboratory and Joint Institute for Marine and At
Isaac C Kaplan, Center for Limnology, University of Wisconsin
Kerim Aydin, Alaska Fisheries Science Center
George M Watters, Pacific Fisheries Environmental Laboratory
Robert J Olson, Inter-American Tropical Tuna Commission
James F. K. Kitchell, Center for Limnology, University of Wisconsin

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Abstract

We use food-web models to develop visualizations to compare and evaluate the interactions of tuna fisheries with their supporting food webs in the eastern tropical Pacific (ETP) and the central north Pacific (CNP) Oceans. In the ETP and CNP models, individual fisheries use slightly different food webs that are defined by the assemblage of targeted tuna species. Distinct energy pathways are required to support different tuna species and, consequently, the specific fisheries that target different tuna assemblages. These simulations suggest that catches of tunas, sharks, and billfishes have lowered the biomass of the upper trophic levels in both systems, whereas increases in intermediate and lower trophic level animals have accompanied the decline of top predators. Trade-offs between fishing and predation mortality rates that occur when multiple fisheries interact with their respective food webs may lead to smaller changes in biomass than if only the effect of a single fishery is considered. Historical simulations and hypothetical management scenarios further demonstrate that the effects of longline and purse seine fisheries have been strongest in upper trophic levels, but that lower trophic levels may respond more strongly to purse-seine fisheries. The apex predator guild has responded most strongly to longlining. Simulations of alternative management strategies that attempt to rebuild shark and billfish populations in each ecosystem reveal that (1) changes in longlining more effectively recover top predator populations than do changes in purse seining and (2) restrictions on both shallow-set longline gear and shark finning may do more to recover top predators than do simple reductions in fishing effort.

Key words

ecosystem modeling, food webs, longline fishing, purse-seine fishing, tunas, trophic levels, Pacific Ocean
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Ecology and Society. ISSN: 1708-3087