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

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.

[1]  Kerim Aydin,et al.  Physical forcing and the dynamics of the pelagic ecosystem in the eastern tropical Pacific: simulations with ENSO-scale and global-warming climate drivers , 2003 .

[2]  Carl J. Walters,et al.  Cultivation/depensation effects on juvenile survival and recruitment: implications for the theory of fishing , 2001 .

[3]  A. Wright,et al.  Drift-net fishing in the South Pacific: From controversy to management , 1991 .

[4]  D. Pauly,et al.  Fishing down marine food webs , 1998, Science.

[5]  Ransom A. Myers,et al.  Collapse and Conservation of Shark Populations in the Northwest Atlantic , 2003, Science.

[6]  Michael J. Fogarty,et al.  LARGE-SCALE DISTURBANCE AND THE STRUCTURE OF MARINE SYSTEMS: FISHERY IMPACTS ON GEORGES BANK , 1998 .

[7]  Steven J. D. Martell,et al.  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 , 2002 .

[8]  K. Bjorndal,et al.  Historical Overfishing and the Recent Collapse of Coastal Ecosystems , 2001, Science.

[9]  D. Kulka,et al.  Was the Increase in Shrimp Biomass on the Northeast Newfoundland Shelf a Consequence of a Release in Predation Pressure from Cod , 2000 .

[10]  H..  Depth , capture time , and hooked longevity of longline-caught pelagic fish : Timing bites of fish with chips , 2004 .

[11]  John D. Stevens,et al.  The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems , 2000 .

[12]  M. Tegner,et al.  Ecosystem effects of fishing in kelp forest communities , 2000 .

[13]  S Rogers,et al.  The effects of fishing on marine ecosystems and communities , 2000 .

[14]  Robert J. Olson,et al.  A model of the pelagic ecossystem in the eastern tropical Pacific Ocean , 2003 .

[15]  Carl J. Walters,et al.  Adaptive Management of Renewable Resources , 1986 .

[16]  Villy Christensen,et al.  Representing Density Dependent Consequences of Life History Strategies in Aquatic Ecosystems: EcoSim II , 2000, Ecosystems.

[17]  C. Walters,et al.  Reconstructing ecosystem dynamics in the central Pacific Ocean, 1952-1998. I. Estimating population biomass and recruitment of tunas and billfishes , 2002 .

[18]  B. Worm,et al.  Rapid worldwide depletion of predatory fish communities , 2003, Nature.

[19]  C. Walters.  Folly and fantasy in the analysis of spatial catch rate data , 2003 .

[20]  R. Hilborn,et al.  ALTERNATIVE FISHERIES AND THE PREDATION RATE OF YELLOWFIN TUNA IN THE EASTERN PACIFIC OCEAN , 2002 .

[21]  M. Hall.  An ecological view of the tuna--dolphin problem: impacts and trade-offs , 1998, Reviews in Fish Biology and Fisheries.

[22]  Villy Christensen,et al.  ECOPATH II − a software for balancing steady-state ecosystem models and calculating network characteristics , 1992 .

[23]  A. H. Richards.  Problems of drift-net fisheries in the South Pacific , 1994 .

[24]  G. Daskalov,et al.  Overfishing drives a trophic cascade in the Black Sea , 2002 .

[25]  R. Hilborn,et al.  SHARKS AND TUNAS: FISHERIES IMPACTS ON PREDATORS WITH CONTRASTING LIFE HISTORIES , 2002 .

[26]  K. Frank,et al.  Ecosystem changes and the effects on capelin (Mallotus villosus), a major forage species , 2001 .

[27]  S. Yaffee.  Three Faces of Ecosystem Management , 1999 .