Alternative fisheries and the predation rate of yellowfin tuna in the eastern pacific ocean

Apex predators in pelagic ecosystems may play key roles in determining food web structure and ecosystem dynamics. Commercial fisheries can thereby have large effects on pelagic ecosystems by selectively harvesting apex predators. We assessed the potential ecological consequences of fishing due to alternative harvest practices currently employed in the capture of yellowfin tuna (Thunnus albacares) in the eastern Pacific Ocean (EPO). Specifically, we estimated the demand on the prey resources consumed by yellowfin tuna and how this demand is altered by three methods of purse seining and by long‐lining. These fishing techniques are contrasted by stark differences in age selectivity; longline and purse seine sets on yellowfin associated with dolphins (“dolphin sets”) capture old fish, whereas purse seine sets on free‐swimming tuna schools or floating objects (“dolphin‐safe sets”) capture young yellowfin. Yellowfin tuna predation rates were estimated from a bioenergetics model coupled with a population model based on natural mortality estimates and age‐specific vulnerabilities for each gear type. Maximum equilibrium fishery catch (measured in megagrams per year; 1 Mg = 1 metric ton) was lowest for dolphin‐safe sets and highest for dolphin sets and longline fishing. Moreover, the present combined fishery yield could not be sustained by either of the dolphin‐safe sets, but could be sustained with either dolphin sets or long‐lining. The predation rate of the yellowfin population was highly sensitive to the magnitude and type of fishing mortality: yellowfin predation rate declined by ∼119 Mg per megagram of floating object set catch, 72 Mg per megagram of school fish set catch. In contrast, predation declined by only 34 Mg and 17 Mg per megagram of dolphin set and longline catch, respectively. When considering all fishing techniques simultaneously in proportion to the present fishery, we estimated that total yellowfin predation is presently only one‐third the magnitude expected from an unfished population. This reduction in predation was unevenly distributed among prey items; the prey of large yellowfin experienced 80% less predation, whereas the prey of small yellowfin experienced only 50% less predation, compared to an unfished population. We conclude that fisheries can have large effects on trophic linkages in pelagic food webs even when fishing rates are sustainable, and that these ecological effects vary considerably for different types of fishing gear and fishing practices.