Incorporating ecosystem forcing through predation into a management strategy evaluation for the Gulf of Alaska walleye pollock (Theragra chalcogramma) fishery

An emerging goal of national and international fisheries management organizations is the development of an ecosystem-based approach to fisheries management. The mechanisms determining changes in an ecosystem and their impacts on marine populations are not well established for most species, but the expectations of managers are that these impacts should be accounted for when selecting management strategies. Predation is an important component of ecosystem dynamics, and predator–prey functional responses can be used to represent predation in multi-species population dynamics models. The predators of walleye pollock, Theragra chalcogramma, in the Gulf of Alaska include arrowtooth flounder (Atheresthes stomias), Pacific cod (Gadus macrocephalus), and Pacific halibut (Hippoglossus stenolepis). Management strategy evaluation is used to examine the impact of changes over time in natural mortality-at-age related to trends in predator abundance on the performance of the current management strategy for the fishery for walleye pollock in the Gulf of Alaska. The current management strategy is evaluated under nine scenarios which capture uncertainty regarding the predator–prey functional relationship and the level of future fishing mortality on the predators. The current management strategy, which ignores the possibility of changes over time in natural mortality, kept the spawning biomass high relative to the target level and had a low risk of fishing mortality exceeding the overfishing limit, due to low catches relative to the total biomass.