Pacific herring (Clupea pallasii) as a key forage fish in the southeastern Gulf of Alaska

Abstract

Pacific herring (Clupea pallasii) is a schooling planktivorous fish consumed by numerous fish, seabirds, and marine mammals. This paper aimed to determine whether Pacific herring serves as a key forage fish (i.e. strongly supports predator populations) in the southeastern Gulf of Alaska. All analyses were conducted using mass- and energy-balanced ecosystem models constructed in Ecopath with Ecosim. Supportive Role to Fishery (SURF) index values were computed using predator diets and food web structure encoded in static ecosystem models. Ecosystem impacts of herring stock depletion and collapse were evaluated using quantitative criteria (thresholds) applied to dynamic ecosystem simulations. SURF index values from mass-balanced models lay below the threshold required to designate herring as a key forage fish. However, values from an energy-balanced model supported the key status of herring. Dynamic ecosystem simulations in mass- and energy-balanced models revealed strong negative effects of herring depletion on several predators. In most energy-balanced models, simulation results designated herring as a key forage fish despite indications of functional redundancy in the forage fish guild. Impacts of herring depletion on predators were stronger and more numerous in energy-balanced models, suggesting that the high energy content of herring enhances its importance to predators. Simulation results also demonstrated positive impacts of herring depletion on two zooplankton groups due to release from predation pressure. The status of Pacific herring as a key forage fish apparently depends on its energy content relative to other forage fish. Nevertheless, the results of this study support precautionary, ecosystem-based management of Pacific herring fisheries.