Prey switching and consumption by seabirds in the central California Current upwelling ecosystem: Implications for forage fish management

Effective ecosystem-based fishery management involves assessment of foraging interactions among consumers, including upper level predators such as marine birds and humans. Of particular value is information on predator energetic and consumption demands and how they vary in response to the often volatile dynamics of forage populations, as well as the factors that affect forage availability and potential prey switching. We examined the prey requirements of common murre (Uria aalge), Brandt's cormorant (Phalacrocorax penicillatus), and rhinoceros auklet (Cerorhinca monocerata) in the central California Current over a 30-year period, 1986-2015. We developed a bioenergetics model that incorporates species-specific values for daily basic energy needs, diet composition, energy content of prey items and assimilation efficiency, and then projected results relative to stock size and levels of commercial take of several species. The most common forage species consumed were juvenile rockfish (Sebastes spp.), northern anchovy (Engraulis mordax), smelt (Osmeridae), and market squid (Doryteuthis opalescens). Total biomass of forage species consumed during the breeding season varied annually from 8500 to >60,000 metric ton (t). Predator population size and diet composition had the greatest influence on overall prey consumption. The most numerous forage species consumed in a given year was related to abundance estimates of forage species derived from an independent ecosystem assessment survey within the central place foraging range of breeding avian predators. The energy density of dominant prey consumed annually affected predator energy expenditure during chick rearing and whether prey switching was required. Increased forage species take by predators, as revealed by seabirds, may be adding consumptive pressure to key forage fish populations, regardless of the potential additional impacts of commercial fisheries. Improving estimates of consumption by predators and fisheries will promote more effective management from an ecosystem perspective.