Seabird–sockeye salmon co-variation in the eastern Bering Sea: Phenology as an ecosystem indicator and salmonid predictor?

Abstract

Seabirds (Rissa spp. and Uria spp.) and sockeye salmon (Onchorhynchus nerka) of the eastern Bering Sea share similarities in their trophic ecology. We tested the role of seabirds as indicators of food web conditions that affect sockeye salmon at sea survival by investigating co-variation between seabirds breeding on the Pribilof Islands and returns of Bristol Bay sockeyes. We examined seabird phenology (hatching dates of eggs) against sockeye returns based on the year of ocean entry. Annual seabird hatching date was inversely related to sockeye returns, with the strongest co-variation found for sockeye which entered the ocean at 2 years of age (age 2x smolts). The mechanism supporting this covariation is unknown, but both birds and salmon may be responding to changes in prey availability (a ‘‘bottom-up’’ effect). The co-variation between seabird hatching date and sockeye returns supports the idea that variation in seabird breeding parameters indicates food web conditions that also affect other upper trophic level predators in marine systems. Coupling seabird phenology with existing annual predictions for Bristol Bay salmon may improve forecasts and fishery management.