Abstract
Large population declines (>50%) since the early-1970s of some eastern Bering Sea and Aleutian Island apex predator populations (Steller sea lions, murres, and kittiwakes) suggest that major changes have occurred in the structure of the Bering Sea ecosystem. One cause of the decline in mammalian and avian predator populations may be a decrease in availability or abundance of preferred prey (e.g. capelin, juvenile walleye pollock). However, adult groundfish biomass has generally been at high levels, and periods of high adult groundfish biomass coincide with periods of decline of sea lion and seabird populations. Adult groundfish may, therefore, be out-competing other predators for their common prey (i.e. small schooling fish). Three factors may have led to increases in adult groundfish biomass in the southeastern Bering Sea, changes in environmental conditions, present commercial fishing practices, and predator release resulting from overharvesting of marine mammals and some fishes during 1955–75. If the decline in whale and fur seal populations during 1955–75 contributed to the current high biomass of groundfish, then marine mammals once (but no longer) exerted a structuring effect on the Bering Sea ecosystem. The current high abundance of piscivorous adult groundfish in the eastern Bering Sea may, therefore, impede the recovery of marine mammal and bird populations to historical levels.
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