Ecosystem change and the decline of marine mammals in the Eastern Bering Sea: Testing the ecosystem shift and commercial whaling hypotheses

Trites, A., Livingston, P., Mackinson, S., Vasconcellos, M., Springer, A. and Pauly, D. 1999. Ecosystem change and the decline of marine mammals in the Eastern Bering Sea: Testing the ecosystem shift and commercial whaling hypotheses

Some species in the Bering Sea underwent large changes between the 1950s and the 1980s. Among the best documented are the declines of Steller sea lions and northern fur seals, and the possible increase and dominance of ground fish - pollock and large flatfish. A frequently proposed explanation is that human exploitation of top predators and/or a shift in the physical oceanography altered the structure of the eastern Bering Sea ecosystem.

We employed two inter-related software packages (Ecopath and Ecosim) to describe quantitatively the eastern Bering Sea ecosystem during the 1950s, before large-scale commercial fisheries were underway, and during the 1980s, after many marine mammal populations had declined. We grouped the hundreds of species that make up the Bering Sea ecosystem into 25 functional groups.

Some ecosystem indices derived from our ecosystem models indicate that the eastern Bering Sea was more mature in the 1950s than in the 1980s. However, we are less certain about the actual state of the Bering Sea in the 1950s due to the relative paucity of data from that time. The ecosystem indices for both the 1950s and 1980s models suggest that the Bering Sea is relatively resilient and resistant to perturbations. Removing whales from the 1950s ecosystem had a positive effect on pollock by reducing competition for food. However, whaling alone is insufficient to explain the 400% increase in pollock biomass that may have occurred between the 1950s and the 1980s. Nor can commercial fisheries account for these observed changes. The magnitude of changes that occurred in the biomass of all the major groups in the eastern Bering Sea cannot be explained solely through trophic interactions. We suggest that other factors comprising a regime shift, such as changes in water temperature or ocean currents may have been at play.

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