Ecosystem Change and the Decline of Marine Mammals in the Eastern Bering Sea : Testing the Ecosystem Shift and Commercial Whaling Hypotheses Trites, Andrew W., 1957-; Livingston, Patricia A.; Mackinson, Steven; Vasconcellos, Marcelo, 1971-; Springer, Alan M.; Pauly, D. (Daniel)
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. Our mass-balance ecosystem models showed that most of the top predators (trophic level IV) declined from the 1950s to the 1980s. They included Steller sea lions, seals, sperm whales, deep-water fish and other demersal fishes. The only top predators to increase were large flatfish such as arrowtooth flounder. At the mid-trophic level (III), baleen whales and pelagic fishes declined, while small flatfish, pollock, and walrus and bearded seals increased. Based on our model assumptions, pollock contributed over 50% of the total flow of energy at the mid trophic levels during the 1980s compared to only 10% in the 1950s model. In contrast, pelagic fishes contributed nearly 50% of the flow in the 1950s. At trophic level IV, no one species dominated the flow of energy during the 1950s. However, large flatfish contributed over 60% of the total energy flow in the 1980s model. Large flatfish and adult pollock that dominate the Bering Sea in the 1980s appear to be significant competitors of seals. Large flatfish are also competitors of Steller sea lions and there are large overlaps in the diets of pollock and baleen whales. Changes in the biomass of marine mammals appear to have little effect on the biomass of other groups in the Bering Sea. Reductions in prey abundance can quickly reduce marine mammal populations, but marine mammals are unable to quickly recover when abundant food becomes available. Our models suggest that Steller sea lion populations would be larger if adult pollock and large flatfish were lower in abundance due to competitive release of important prey. Most impacts on the modeled ecosystem can be associated with changing the biomass of lower trophic levels. Total catch in the eastern Bering Sea rose from 0.33 to 2.62 t•km-2 between the 1950s and the 1980s. Exploitation during the 1950s used 47% of the net primary production, with most of it flowing through the harvested whales. Shifting the emphasis from exploiting marine mammals in the 1950s to catching fish in the 1980s lowered the amount of primary production required to sustain harvests to 6%. 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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