Are declines in higher trophic levels the result of fishing?
The decline in Steller sea lions, particularly in the eastern Aleutian Islands and the western GOA, dramatically demonstrates the need for a better understanding of causal relationships between the dynamics of upper trophic level organism and changes in the abundance and species composition of lower trophic level taxa. By 1990, the population of Steller sea lions had declined by about 80%, prompting their listing as threatened under the Endangered Species Act (ESA). As required under this listing, research was initiated with the aim of investigating the functional linkages in the BSAI ecosystems and of identifying factors responsible for their reorganization. Mechanisms hypothesized to explain the decline of Steller sea lions can be broadly divided into bottom-up and top-down; bottom-up hypotheses included nutritional limitation caused by declines in prey taxa abundance resulting from an ecological regime shift or increased commercial fishing pressure of preferred prey. Top-down hypoth