Humans, like fish, birds, and marine mammals, are also apex predators in marine systems as harvesters of marine resources. Therefore, beginning in 2003, GAP began addressing the importance of human harvest of fish resources within the study area by exploring data sets regarding commercial catch locations, amounts, and species.
The first approach to investigating human impact was to estimate the trophic levels of commercial groundfish removals in the Gulf of Alaska (PDF). Trends in trophic level estimates of commercial fishery catches are used as ecosystem-based indicators for sustainability, but these estimates often do not incorporate species-specific interannual and ontogenetic feeding patterns. Changes in the trophic structure of an ecosystem can also be altered by climate changes; e.g., following a shift to a warm phase of the Pacific Decadal Oscillation in 1976/77. Therefore, it is essential to understand trophic patterns and interactions of key species to elucidate potential impacts of fishery removals on the ecosystem. This study provided a finer resolution of ontogenetic and temporal variations in the trophic position of four groundfish species in the central Gulf of Alaska (walleye pollock, Pacific cod, arrowtooth flounder, and Pacific halibut) using stable isotope analysis to assess total length and diet source.