U.S. GLOBEC: A Retrospective Study of Top Predator Trophic Positions, Productivity, and Growth in the Gulf of Alaska for 1960-75 and 1975-90. (Merrick, R. [Alaska Fisheries Science Center], K. Hobson [Canadian Wildlife Service], J. Piatt [Biological Resources Division, USGS]) Conditions in the Northeast Pacific Ocean began a shift in 1975-76 to a different oceanic regime which continued through at least the early 1990s, and appears to have resulted in increased productivity of Alaskan salmonids and groundfish. However, pinnipeds and sea birds, and some forage fish species exhibited sharp population declines (~50%). While trends of pinniped and sea bird populations have been suggested as related to changes in the availability or abundance of prey, the opposing trends of these predators and the major salmonid and groundfish stocks suggest that the relationships between marine predators and prey are more complex than are presently understood. Ocean ecosystem dynamics in the Northeast Pacific may affect salmonids and upper trophic level consumers in entirely different (and perhaps opposing) ways. The research proposed here will investigate through retrospective analysis whether changes in the oceanography and trophic structure of the Gulf of Alaska after 1975-76 decreased the Gulf's carrying capacity for marine mammals and birds.

This research will first involve a reconstruction of the trophic positions of a pinniped (Steller sea lion), piscivorous (common murre) and planktivorous (auklet) seabirds, and some of their major prey (juvenile and adult walleye pollock, juvenile and adult sockeye salmon, and adult herring) using analyses of changes in nitrogen isotope ratios for periods prior to and after the onset of the 1975-76 regime shift. Analyses will be performed using archival material (i.e., bones, teeth, feathers, otoliths, scales, and other tissue) obtained from animals collected during the two periods. Sea lion growth during the two periods will be assessed using the width of dental growth layer groups as an index. Age-specific sea lion trophic positions and growth indices will then be compared to assess relationships between the two. Finally, data on predator trophic positions, productivity, and sea lion growth will be compared to indices of oceanographic conditions (e.g., the Northeast Pacific Pressure Index) and prey productivity (e.g., year-class size) to assess the influence of bottom up processes on the carrying capacity for upper-trophic level species in the Gulf of Alaska. Results of this study will add to the understanding of the potential effects of climate variability and decadal climate change on top predator productivity in the eastern North Pacific. This study also adds to the ongoing exploration of the relationship between food web changes and the precipitous declines in Gulf of Alaska top predator populations being conducted by various agencies in the U. S., Canada, and Russian Commonwealth.

This page was last updated on March 15, 2007.

Maintained by:
Hal Batchelder
College of Oceanic and Atmospheric Sciences
Oregon State University
Corvallis, OR 97331-5503
phone: 541-737-4500; FAX 541-737-2064