School of Fisheries and Ocean Sciences, Marine Advisory Program, University of Alaska Fairbanks
Kodiak, AK, USA
For over three decades we have watched the drastic decline and slow recovery of Steller sea lions (Eumetopias jubatas) in western Alaska. Many hypothesize that Steller sea lions were nutritionally stressed in 1970's and 1980's due to natural- or human-induced limitations in the quality or quantity of their prey. Evidence suggests conditions for Steller sea lions improved in the 1990's as the decline abated and first signs of population growth were detected in counts made since 2000.
With unprecedented funding in the 2000's, the research community has developed new technologies and refined measures and indices of individual Steller sea lion health. But due to the species' protected status, assessment of population vital rates has been limited to counting pups and non-pups onshore and tracking movements and survival of marked individuals. Another approach to assessing potential limitations to population growth is to look more broadly at the system supporting Steller sea lions for clues from sympatric consumers of Steller sea lion prey.
Since 1999, the UAF Gulf Apex Predator-prey (GAP) study has taken a multi-trophic approach to assessing conditions for Steller sea lions, their prey, and potential competitors in the central Gulf of Alaska. In addition to assessing seasonal dietary and population trends of Steller sea lions, GAP researchers are studying a diverse suite of sympatric apex predators and the prey resources they share in waters off northeastern Kodiak Island.
GAP's aerial surveys have documented seasonal variability in haulout use by Steller sea lions in the Kodiak area and verified the continued decline in overall counts in the area reported by NMFS. Contrary to expectations, from 1999-2002 Kodiak's Steller sea lions were found to be exploiting one of the most diverse diets previously reported. Associated GAP prey studies have documented the seasonal quantity and quality of many of their key prey species. GAP studies have also identified a variety of sympatric piscivores that also consume Steller sea lion prey species in the study area, including tufted puffins, black-legged kittiwakes, harbor seals, arrowtooth flounders, and other piscivorous fish and whales. By monitoring the productivity and foraging ecology of these fish, bird, and mammal species, GAP is developing an indirect means of monitoring the productivity of Kodiak waters and assessing the likelihood that prey is currently a limiting factor to piscivorous consumers in the area.
Taking an ecosystem-based approach to assessing population-level questions is analogous to considering a patient's environment when diagnosing individual disorders.