OVERVIEW

Project summary

The precipitous decline of the western stock of Steller sea lions (Eumetopias jubatus) has been well documented (Calkins and Goodwin 1988, Loughlin et al. 1992, Sease and Loughlin 1999, Hill and DeMaster 1999, NMFS 2000). The number of adults and juveniles declined approximately 50-75% between 1960 and 1990 (Merrick et al. 1987, Loughlin et al. 1992, Calkins et al. 1999). They continued to decline at an average annual rate of 3-5% between 1990 and 2000 (Sease et al. 2001). Factors contributing to their decline through the 1980's remain unclear although reduced juvenile survival is considered a likely proximate cause (Merrick 1995, Sease and Merrick 1997). It has been widely hypothesized that the western stock of Steller sea lions was nutritionally stressed by reduced availability, quality, and/or diversity of their prey (ASG 1993, Calkins, et al. 1998, NMFS 1995, Sease and Merrick 1997, Merrick et al. 1995). Although evidence from the 1970's and 1980's support this "prey-limitation hypothesis", studies in the 1990's do not (Millette et al.1999, PSMFC 1997), suggesting other factors may now be involved.

We currently lack adequate understanding of the dynamic ecological processes affecting Steller sea lions to determine or reduce impediments to their recovery. In the absence of such information, the Endangered Species Act requires that federal actions such as Fishery Management Plans be precautionary to avoid potentially jeopardizing the stock's recovery. This approach presumes that Steller sea lions are currently prey-limited and that commercial harvest of known prey may negatively affect population health at some scale. Alternatively, Steller sea lion recovery may be limited by predation, competition with other apex predators, disease and contaminants, and/or environmental factors that have acted individually or together to reduce juvenile survival. Congressional support for testing these and other hypotheses has resulted in over $80 million of research support since 2000.

The University of Alaska Fairbanks' Gulf Apex Predator-prey study (GAP) will address these hypotheses and assumptions through holistic studies of the processes and interspecific interactions occurring seasonally and between years in small geographic areas.

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Initiated in 1999, GAP's primary goal is to document trophic relationships between Steller sea lions, their prey, predators, and potential competitors in waters near Kodiak Island, an area of continued sea lion declines and extensive commercial fishing. Broadly, through integrated studies that overlap spatially and temporally, GAP will assess the degree of dietary overlap among Kodiak's sympatric apex predators while exploring processes that drive populations of their prey within a dynamic marine environment. Collectively, GAP studies will indirectly test the hypothesis that Steller sea lions are prey-limited by documenting a) sea lion prey, b) the abundance, distribution, and quality of those prey species, and c) the productivity and health of other consumers of the same prey. GAP studies will also collect baseline data needed to test the hypothesis that predation by killer whales or sharks may be limiting Steller sea lion recovery in the Gulf of Alaska. And by simultaneously monitoring environmental and oceanographic parameters over time in this area, GAP researchers will test the hypothesis that environmental and oceanographic change affects Steller sea lions, their prey, predators, and potential competitors.

Funding from National Marine Fisheries Service (NMFS) in FY02 will provide continued support of GAP's longterm goals and objectives. Some modifications and additions to the program's previous methodology are proposed in response to FY01 findings. Specifically, seasonal prey surveys will be extended to include waters surrounding Marmot Island, nearshore prey communities will be surveyed on a finer spatial and temporal scale, sea lion use of ephemeral prey resources will be monitored, and the diet and foraging patterns of sympatric harbor seals will be assessed.

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Program goal and objectives

GAP's primary goal is to document trophic relationships between Steller sea lions, their prey, predators, and potential competitors in waters near Kodiak Island, an area of continued sea lion declines and extensive commercial fishing. Through integrated studies that overlap spatially and temporally, GAP will assess the degree of dietary overlap among Kodiak's sympatric apex predators while exploring processes that drive populations of their prey within a dynamic marine environment. Specifically, GAP studies will collectively test the hypothesis that Steller sea lions are prey-limited by documenting a) seasonal sea lion diets, b) the abundance, distribution, and quality of known prey species, and c) the abundance, distribution, and productivity of other consumers of the same prey. We will also collect baseline data needed to test the hypothesis that predation by killer whales or sharks may be limiting Steller sea lion recovery in the Gulf of Alaska. And by simultaneously monitoring environmental and oceanographic parameters over time in this area, we will test the hypothesis that environmental change affects Steller sea lions, their prey, predators, and potential competitors.

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Statement of work

The GAP program consists of a number of independent but integrated studies focused on coastal waters northeast of Kodiak Island. This small geographic scale enhances the logistic feasibility of seasonal surveying and provides insights into complex issues of broader ecological and managerial concern. The Kodiak area was chosen for study because its sympatric assemblage of marine predators includes declining and well-studied pinniped populations, increasing piscivorous whale populations, highly productive seabird colonies, and a variety of commercial, subsistence, and sport fisheries that depend on a common and dynamic assemblage of prey. Historically, the Kodiak region has supported the north Pacific's largest Steller sea lion and harbor seal rookeries, numerous seabird colonies, a commercial whaling station, long-term marine resource monitoring, communities dependent on commercial and subsistence fishing, and a port which has ranked among the nation's leaders in commercial fish landings for decades. Kodiak's infrastructure supports field services and sampling opportunities (ship and air charters, processing plants), research labs and offices (University of Alaska Fairbanks (UAF), National Marine Fisheries Service (NMFS), and Alaska Department of Fish and Game (ADFG)), and year-round residency of project personnel thus reducing logistical difficulties and operational expenses.

Individual GAP studies (fish, marine mammals, seabirds) are conducted within this area on several spatial scales. Several collaborating researchers have chosen to conduct related studies near Kodiak synchronously on one or more of these scales. Such collaborative studies, overlapping and coordinated in space and time, are key to synergistic understanding of the complex and dynamic ecosystem in which Steller sea lions live.

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