Higher Aggregation of Key Prey Species Associated with Diet and Abundance of the Steller Sea Lion Eumetopias jubatus across the Aleutian Islands

Abstract

AbstractThe Aleutian Islands ecosystem is a highly dynamic marine environment that supports commercially important fish species, such as Atka Mackerel Pleurogrammus monopterygius and Northern Rockfish Sebastes polyspinis, and several large marine mammals, including the Steller sea lion (SSL) Eumetopias jubatus. To protect SSL foraging behavior and prey, trawl exclusion zones (TEZs) were established around SSL rookeries and haulouts across the Aleutian Islands and are closed to bottom trawling. The Atka Mackerel fishery is concentrated at small, local patches (~37.04 km [~20 nautical miles]) across the Aleutian Islands, often near and adjacent to TEZs. Food web models and fishery stock assessments predict sufficient prey for both the fishery and foraging SSLs at the level of large management areas (each area is ~463 km [~250 nautical miles] longitudinally), but little is known about local fish distribution (~37.04 km) in proximity to SSL rookeries and haulouts. We used CPUE to examine the spatial distribution of two SSL prey species (Atka Mackerel and Northern Rockfish) and marine invertebrates (as a proxy for habitat quality) at large (across the Aleutian Islands) and small (~37.04‐km) scales. Although Atka Mackerel abundance estimates were similar across the Aleutian Islands, the density of Atka Mackerel differed. In the eastern Aleutian Islands, where SSL population trends have stabilized, Atka Mackerel aggregations were dense. In contrast, in the far western Aleutian Islands, where SSL populations are still in decline, Atka Mackerel distributions were diffuse and Northern Rockfish exhibited a greater density. Results indicated no significant difference in CPUE between areas open to fishing and closed areas (i.e., TEZs) at all study sites for Atka Mackerel and at all but one study site for Northern Rockfish, whereas invertebrate densities were significantly higher inside of TEZs at all study sites. We discuss how fish distributions across the Aleutian Islands may impact the success and behavior of foraging SSLs.