Steller sea lions and fisheries : competition at sea?

• Main Author: Hui, Tabitha Cheng Yee
• Format: Thesis
• Language: English
• Published: University of British Columbia 2011
• Subjects: Atka; Bering Sea; Gulf of Alaska; Pacific; Alaska; Aleutian Islands
• Online Access: http://hdl.handle.net/2429/32588

A leading hypothesis to explain the decline of Steller sea lions (Eumetopias jubatus) in western Alaska is the reduction of prey abundance or change in prey distributions caused by commercial fisheries. We sought to improve on past studies that attempted to assess competition between sea lions and fisheries by estimating the local amounts of prey accessible to sea lions. We explored the relationships between sea lion population trends, fishery catches and the prey biomass accessible to sea lions around 33 rookeries from 2000-2008. We focused on three commercially important species that dominate the sea lion diet: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass (estimated using our accessibility model and also within 10, 20 and 50 km of each rookery). Of the 304 statistical models we constructed to compare accessible prey biomass and catch to sea lion population trends, only three relationships were significant. These three suggest that sea lion population change rates increased (became less negative) with increasing accessible pollock biomass in the Aleutian Islands and with cod biomass in the Gulf of Alaska. No relationships were found between sea lion population trends and Atka mackerel biomass. Given that the majority of the relationships we explored were insignificant, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s. Sea lion population trends appeared to be affected by some unknown factor associated with regional differences. Removing fish catches or adding catch to our predicted distributions of groundfish abundances had no measurable effect on sea lion population trends. These observations suggest that sea lion populations were largely unaffected by fishery removals during this period.