Tuesday, August 7, 2007 - 3:40 PM

COS 53-7: Age-structured modeling provides evidence for a 28-year decline in the natality of western Steller sea lions

E. E. Holmes, Northwest Fisheries Science Center, Lowell Fritz, National Marine Mammal Laboratory, Anne York, York Data Analysis, and Kathryn Sweeney, University of Washington.

Since the mid-1970s, the western Steller sea lion (Eumetopias jubatus), inhabiting Alaskan waters from Prince William Sound west through the Aleutian Islands, has declined by over 80%.  Changing oceanographic conditions, competition from fishing operations, direct human-related mortality and predators have been suggested as factors driving the decline, but the complexity and indirectness of their effects on Steller sea lions (SSL) have made it difficult to associate these factors with changes in abundance.  In order to determine how SSL survivorship and natality have changed during their 30-year decline, we measured historical changes in age-structure using measurements of SSL length distributions in aerial photographs taken since 1985 and used an age-structured model with temporally varying vital rates to analyze this time series along with a 1976-2004 time series of population counts.  We compared results using four different Leslie matrix models.  Predictions of the models were cross-validated against independent data on age-structure, survivorship, and pup to female ratios at three time points between 1976 and 2004.  The overall analysis found strong support for declining birth rate from 1976 to 2004 concurrent with a steady increase in adult and juvenile survivorship after a severe drop in the early 1980s.  The best fitting model predicts that in 2004, natality in the Central Gulf of Alaska is 36% lower than in the 1970s, while adult and juvenile survivorship are close to or slightly above 1970s levels.  The dominant eigenvalue for the estimated 2004 Leslie matrix is 1.0014, indicating a stable population, but the stability depends on very high adult survival to compensate for low natality.  These results suggest that low natality not survivorship is limiting the recovery of the western Steller sea lion, and that some, as of yet undetermined, factor is driving down late-term pregnancy rates in this Bering sea apex predator.

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