Research Document - 2007/076
Preliminary Assessment of the Recovery Potential of Northern Fur Seals (Callorhinus ursinus) in British Columbia
By P.F. Olesiuk
Abstract
The northern fur seal (Callorhinus ursinus) is the most abundant and widely distributed pinniped in the North Pacific Ocean. The species currently breeds on 6 rookeries, and although both sexes generally exhibit a high degree of fidelity to natal sites, there is sufficient exchange among sites to prevent genetic differentiation. Movements of animals between or colonization of new rookeries can affect population dynamics, so fur seals should be recognized as a single population. Pups are the only component of the population confined to land where they can be censused, and have been used to track population trends. Total pup production has declined by 38% over the last 30 years (3 generations) due to a decrease at the largest breeding site on the Pribilof Islands. Pup production at other rookeries has been stable or increasing, such that the proportion of pups born on the Pribilof Islands has dropped from 76% to 53% over the last 30 years. Due to the cessation of subadult male harvests on major rookeries, there has also been a shift in the sex- and age-structure of the population, from one skewed toward females to more natural sex ratio, resulting in a decrease in per capita pup production. Total population size is estimated to have dropped from 1.67 million to 1.22 million over the last 30 years, representing a decline of about 27% in total abundance, and a decline of about 23% in the number of mature individuals in the population.
Population projections for St. Paul Island suggest the Pribilof stock has been experiencing chronic declines since the 1950s. Females killed during a herd reduction program and taken for research accounted for 70% of the steep decline that occurred in the late 1950s and early 1960s, but models suggest that juvenile survival was below equilibrium levels and the population would have declined to some extent even in the absence of these kills (York and Hartley 1981; Trites and Larkin 1989). The reason for the lack of recovery from the herd reduction kills and for the continued decline is unknown. Simulations indicate a decline in pup production of the magnitude observed could result from juvenile survival remaining at 18% below equilibrium levels, an 8% decrease in adult survival, or a 12% decrease in pregnancy rate, all of which are biologically realistic but would be impossible to detect from data currently being collected. Following the termination of commercial subadult male harvests in 1984, the prevalence of adult males is projected to have increased sharply, but these predictions are difficult to validate as counts of idle and harem bulls provide only an index of their actual abundance. Given the larger size of males, the models indicate mean body mass has also increased, such that the biomass of fur seals in the Bering Sea may not have changed much over the last 30 years.
Northern fur seals wintering along the west coast of North America (California to SE Alaska) are comprised mainly of adult females (64%), with some juveniles (36%) and a few adult males (0.5%), representing about 74% of the adult female population and 52% of the entire population in the eastern Pacific Ocean. Seals arrive on the coast in December-January and depart in June-July, spending an average of 4.8 months in coastal waters. Overall abundance remains fairly stable from February-May, during which about 375,000 fur seals occupy coastal areas. The distribution of pelagic seals shifts northward along the coast during the winter, peaking off California in February, Washington in April, and British Columbia and SE Alaska in May. Roughly 123,000 fur seals inhabit Canadian waters at peak abundance in May, with highest densities on LaPerouse Bank off SW Vancouver Island. The main prey are northern anchovy and hake in the southern part of the wintering range, and herring, salmon and rockfish in the northern part, while squid is important in offshore areas.
The reasons for the decline of fur seals breeding on the Pribilof Islands is unknown. Only small numbers of subadult males are taken for subsistence and bycatch in fisheries is minimal, so its unlikely direct human-induced mortality is driving the decline. Prey availability in the Bering Sea may have changed, perhaps due to changes in ocean conditions or as a result of commercial fisheries, as evident from declines of other pinnipeds in the Gulf of Alaska and Bering Sea (DeMaster et al. 2006). Female fur seals from the Pribilof Islands may also be experiencing greater intra- and inter-specific competition for prey resources. Relative numbers and biomass of male fur seals in the Bering Sea is predicted to have increased substantially since commercial harvests were terminated. California and Steller sea lions, which often feed on the same prey as northern fur seals, and have increased in abundance along the west coast of North America. The degree of competition between these apex predators warrants further study.
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