Annual Report 1998

Principal Investigators: JASON D. BAKER AND THOMAS R. LOUGHLIN

 PROGRESS: The goal of this project is to establish whether there is a link between abundance of the primary prey of northern fur seals in the Bering Sea, juvenile walleye pollock, and the number of foraging trips made by lactating female fur seals (an index of foraging success). Growth lines formed during a northern fur seal's birth year indicate the number of feeding trips made by the seal's mother. If a link between abundance of pollock and number of feeding trips is established, a long-term time series can be reconstructed on the mean number of trips made by female fur seals by examining teeth collected since the late 1940's on St. Paul Island. This time series would serve as an index of prey availability to fur seals in the Bering Sea over a period spanning several decades.
Last year, 130 fur seal teeth from four cohorts were sectioned and the number of nursing lines estimated. This year, 54 more teeth from two more cohorts were sectioned, bringing the total to 234 teeth from six cohorts. The number of feeding trips was analyzed in relationship to relative abundance of age-0 and age-1 walleye pollock in the same years. A draft manuscript was completed (see below) and is ready for submission for publication.

 SCIENTIFIC ACCOMPLISHMENTS: According to recent diet studies, juvenile walleye pollock (Theragra chalcogramma) is the primary prey species of lactating northern fur seals (Callorhinus ursinus) from St. Paul Island, Alaska. Using data collected and analyzed this and last year, we tested the hypothesis that the abundance of juvenile pollock is strongly linked to the foraging success of fur seals. We compiled estimates of age-0 and age-1 pollock abundance from research surveys and fishery data for 6 years (1978,1979, 1981, 1982, 1991 and 1994) and compared them to the mean number of foraging trips completed by females between parturition and weaning: an index of foraging success (see table). Although the abundance of age-0 and age-1 pollock varied greatly among study years, there was no consistent relationship with the number of fur seal foraging trips . In 1979, there was an enormous quantity of age-1 fish available and fur seals completed significantly more foraging trips that year (see figure). However, there were no similar peaks in the number of foraging trips in other years with high fish abundance (1978 and 1982). Most notably, juvenile pollock were scarce in 1994 but the number of foraging trips was average. While previous studies have shown that pollock is a primary prey species for lactating northern fur seals in the eastern Bering Sea, this study does not indicate a strong direct link between fur seal foraging success and trends in pollock abundance. Fur seals may benefit from years with extremely high pollock abundance, but they are not necessarily limited in years with moderate or low pollock abundance, as they rely on a variety of prey species.

 APPLICATIONS: The following manuscript was drafted and will soon be submitted to an appropriate journal: Baker, J. D., and R. V. Caruso. How Influential is the Abundance of Juvenile Walleye Pollock in the Bering Sea on the Foraging Success of Northern Fur Seals? Table 2. Abundance of age-0 and-1 walleye pollock in the eastern Bering Sea using various estimation methods. Based upon the relative magnitude of the estimates, annual relative abundance was categorized for each age class as "very small", "small", "medium", "large" and "very large". Corresponding estimates of the mean number of feeding trips made by lactating northern fur seals in the same years (estimated from offspring teeth) are also shown.

 

	Age-0 pollock		Age-1 pollock		Category		Mean # Trips
Year	Multi-species Model	Year-Class Strength	Direct Survey	Year-Class Strength	Age 0	Age 1	Lines
1978	N/A	17.9	N/A	7.5	very large	medium	12.8
1979	101	6.9	76.9	17.9	medium	very large	14.3
1981	125	3.6	N/A	7.5	medium	medium	11.9
1982	348	12.5	1.2	3.6	large	small	13.2
1991	58	3.6	3.2	6.2	small	medium	13
1994	N/A	1.6	1.7	1.4	very small	very small	13.2

STEPS TO COMPLETION: Submission, revision and publication of the manuscript (above) is the final step to completion of this project. While no strong relationship between the abundance of juvenile pollock and fur seal foraging success was indicated, six years of data may have been too few to detect a noisy relationship. As more fisheries data accumulate and collection of fur seal teeth continues, this relationship can be explored more fully in the future. A further step, the construction of a long-term time series on fur seal feeding trips, would likely reflect foraging success of lactating northern fur seals, but may not reflect availability of juvenile walleye pollock.