HISTORICAL TRENDS IN THE NUMBER OF FORAGING TRIPS

MADE BY LACTATING NORTHERN FUR SEALS

Jason Baker, Robert Caruso, and Tom Loughlin

National Marine Mammal Laboratory

Background

Female northern fur seals make several day-long foraging trips from their breeding islands, periodically returning to nurse their pups. Variability in the duration and total number of trips females complete before weaning their pups likely reflects changes in prey availability in the Bering Sea during July-November. When prey is abundant, females presumably make shorter trips and therefore more total trips before weaning. The number of foraging trips made by female northern fur seals while rearing their pups can be estimated by counting growth lines in their offspring's teeth (Bengtson 1988; Baker 1991).

The goal of this project is to reconstruct a long-term time series 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 data set would then serve as an index of prey availability to fur seals over several decades. Our first step is to establish whether there is a link between abundance of the current primary prey of fur seals in the Bering Sea, juvenile walleye pollock, and the number of foraging trips recorded in teeth.

Approach

Our strategy has been to start by determining the mean number of trips made by females in years when the best data on juvenile walleye pollock in the Bering Sea are available. If we can establish that the number of trips made is indeed correlated with prey (pollock) abundance, we will proceed to expand the time series to include years when no prey surveys were conducted.

The number of foraging trips made is estimated by counting "nursing lines" visible in the canine teeth of fur seals during the period when they are dependent on their mothers. The nursing lines form in the dentin due to the feeding/fasting schedule imposed on the growing pup by the mother's periodic absence on foraging trips. Growth lines in mammalian teeth are preserved for the life of the animal. We are examining teeth from 2-3 year-old male northern fur seals which were killed either in the commercial skin harvest prior to 1985, or killed in the Aleut native subsistence hunt since then. Figure 1 shows a thin longitudinal section of a 3-year-old male's canine tooth. Annual growth lines are evident. Figure 2 is a close up of the first year's growth, with the nursing lines plainly visible.

Mean numbers of nursing lines will be estimated from the teeth and compared with abundance estimates of juvenile walleye pollock in the area around St. Paul Island where females forage.

Progress

We began by limiting our analysis to years when juvenile pollock abundance was estimated using both bottom trawl and hydroacoustic surveys. These surveys have been conducted triennially beginning in 1979 (Traynor 1996, Traynor et al. 1990). Teeth from northern fur seals were available for four of these years, and a sample from each year is currently being analyzed (Table 1). Thirty teeth are being analyzed from each year excepting 1982.

Figure 2. This image is a close up of nursing lines formed in the dentin
during the first year of life. PN indicates pre-natally formed dentin.
N indicates the neo-natal line, which forms around birth.
The lines numbered 1-11 indicate individual foraging trips made
by this seal's mother.

Table 1. Canine teeth sampled for nursing lines.

Cohort Year	Age at Death	Year Killed	No. of Teeth
1979	3	1982	30
1982	2	1984	20
1982	3	1985	20
1991	2	1993	30
1994	2	1996	30
		Total	130

when 20 teeth from 2-year-olds and 20 from 3-year-olds will be analyzed to determine whether age at death of the sampled fur seals influences the estimated mean number of nursing lines. The teeth are sectioned and the number of lines estimated independently by two researchers. Sections of poor quality in which nursing lines cannot be counted are replaced until the sample sizes shown in Table 1 are achieved.

Currently, all the teeth have been sectioned and read. We are now beginning the process of analyzing the nursing line data along with estimates of juvenile walleye pollock abundance.

References

Baker, J.D. 1991. Trends in female northern fur seal, Callorhinus ursinus, feeding cycles indicated by nursing lines in juvenile male teeth. M.S. thesis, University of Washington .

Bengtson, J.L. 1988. Long-term trends in the foraging patterns of female Antarctic fur seals at South Georgia. In Antarctic Ocean and resources variability 286-291. Sahrhage, D. (Ed). Berlin Heidelberg: Springer-Verlag.

Traynor JJ. 1996. Target strength measurements of walleye pollock (Theragra chalcogramma) and Pacific whiting (Merluccius productus). ICES J. Mar. Sci. 53:253-258

Traynor JJ, Karp WA, Furusawa M, Sasaki T, Teshima K, Sample TM, Williamson NJ, Yoshimura T. 1990. Methodology and biological results from surveys of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea and Aleutian Basin in 1988. Int. North Pac. Fish. Comm. Bull. 50: 69-99.