Project Description
The abundance of northern fur seals (Callorhinus ursinus) on the Pribilof Islands of St. Paul and St. George has declined in recent decades. The reasons for the decline are unclear. In contrast, the northern fur seal population on Bogoslof Island has been growing rapidly, and fur seals have recolonized the Farallon Islands in Central California with a small, but growing population.
Annual counts of adult male northern fur seals at the Pribilof Islands have occurred since the early part of the 1900s. These counts are used as an index that reflects the general trend of the population. Pup production estimates started around the same time, but methodology was not standardized until 1961. The most recent population estimates were conducted during 2006 on the Pribilof Islands and during 2007 on Bogoslof Island. Current adult male counts and estimates of pup production on the Pribilof Islands are approximately one third the numbers estimated during the 1950's when the population was assumed to be at its peak.
We will conduct pup production estimates and counts of adult male fur seals at the Pribilof Islands during the summer of 2008. This will allow us to assess the overall size and health of the Eastern Pacific stock of northern fur seals. The NMML is also supporting a small project to survey the population of fur seals on the Farallon Islands in California.
Issues & Justification
Northern fur seal population monitoring activities are regularly conducted on the Pribilof Islands, where significant declines have been documented, but only sporadically on Bogoslof Island, where population growth was observed as recently as 2005. The annual counting of bulls at the Pribilof Islands is used to make predictions regarding the status and health of the population as well as to provide a general index of the trajectory of the population. Because they have been done since the early 1900s these counts can illuminate changes in the population. Pup production estimates provide another, perhaps more reliable, means to monitor the population. Estimated northern fur seal pup production on the Pribilof Islands declined by approximately 9.1% from 2004 to 2006. The 2006 pup production estimate on St. Paul Island is comparable with the level observed in 1917, while the St. George pup production estimate is below the level observed in 1916. During the period of 1916 and 1917, the northern fur seal population was increasing at approximately 8% per year following the cessation of extensive pelagic sealing. By contrast, the relatively small population at Bogoslof Island (eastern Aleutian Islands) was growing at over 12% per year between 1997 and 2005, and preliminary pup production estimates from 2007 indicate that the population has continued to grow. Further assessment of the northern fur seal population on the Pribilof Island is needed to monitor the substantial declines observed in this species, and to address potential redistribution of the population among islands.
Southeast Farallon Island in Central California was a historically significant breeding colony for northern fur seals, with tens of thousands of breeding animals, but they were extirpated due hunting by the late 19th century. In 1996 the first breeding fur seals returned to Southeast Farallon Island after a nearly 200-year absence, and the rookery is showing signs of increasing use.
These counts are also the opportunity for a review of the condition and maintenance needs of the catwalks on St. Paul. Funding in this project is also included for support of the infrastructure at both sites. Funding to purchase two trucks is included.
Goals
- Count territorial bulls and idle bulls at the Pribilof Islands (St. Paul and St. George Islands).
- Conduct estimates of pup production at the Pribilof Islands (St. Paul and St. George Islands).
- Conduct surveys to document fur seal abundance at the Farallon Islands.
Methods
Males are counted from catwalks and remote observation points above the rookeries.
Northern fur seal pups will be captured and marked by shearing a small portion of guard hair on the top of their heads. Small groups of pups are rounded up by the researchers for marking after the adult fur seals have been safely moved away. The shearing exposes the lighter underfur and produces an easily identifiable "mark". The marking effort will be allocated throughout the rookery so that each pup has an approximately equal chance of being marked. Past research indicates that marking approximately 10% of the pups born on sample rookeries will yield adequately precise estimates of the total number of pups. A few days after shearing, the number of pups sheared versus unsheared will be counted by independent observers (sampled by visual observations) on two occasions to estimate the proportion of sheared pups on the rookery. The estimate of the number of live pups present on each rookery at the time of shearing is equal to the number of pups sheared divided by the proportion of sheared pups observed among all those resighted (Petersen population estimate). The variance of the total estimate of pup production is one-half the squared difference of the two estimates.
Pup production estimates also require a count of dead pups which provide an estimate of mortality rate. Counts of dead pups will also be made (simultaneously with the second sample of sheared and unsheared pups). An estimate of the total number of pups born is calculated by adding the number of pups present at the time of counting to the count of dead pups.
Fur seals at the Farallon Islands will be counted from remote viewing locations.