Alaskan Fur Seals - Earthwatch Institute

APPENDIX
ALASKAN FUR SEALS: THE RESEARCH
The following information was taken from the research proposal submitted by the Earthwatch Scientist(s) to
Earthwatch Institute. Included is a description of the research conducted through this project, some results to
date, and other information regarding the accomplishments of the project and the staff. Specific details
regarding research sites, methods, etc. is subject to change slightly from year to year and such changes may
not be incorporated into this document.
BACKGROUND, OBJECTIVES, AND METHODS
The focus of the Alaskan Fur Seals research project is long-term monitoring of population age structure in the
northern fur seals (Callorhinus ursinus) of the Pribilof Islands in Alaska. For long-lived animals such as marine
mammals, understanding the age structure of the population is critical in assessing its future viability and health.
Healthy populations have a mix of age groups. Decreases in different age groups have different consequences.
For example, lack of recruitment (maturing young animals) in a population results in a gradual increase of the
average reproductive age. In the short term, the result may be a slight decrease in the overall population. The
delayed response, however, can be an accelerated population decrease and possibly a crash once the depressed
numbers of younger animals mature and fail to sufficiently replace the primary breeding age group. Whether or
not northern fur seals are currently experiencing age-biased population changes is unknown. If it were known, it
could shed light on the cause of the current decline as well as indicate future population trends. The main goal of
our current study is to test whether we can quantify changes in the age structure of breeding age northern fur seal
females. The study began in 2004 and the methodology has been refined and proven successful (Robson et al
unpublished data, 2007).
Our ability to understand the factors influencing the current fur seal population decline is complicated by a lack
of information on vital rates in the Pribilof population. At the moment, estimates of the total population are made
using a correction factor derived from survival and fecundity estimates. These estimates are based on data
collected from females collected at sea between 1958 and 1974 and whether or not they remain valid or have
changed as the population declines is unknown. There is no current data available to compare age composition
and fecundity to the 1950s data. As a result, there is insufficient information to evaluate whether reduced juvenile
or adult survival is an underlying cause of the decline, or whether fewer females are returning to land and giving
birth to pups. It is also possible given the precipitous rate of decline observed in recent years, that both factors
may be in play. Little is known about juvenile survival after pups leave the Pribilof Islands, although substantial
research efforts are currently underway to track the movement of pups during the winter migration. However,
similar research efforts to study vital rates in the fur seal population are only just beginning. It is noteworthy in
this regard that anecdotal observations of groups of mostly white-whiskered (older) females on the back edge of
rookeries, appearing to be without pups, have been made in recent years by both local (G. Fratis, Island Sentinel
Database, St. Paul Island) and scientific researchers (A. York, Pers. Comm.).
The urgent need for updated information on fur seal vital rates is complicated by the logistical difficulty, expense
and potential disturbance associated with the longitudinal studies required to obtain survival and fecundity
estimates from marked individuals. Efforts are underway to evaluate a range of techniques for use in such longterm
monitoring studies, but the studies are only in their initial stages. In addition, even in the event that longterm
tagging studies are initiated soon, it will be 10 years or more before information on vital rates in the Pribilof
fur seal population are available (e.g. see Boyd et al. 2001).
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