Project Description

Northern fur seals on the Pribilof Islands have experienced a significant decline over the last several decades and the most recent population census showed a 5.8% per year decline in pup production from 1998 to 2004. The cause of the decline is unknown though; contributing factors may include climate change, predation, and interactions with commercial fisheries. Survival and health of breeding adult animals is likely a reflection of the quality of habitat they rely on. Adult females with dependent pups rely on predictable profitable foraging grounds during the breeding season and territorial males must build sufficient food stores prior to the breeding season in order to defend harems. Answering important life history questions such as the age distribution of mortality on breeding colonies may highlight vulnerable portions of the population and may reflect changes in the environment. Teeth were collected from dead adult animals found on rookeries during biennial population assessments between 1973 and 2006. Teeth will be cut, sectioned, aged and stable isotope analysis will be conducted. Results of the project will provide a unique and economical opportunity to potentially glean insight into the age structure of natural mortality on rookeries, potential age structure of the decline, decadal dietary shift that potentially can be associated with decrease quality of prey, long-term oceanographic cycles and climate shifts. In addition because these samples will continue to be collected during biennial population assessments we will create a tool for monitoring future changes in a declining population.

Issues & Justification

With the continued decline of NFS in the Bering Sea and the possibility that climate change and fishery pressure may influence prey distributions, it is important to monitor the health of the breeding population. The set of archived NFS teeth at NMML represents a significant effort to collect specimens over the years and is a unique data set that may provide insight into the age structure of the decline. Differences in the distribution of dying age classes between that expected from natural senescence and the observed (e.g. larger than expected groupings of certain ages) may reflect reduced habitat quality or unique annual environmental influences. Examining the age structure of the carcasses on the rookery during a specific time of year offers an opportunity to observe a window of time that encompasses the recent decline. This data cannot be generated again. Furthermore stable isotope analysis provides dietary information on a number of temporal scales, lifetime, decadal or annual.

Goals

• Use upper canine teeth collected from rookeries on St Paul Island during population assessments (1973-2006) to determine the age distribution of dead adult northern fur seals.
• Determine any significant patterns in mortality within sex and among years/decades.
• Link patterns in age structure to changes in population trends, prey abundance and shifts in oceanographic conditions.
• Conduct stable isotope analysis to determine trophic level shifts as they relate to changes in age structure on a number of temporal scales; lifetime, decadal, or annual.
• Determine if changes in age of dead adults can be correlated with decreased quality of prey, long-term oceanographic cycles or climate shifts.
• Develop methods and analysis which can be used to continue to monitor the age of mortality of males and females during the breeding season.

Methods

Upper canine teeth were opportunistically collected from dead adult northern fur seals during biennial population census between 1973 and 2006. Rookeries were searched during dead pup counts between 15 August and 1 September of each year. Upper canine teeth of non pups were collected, placed in cloth collection bags, boiled and cleaned, dried and stored in labeled plastic bags. This project will examine 2658 samples (761 males, 1897 females). The samples will be sectioned, mounted on slides and aged. Age structure of dead adults will be evaluated for annual, decadal and spatial patterns which may relate to oceanographic and climate shifts and correlated with changes in prey recruitment and distribution. Stable isotopes will be evaluated on a subset of the data to determine trophic level shifts as they relate to decreased quality of prey, long-term oceanographic cycles or climate shifts. A database will be created for the continued collection of tooth samples and analysis for monitoring future changes on the population.