Research Projects > Prey Requirements of the Northern Fur Seal

Prey Requirements of the Northern Fur Seal

Using data from pelagic surveys, captive studies and literature reviews, a bioenergetic model will be developed to estimate the prey requirements of northern fur seals.

Anthropogenic mortality contributed much to the early part of the decline in the size of the northern fur seal population on the Pribilof Islands. However, the population has dropped even further since the mortality ended. This lack of recovery may be a result of changes in the capacity of the environment to support fur seals (i.e., shortage of available prey). A shortage of available prey was also implicated in a drastic reduction in pup production of the northern fur seal population on San Miguel Island, California, during the 1997-98 El Niño. In order to understand the relationship between fur seal population dynamics and the availability of their prey it is necessary to have quantitative estimates of the amount of prey that fur seals require.

Bioenergetic modeling is a common technique for estimating the food requirements of marine mammals, including fur seals. First, energy budgets are constructed for individual animals of all ages and sexes at all times of the year. Then, information on diet composition and population size is used to convert the energy requirements of individuals to the prey requirements of the entire population.

Project Outline:
An existing bioenergetic model of Steller sea lions will be adapted to estimate the prey requirements of northern fur seal individuals and populations. The model will be parameterized using data collected from pelagic surveys of fur seals (1958-1974), as well as from data collected from captive fur seals held at the Pacific Biological Station from 1976-1981. Other model parameters will be drawn from field studies of northern fur seals and other otariids.

Estimates from the model of the prey requirements of northern fur seals will provide a basis for further analyses of the ability of the environment to support fur seal populations, the impact of fur seals on populations of their prey, and potential competition between fur seals and fisheries. An important aspect of the model is a random sampling routine that will allow explicit and quantitative examination of the relative impact of uncertainty in key parameters (e.g., population structure, energy requirements, die composition, energy density of prey, etc.) on the precision of the model's predictions. This uncertainty analysis will provide direction for future research aimed at improving estimates of prey consumption by fur seals.

Principle Investigators:
Andrew Trites, University of British Columbia

Funding Source:
NOAA and the North Pacific Marine Science Foundation