Project Description
NMML has studied the population biology of northern fur seal colonies at San Miguel Island, California for 38 years. The studies have provided a long-term dataset to evaluate factors affecting the growth or decline of the population. The population has experienced two severe declines (>60%) during El Nino events in 1982-1983 and 1997-1998 in which pup production decreased and pup and adult female mortality increased. Recovery from the 1982-1983 event occurred seven years later and the population has not yet recovered from the 1998 El Nino event. In 2004, 50% of mortality of fur seal pups was due to hookworm disease. If the disease continues with the same severity or becomes worse it will result in reduced recruitment during the coming years. Thus, the primary regulators of population growth in this species are severe El Nino events and hookworm disease. Whereas the population is likely to decline due to low recruitment resulting from high pup mortality in the late 1990's to the present, the resurgence of Pacific sardine in the California Current and a colder oceanic regime shift that occurred in 1999 should increase the survival of pups and adults in the coming years.
Issues & Justification
San Miguel Island, California is the southern extent of the breeding range for northern fur seals and the population is managed as a separate stock under the MMPA. The population is small (~ 10,000 individuals), and is vulnerable to catastrophic decline caused by environmental perturbations or disease. We propose to continue marking studies to estimate the population vital parameters as the population recovers from the severe decline in 1998 and responds to the effects of high pup mortality from disease. Marking and health assessment studies will provide opportunities to evaluate if the resurgence of sardine and a colder oceanic regime in the California Current will increase survival and natality rates. The expansive range of northern fur seals requires a multi-ecosystem approach to understand factors affecting the U.S. northern fur seal population. Thus, we will collaborate with the Alaska Ecosystem Program at NMML to provide a comparative framework to evaluate factors influencing northern fur seal populations in different population growth phases and ecosystems.
Goals
- Mark cohorts of pups and re-sight marked animals to estimate age-specific survival.
- Evaluate new methods of long-term marking.
- Evaluate population trends using live and dead pup surveys.
- Evaluate pup health through morphometric and physiological indices to evaluate the population's response to short- and long-term environmental changes (e.g. El Nino, Pacific Decadal Oscillations).
- Develop a model to estimate survival.
Methods
Marking:
200 northern fur seal pups will be tagged with flipper tags or another permanent marking method in September-October each year. Pups are randomly selected and held in a holding pen for tagging. They are weighed, sexed, measured, tagged, and released. Although tags have proven to provide survival and natality data for young fur seals, worn tag numbers and tag loss create problems estimating survival for older animals. New marking methods were recommended during the Northern Fur Seal Census and Tagging Workshop in September 2005. Monel and plastic super flex tags will be applied to pups to evaluate these tags over the next five years to evaluate the retention of different tag types, as well as to estimate tag loss.
Re-sighting:
Northern fur seals are present at San Miguel between May and November. Observations of tagged animals will be conducted during that period using a mobile blind, which allows observers to move within the colony with little disturbance. A minimum of six surveys for tagged animals will be conducted each year. Tag number, physical characteristics, and reproductive status will be recorded to estimate survival and natality. The Program MARK will be used to estimate survival and natality rates.
Population Surveys:
Each year, the number of live pups in each colony will be counted to continue the long-term database of pup production. Live pups are counted in early August. In addition to live pup surveys, pup mortality surveys will be conducted up to six times during the breeding season. Dead pups will be counted and removed from the colony to avoid re-counting on subsequent surveys. The total live and dead pup counts will provide an index of pup production for the San Miguel Island and Castle Rock colonies.