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California Current Ecosystem Program

San Miguel Island Field Work

In early October members of the California Current Ecosystem Program returned from San Miguel Island, where they had branded California sea lion pups and tagged northern fur seal pups. During their field work there they found the northern fur seal pup weights to be significantly less than the long-term average, indicating that the pups had grown more slowly than normal in the 3 months since birth. The pups subsequently weaned and departed the island in late October and November. In November and December higher than normal numbers of fur seal pups were reported to the Southwest and Northwest Regions’ Stranding Networks. Most of the pups were alive but emaciated at the time of stranding along the coasts of central and northern California, Oregon, and Washington. The Marine Mammal Center in San Francisco admitted 29 fur seal pups during November and December. Although some died, most have responded well to rehabilitation and a steady diet of squid and herring. Pups which died after admission to the Marine Mammal Center showed no significant pathology at the time of death. It is unclear what caused the low growth of pups on the island, but it appears there will be very low survival of pups from the 2006 cohort produced at San Miguel Island.

In November 2006, 10 adult female California sea lions were instrumented with satellite telemetry instruments and dive recorders at San Miguel Island. During the same period, 12 adult females were instrumented at nearby San Nicolas Island by Carey Kuhn of the University of California, Santa Cruz. This is the second year of a 2-year project to describe the movements of reproductive females in the two largest populations of California sea lions in U.S. waters. Although the two islands lie within 60 miles of each other, preliminary results from deployments in 2005 indicate that there may be segregation of foraging areas by females from the two colonies. Initial positions show San Miguel females dispersing northwest of San Miguel Island, traveling as far north as Monterey Bay and the Farallon Islands on foraging trips that last 3-5 days. Females are traveling along the coast and over the shelf but also occur over the slope and offshore. Females from San Nicolas Island remain mostly in the California Bight and in and around the Channel Islands. We will recover the instruments from the current deployment in January 2007 and will begin analyses of the data.

Seven California sea lion pups between 5 and 6 months old were instrumented with satellite telemetry instruments at San Miguel Island. This is the final stage of a study investigating the development of foraging behavior of juvenile California sea lions at San Miguel Island. The goal of this study is to describe the development of diving in California sea lion pups and to determine when pups begin independently feeding. Five of the pups were paired with instrumented females to investigate mother and pup foraging behavior.

Blood and hair samples were collected from pup and adult female sea lions and fur seals for stable isotope analysis. Unlike fecal samples, the diet of a predator can be assessed over different time scales (and not just the last feeding trip) using stable isotopes. Isotopic measurements of multiple tissues (e.g., serum, plasma, and red blood cells from blood and fur) from the same individual can provide dietary information on various temporal scales because of the differing metabolic rates of each tissue. Results from these data will be examined to discern differences in the foraging ecology of individuals, between sexes, and among age groups.

In November 2006, nine adult female northern fur seals and nine pups were instrumented with satellite telemetry instruments at San Miguel Island. This is the second year of a collaborative study between NMML’s Alaska Ecosystem and the California Current Ecosystem Programs. The goal of the study is to describe the migratory movements of adult females and pups throughout the range of northern fur seals in the eastern north Pacific and annual variability in migration patterns. Preliminary data show that pups from San Miguel Island dispersed from the island in late November and traveled northward along the coast. By December, most of the pups were located north of San Francisco, California. The adult females have moved northward also, traveling along the slope and offshore. By December, the females were farther offshore than the pups but in the same area, north of San Francisco Bay.

By Robert DeLong



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