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Fieldwork

California Sea Otter Numbers Slide for Second Straight Year


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male sea otters
Otters: Raft of male sea otters in Elkhorn Slough, CA. The California sea otter population has decreased for a second consecutive year, according to last May's annual sea otter survey. Photograph by Warren Worthington.
Equipped with binoculars and spotting scopes, scientists and skilled volunteers paired up onshore for this year's spring survey of California sea otters. Other crews were airborne. Together, they scanned 375 mi along the California coast, from Half Moon Bay southward to Santa Barbara, during the month of May.

This year's sea otter tally showed a decline for a second consecutive year, according to U.S. Geological Survey (USGS) researchers who led the survey, which was conducted cooperatively with the California Department of Fish and Game, the Monterey Bay Aquarium, the U.S. Fish and Wildlife Service, and other agencies and organizations. The number of otters counted in the recently completed spring survey was 1 percent below last year's count, from 2,161 otters in 2001 to 2,139 in 2002. The 2001 survey also indicated an overall decrease from the previous year, but by 6.7 percent.

Researchers and managers are concerned about the overall slow rate of growth for the threatened California sea otter. Cooperative research efforts are ongoing to try to understand why the otter's recovery has stalled since reaching 2,377 individuals in the 1995 survey. USGS scientists developed the standardized methods for counting California sea otters that have been in use since 1982. Spring surveys of the otters indicate a growth rate of about 5 percent until 1995. Since then, the rate has declined by an average of about 1 to 2 percent per year.

To further examine the data, the researchers used graph points computed by averaging three consecutive years of survey data (see graph, below). "Three-year running averages of our spring survey data plot a decline from about 1995 to 1998, then a leveling off of the population from then to the present," says survey organizer and compiler Brian Hatfield, a USGS biologist at the Western Ecological Research Center Field Station in San Simeon, CA.

graph of 3-year averages of number of sea otters counted during spring surveys, over the period of 1983 through 2002, showing rise in otter population from 1983 to 1995 followed by a subsequent decline
Counting otters: Number of sea otters counted during spring surveys, plotted as 3-year running averages. (Example: value for 2001 is the average of the 2000, 2001, and 2002 counts.) Indep, independent.

The recent decline and lack of growth coincide with an increase in mortality, as indicated by the number of beach-cast sea otter carcasses. Since 1995, a relatively high number of dead otters have washed ashore; in 2001, there were 183 sea otter strandings, Hatfield notes. Through the end of May of this year, scientists had documented 92 strandings—a pace already exceeding the number that were stranded last year. Necropsies of these otters tell the researchers the fate of at least some of the animals.

"Of special significance is the loss of young and prime-age adults needed to replace mature otters. Young adults are dying at a high rate," says Jim Estes, a research ecologist at the USGS Western Ecological Research Center Field Station in Santa Cruz, CA.

Entanglement or entrapment in coastal fishing gear, starvation, disease, and contaminants may all have contributed to the recent sea otter decline, says Estes, who has studied sea otters and their role in kelp forests in California and Alaska for the past 30 years.

The survey information gathered by this cooperative effort is used by Federal and State wildlife agencies in making decisions about the management of this sea mammal.


Related Sound Waves Stories
Congressional Briefing on California Sea Otter Research
March 2002
What's Wrong with the California Sea Otter?
February 2002

Related Web Sites
Western Ecological Research Center
U.S. Geological Survey (USGS), Sacramento, CA

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in this issue: Fieldwork cover story:
Sea Otter Numbers Slide for Second Straight Year

Lake Mead Sediment Study

Salmon & Trout Habitat

Outreach Students Tour M/V Auriga

Williams' Presentation on Videotape

Meetings Benthic-Habitat Symposium

Caribbean Climate Change Conference

Regional Syntheses

Scientists Speak in Reston & D.C.

Awards Remote-Sensing Conference

Staff & Center News Sea-Survival Course

Regional Geologist Visits WHFC

New WHFC Employees

New Student Interns

WHFC Labs Pass Audit

Award-Winning Student Volunteer

Publications Contaminated Sediment - Special Issue of Continental Shelf Research

July Publications List


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