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What's Wrong with the California Sea Otter?

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two sea otters feeding
In decline?: California sea otters had been hunted to near extinction by 1911, when unregulated hunting of sea otters was stopped. Slow growth occurred with protection, but the population currently appears to be roughly stable or possibly in slow decline. Scientists are studying information accumulated over decades for clues that might explain why the California sea otters' population growth has been so sluggish. Photograph courtesy of Friends of the Sea Otter.
Some of the most important insights about wildlife populations come from retrospective analyses of data and records that accumulate over years, decades, and even centuries. Biologists from the USGS Western Ecological Research Center, California Department of Fish and Game, and Smithsonian Institution recently undertook such analysis in an effort to better understand why the threatened California sea otter is doing so poorly.

Information on the abundance and distribution of California sea otters has been recorded since the early 1900's, and beach-cast sea otter carcasses have been recovered and salvaged since 1968. The salvage records contain many different kinds of information, including age, sex, location, date, body condition, and cause of death. By 1999, the salvage database had grown to include 3,105 records.

Our analysis was more or less a "fishing expedition"—a search for the proverbial needle in a haystack— which we hoped would provide clues about why the California sea otter population has grown at such a sluggish rate during the best of times and why it is currently in decline. Our approach was to characterize overall patterns in this unique database, and especially to contrast periods of population growth and decline.

These efforts provided important insights. Overall, we were able to determine that the carcasses of an estimated 40 to 60 percent of dead sea otters were not recovered and that 70 percent of the sea otters whose carcasses were recovered died from unknown causes, thus creating considerable uncertainty about population-level assessment from the carcass record. Nonetheless, the high proportion of prime-aged adults (3 to 10 years old) in this record explains why the population has not grown more rapidly.

Prime-aged adults, and animals for which cause of death was unknown, were more common in the carcass record during periods of population decline. Carcasses of sea otters killed by white-shark attacks also were more common during the periods of decline. A disproportionately large number of carcasses were recovered during spring and summer, and this pattern was more pronounced during the periods of population decline. Per capita pup production and mass/length ratios of adult carcasses declined over the 31-year period examined in the study, indicating that environmental conditions for otters are deteriorating. However, neither measure varied consistently between periods of population increase and decline.

Neither sex composition nor the proportion of carcasses of sea otters that died of infectious disease varied significantly between the periods of population increase and decline, although the overall high proportion of deaths from infectious disease suggests that this factor has contributed to the chronically sluggish growth rate of the California sea otter population. The population decline from 1976 to 1984 was likely due to sea otters drowning after being caught unintentionally in gill nets; population growth resumed when gill nets were transferred farther offshore. There is correlative evidence that the population decline from 1995 to 1999 is associated with a developing live-fish fishery, although further information is needed to establish a causal relationship. (The live-fish fishery is a relatively new industry along the central California coast that uses box traps to catch live fish for sale to restaurants specializing in very fresh fish. Caught in shallow water without hooks, the fish can be returned unharmed if undesirable or undersize.)

Related Web Sites
Western Ecological Research Center
U.S. Geological Survey (USGS)
Department of Fish and Game
State of California
Friends of the Sea Otter
non-profit organization

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Santa Barbara Shorebirds

Northwest Australia

Research California Sea Otter

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Woods Hole Food Drive

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Curt Mobley

Gulf of Mexico Integrated Database Workshop

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Staff & Center News Mike Carr & Homa Lee

Netherlands Visiting Scientist

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