The spring 1999 survey of 2,090 California sea otters indicates the population has declined overall by 1.14 percent since the 1998 spring survey, which revealed that there were a total of 2,114. There was, however, a disconcerting decline in the group categorized as independents, which includes all adults and subadults. That category of sea otters declined nearly 5 percent, from 1,955 in spring 1998 to 1,858 in spring 1999.
Spring surveys of California sea otters have shown a steady decrease from a high of 2,377 sea otters counted in 1995. The surveys, conducted cooperatively by scientists of the U.S. Geological Survey, California Department of Fish and Game and Monterey Bay Aquarium with the help of experienced volunteers, cover about 375 miles of California coast, from Half Moon Bay south to Santa Barbara.
"This continuing pattern of decline is of grave concern, especially given our lack of understanding of the cause. The information we gather from these surveys will be used by federal and state wildlife agencies in making important decisions about the management of this sea mammal," said Jim Estes, a USGS scientist with the Western Ecological Research Center in Santa Cruz.
Scientists do not understand why the California sea otter is declining, said Estes. Sea otter deaths, as indicated from beach-cast carcasses, have increased over the same time period that the survey numbers have declined. Additionally, during 1998, 213 stranded sea otters were reported, the highest number ever recorded.
Just what factors are responsible for the increase in deaths is unclear. Disease, contaminants, starvation and entanglement or entrapment in coastal fishing gear may be contributing to the sea otter decline, Estes said. Disease is responsible for roughly 40 percent of the deaths, a rate that is high when compared to disease- caused deaths in other wild predators. As yet, however, there is no evidence that the rate of deaths from disease has increased. The USGS National Wildlife Health Center in Madison, Wis., California Department of Fish and Game, and University of California, Davis, have been examining fresh carcasses for disease.
Reproductive failure does not appear to be a factor in the decline, as the birth rate has remained largely unchanged, said Estes. Intense winter storms generated by El Niño events of 1982-83 and 1997-98 probably caused an abnormally large number of females to lose their dependent pups.
In 1977, the California sea otter was listed as threatened under the Endangered Species Act. Sea otters historically ranged across the North Pacific Ocean from about the mid-section of the Pacific coast of Baja California, Mexico, to northern Japan. By the end of the 19th century, maritime fur traders hunted sea otters to the brink of extinction. About a dozen remnant colonies survived at the time of protection in 1911; one remnant colony occurred along the remote Big Sur Coast of Central California.
Low population growth rate has long been a factor in the lagging recovery of the California sea otter, said Estes. The population of sea otters in central California has increased this century at an average annual rate of about 5 percent, much slower than those populations of Alaska, Canada and Washington with maximum growth rates of 15-20 percent.
Nevertheless, said Estes, at a growth rate of 5 percent per year, both range and population size of California sea otters increased steadily until the mid-1970's, when growth ceased and the population began to decline. Concern arose because sea otters were becoming entangled and drowning in nets in a coastal set-net fishery for California halibut. The State of California imposed emergency restrictions on set-net fishing, and soon thereafter the California sea otter population increased.
The spring 1999 survey was conducted following a survey protocol that USGS sea otter researchers developed in 1982. Each spring and fall, teams composed of two observers each use binoculars and spotting scopes to count individuals from shore and from fixed-wing aircraft. The counts made from shore are plotted on maps and then entered into a spatial data base. The aerial counts are entered directly into a geographic information system-linked data base in the aircraft.
Materials provided by United States Geological Survey. Note: Content may be edited for style and length.
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