2009 Spring Survey Shows Drop in California Sea Otter Numbers
The sluggish recovery of the southern sea otter of California, a threatened population on the Endangered Species list, appears to have stalled once again.
U.S. Geological Survey (USGS) scientists say the latest 3-year running average (2,813 sea otters) was 0.5-percent lower than last year, the first time the trend has been negative since the late 1990s. A leveling off of population growth has occurred over the past 3 years.
For southern sea otters to be considered for removal from the Endangered Species list, the 3-year running averages would have to exceed 3,090 for 3 continuous years, according to the U.S. Fish and Wildlife Service's Southern Sea Otter Recovery Plan.
"This slight dip of the sea otter growth trend has happened before, most recently in the mid- to late 1990s, so hopefully this will be just a brief setback to the recovery of the population," said survey organizer Brian Hatfield, a USGS biologist in California. "The fact that the pup counts have continued to increase slowly is encouraging."
The latest 3-year average was obtained by combining the spring census totals from the years 2007, 2008, and the recently completed spring 2009 census. During the 2009 census, observers counted 2,654 California sea otters, 3.8-percent fewer than the 2008 spring count of 2,760. Scientists use 3-year running averages of spring census totals to assess population trends because these averages are more reliable than individual year totals, which can vary with weather conditions, sea otter distribution, and other factors.
"This year's census results demonstrate that sea otters continue to experience levels of mortality sufficient to limit their recovery," said Tim Tinker, lead scientist for the USGS sea otter research program in California. "This highlights the need for continued efforts to understand and mitigate threats to sea otters and other species in the nearshore ecosystem." Ongoing research by USGS scientists and research partners is aimed at determining important sources of mortality in sea otters and the underlying reasons for the sluggish rate of recovery and varying population trends.
"One interesting finding this year was that the big male groups that we have seen at the extreme ends of the range over the last 5 years were largely missing on this survey," pointed out Hatfield, "which raises questions about the factors driving sea otter distribution and behavior." Some of the variation in numbers at smaller scales reflects movements of animals between areas, especially in the case of males. For example, numbers were higher this year in Estero Bay but lower southeast of Point Conception. USGS studies of radio-tagged animals have shown that males frequently move long distances between the range peripheries and sandy embayments, such as Estero Bay, Pismo Beach, and Monterey Bay.
"The apparent redistribution of males this year from the ends of the range to areas well within the existing range demonstrates that range expansion is not a steady progression into new areas but rather a more dynamic process involving advances and retreats," said Lilian Carswell, Southern Sea Otter Recovery and Marine Conservation Coordinator with the U.S. Fish and Wildlife Service.
The spring 2009 California sea otter survey, conducted between May 4 and June 11, covered more than 375 mi of the California coast. The census results provide counts used to evaluate trends and are not absolute population estimates. The census is a cooperative effort of the USGS, the California Department of Fish and Game's Marine Wildlife Veterinary Care and Research Center, the Monterey Bay Aquarium, the U.S. Fish and Wildlife Service, and many experienced and dedicated volunteers. The information gathered from spring surveys is used by Federal and State wildlife agencies in making decisions about the management of this small sea mammal.
in this issue:
Drop in California Sea Otter Numbers