California Sea Otters: Population Recovery Continues at Slower Rate


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California Sea Otters: Population Recovery Continues at Slower Rate

By Tim Tinker, Brian Hatfield, and Gloria Maender
September 2008

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The southern sea otter of California, a threatened population on the Endangered Species list, continues to recover, but its rate of recovery appears to have slowed.

U.S. Geological Survey (USGS) scientists say that the latest 3-year average—the average of the totals from the spring counts of 2006, 2007, and 2008 (2,826 sea otters)—is 0.3 percent higher than last year's 3-year average, representing a slower rate of increase than they have seen in recent years. Scientists use 3-year running averages of spring-census totals to assess population trends because these averages are more reliable than individual year totals.

Above: How many sea otters are floating among the bull kelp (Nereocystis leutkeana) in this photograph taken through a spotting scope by Brian Hatfield during a southern-sea-otter survey? View the answer at URL http://www.werc.usgs.gov/otters/find-the-otters.htm. [larger version]

For southern sea otters to be considered for delisting, 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. Differences in weather conditions, otter distribution, and other factors contribute to the year-to-year variance in survey numbers.

Above: Number of southern sea otters counted during spring range-wide censuses, plotted as 3-year running averages. For example, values for 2007 are averages of the 2006, 2007, and 2008 counts.

During the 2008 census, observers counted 2,760 California sea otters, 8.8 percent fewer than the 2007 spring census of 3,026. "Because of the inherent variability in the surveys, the lower count this spring is not alarming to me. But what does raise an eyebrow is the leveling off of the 3-year average," said survey organizer Brian Hatfield, a USGS biologist in California. "As usual, the next spring sea-otter survey or two will tell us more about the current population trend."

Above: Sketch map of California coast, showing spatial variation in population density of sea otters. Local densities were calculated as 3-year average number of sea otters counted at each 500-m interval of coast, smoothed by averaging over 10 km of coastline. [larger version]

"The population dynamics are actually quite variable across the range," said Tim Tinker, lead scientist for the USGS sea-otter-research program in California. "Over the past 5 years we have seen relatively high growth rates at the south end of the range, and to a lesser extent at the northern-range periphery, but we have seen very low or negative growth in the central part of the range, where sea-otter densities are highest and where most of the reproduction occurs."

"This year's census results highlight the need for continued attention to the recovery of this threatened subspecies, and the importance of targeted research and recovery actions," added Tinker. Ongoing collection and analysis of demographic data by USGS scientists are aimed at understanding the underlying reasons for the sluggish rate of recovery and variable population trends.

Some of the variation in numbers at smaller scales reflects movements of animals between areas, especially males. For example, counts were lower this year in Estero Bay but higher between Pismo Beach and Point Sal. USGS studies of radio-tagged animals have shown that males frequently make long-distance movements between sandy embayments, such as Estero Bay, Pismo Beach, and Monterey Bay. Additionally, the population distribution has expanded farther to the north and south and now stretches from the mouth of Tunitas Creek in San Mateo County southward to Coal Oil Point in Santa Barbara County. The rate of expansion at the south end of the range continues to outstrip the northward range expansion.

"Range expansion is clearly important for population growth and recovery," said Lilian Carswell of the U.S. Fish and Wildlife Service (USFWS), "and it can also give us insight into how sea otters benefit from or are harmed by environmental factors that differ from those at the center of the range. Comparative studies between these areas can yield information on the dynamics that are affecting population growth and point to needed management actions."

Above: Population trends and recovery criteria for the southern sea otter in California—a threatened population on the Endangered Species list. For southern sea otters to be considered for delisting, the 3-year running averages would have to exceed 3,090 for 3 consecutive years.

The spring 2008 California sea-otter survey was conducted May 2-24 over about 375 miles of California coast. The census results provide counts used to evaluate trends and are not absolute population estimates. The census is a cooperative effort by the USGS, the California Department of Fish and Game's Marine Wildlife Veterinary Care and Research Center, the Monterey Bay Aquarium, 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.

For additional graphics and more detailed information about the 2008 sea-otter survey, see the Spring 2008 Mainland California Sea Otter Survey Results. For general information about USGS sea-otter studies and links to online resources, including video footage of southern sea otters, visit Sea Otter Studies at WERC.

In recognition of Sea Otter Awareness Week (September 21-27), a series of podcast interviews with Tim Tinker will be posted at the USGS CoreCast Web site beginning September 22, 2008, and Tinker will give a public lecture on sea otters at the Monterey Bay Aquarium Research Institute in Moss Landing, California, at 7 p.m. on Wednesday, September 24. (See "Sea Otters Take Center Stage in September," this issue.

Sea Otters Take Center Stage in September
July 2008

California Sea Otters - 2007 Survey Count Reaches New High
September 2007

California Sea-Otter Numbers Dip Again in 2006, But Overall Population Trend Remains Up
August 2006