Annual Southern Sea Otter Survey: Despite Small Population Dip, Species Moves a Step Closer to Recovery
According to data released in September by the USGS and partners, the three-year average of the total counts of southern sea otters was down from last year’s high, although it still exceeded the U.S. Fish and Wildlife Service’s (FWS) delisting threshold for a second straight year.
Researchers surveyed sea otter populations along the mainland coast of California, from Pigeon Point in the north to Gaviota State Park in the south, and also the distinct population at San Nicolas Island in the Southern California Bight. This year’s overall count of 3,186 exceeded the 3,090 threshold set by FWS. The otters’ numbers must surpass the threshold of 3,090 for another year before the FWS would consider delisting the subspecies, which is currently designated as “threatened” under the Endangered Species Act. The count is down by 3 percent from 3,272 in the 2016 survey. This dip reflects a decrease in the numbers of sea otters counted along the California mainland.
In contrast, sea otter abundance at San Nicolas Island continues to show an increasing trend, growing over the past decade at a mean rate of 10 percent per year. This population was established by translocation in the late 1980s and struggled at low numbers through the 1990s.
“The lower mainland count this year could be due to poorer counting conditions and very sparse kelp canopies, which likely influenced sea otter distribution,” says Tim Tinker, a research ecologist who leads the USGS California sea otter research program. “However, we cannot rule out the possibility that increased mortality also played a role.”
The number of sea otters dying from shark bite wounds has been high for the last decade. For the past few years, a boom in the abundance of sea urchins (a staple prey item for sea otters) may have offset this shark-caused mortality by supporting improved survival of pups and juveniles. However, the urchin-fed baby boom appears to be slowing, while deaths by shark bite have not. Together with mortality from harmful algal blooms and disease-causing pathogens, sea otters face many threats.
“The high numbers of shark-bitten otters at the north and south ends of the range continue to be cause for concern,” says Mike Harris, a biologist with the California Department of Fish and Wildlife (CDFW), “especially since these are the same areas in which local populations seem to be declining.”
Shark bite mortality has been most severe in the regions from Pigeon Point to Monterey Bay and Cayucos to Point Conception, both of which are areas where sea otter numbers have declined sharply in recent years. Declines at the range ends have implications for the long-term outlook for sea otter recovery. “Negative population trends at the edges of the range are probably responsible for the lack of range expansion over the last decade,” says Tinker. “These are the portions of the population that typically fuel the colonization of new habitats.”
“Although the three-year running average of sea otters at San Nicolas Island continues to indicate strong positive growth, the most recent count also took a dip that may be related to very sparse surface kelp canopy observed this spring or possibly to some emigration to the mainland or to other Channel Islands in southern California,” says Brian Hatfield, the USGS biologist who coordinates the annual census.
Since the 1980s, USGS scientists have computed the annual population index and evaluated trends in the southern sea otter population. For southern sea otters to be considered for delisting under the Endangered Species Act, the population index would have to exceed 3,090 for three consecutive years, according to the threshold established under FWS Southern Sea Otter Recovery Plan. To reach the optimum sustainable population level under the Marine Mammal Protection Act, which is the number of animals that will result in the maximum productivity of the population while considering carrying capacity and ecosystem health, the southern sea otter population would likely have to reach at least 8,400 animals in California alone.
"Conservation efforts over the past century have resulted in the southern sea otter's growth from a tiny remnant population of about 50 animals to the much larger one we see today, where the population index has exceeded 3,090 for the second consecutive year, said Lilian Carswell, Southern Sea Otter Recovery Coordinator for FWS. "That growth has been very encouraging, and we hope to see similar successes at the range ends as strategies evolve to overcome threats from shark bite mortality. Range expansion is essential not only for long-term resilience of the subspecies but also for restoration of the nearshore marine ecosystems where southern sea otters historically occurred.”
The sea otter survey and stranding programs are just one part of a larger research program investigating sea otters and their role as predators in coastal ecosystems. In Elkhorn Slough, located between Santa Cruz and Monterey, a recent study suggests that sea otters’ appetite for crabs can improve the health of seagrass beds. USGS scientists are collaborating with biologists from the Monterey Bay Aquarium Research Institute, the Elkhorn Slough National Estuarine Research Reserve, University of California, Santa Cruz, and the CDFW to study the population in this unique habitat. Another study from UCSC, USGS, and the Monterey Bay Aquarium are investigating how sea otters near Monterey are responding to a dramatic increase in sea urchins, which may be in part a result of loss of predatory sea stars from wasting disease. The scientists are studying whether sea otters play a key role in preventing urchins from over-grazing kelp forests in the absence of sea stars.
Sea Otter Facts
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