Survey results show healthy core population
The Southern sea otter population is healthy at the core of its range along the California coast, but the aquatic mammals are still struggling to expand north and south, probably because of predation by sharks, scientists said as they released the results of the latest annual otter survey.
“The population index has exceeded 3,090 for the first time, and that’s encouraging,” said Lilian Carswell, Southern Sea Otter Recovery Coordinator for USFWS, referring to a threshold number for recovery. If the population stays above that number for three years in a row, the U.S. Fish and Wildlife Service could consider a delisting proposal.Sea otters were presumed to be extinct in California in the early 1900s, but a remnant population of 30 animals was discovered and protected in the 1930s near Bixby Bay, north of Big Sur. They were listed as a threatened species in 1977, deemed at risk from oil spills.Healthy long-term populations require range expansion, “which means that sea otters will somehow have to get past the shark gauntlets near the ends of the current range. Carswell said. ‘Over the longer term, it’s not just sea otter numbers we’re after, but the restoration of ecological relationships in the ecosystems where sea otters and other nearshore species coevolved.”Sea otters live in a complex food web with sea stars, urchins, seagrass, crabs and other shellfish. Systemic ocean ecosystem changes along the West Coast could be under way due to climate change, including a recent sea star die-off and an outbreak of toxin producing algae off the West Coast that could affect parts of the food web the otters depend on. Scientists also study sea otters as an indicator of nearshore ecosystem health, since sea otters feed and live near the coast and often are the first predators exposed to pollutants and pathogens washed down from coastlands, such as the microbial toxin microcystin.“We believe the high count this year is partly explained by excellent viewing conditions, but it also appears to reflect increased food availability in the range center,” said Dr. Tim Tinker, a research ecologist who leads the USGS sea otter research program. “The boom in sea urchin abundance throughout northern and central California has provided a prey bonanza for sea otters, and that means more pups and juveniles are surviving to adulthood.”
This year’s survey results suggest an increasing trend over the last five years of more than 3 percent per year. The population index, a statistical representation of the entire population calculated as the three-year running average of census counts, has climbed to 3,272, up from 2,939 in 2013. The growth is accounted for by an unexpected jump in numbers in the center of the sea otter’s range, an area that spans the Californian coast from Monterey south to Cambria.
While the overall population index continues to trend upward, the northern and southern subsets of the population continue a negative five-year decline, dropping 2.5 percent and 0.6 percent per year. “We are still seeing large numbers of stranded otters near the range peripheries, a high percentage of which have lethal shark bite wounds,” said Mike Harris, a biologist with the California Department of Fish and Wildlife. “These deaths may explain the lack of population growth in those areas.”
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,” explained Tinker. “These are the portions of the population that typically fuel the colonization of new habitats.”
In addition to the sea otter population along the mainland coast, the USGS also surveys the subpopulation at San Nicolas Island in the southern California Bight. This population, established by translocation in the late 1980s, struggled at low numbers through the 1990s, but over the last decade has been growing rapidly with a mean growth rate of 13 percent per year.
“The sea otters at San Nicolas Island continue to thrive, and some may eventually emigrate to and colonize other Channel Islands in southern California,” said 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. For southern sea otters to be considered for removal from threatened species listing under the Endangered Species Act, the population index would have to exceed 3,090 for three consecutive years, according to the threshold established under the Southern Sea Otter Recovery Plan by the USFWS. 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 as many as 8,400 animals in California.