Latest survey shows population growth in some areas
There’s good news and bad news for sea otters along the California coast. A boom in sea urchin numbers in some areas is providing plenty of food for the species, but sharks are taking a bite from the otter population at the northern and southern end of their range, potentially slowing the spread of otters up and down the coast.
Sea otters were presumed extinct in California after the fur trade years, but a remnant population was discovered off the coast of Big Sur in the 1930s, prompting a recovery effort. Otters are a keystone coastal species that maintain the ecological balance in undersea kelp forests.
Overall, results from the latest U.S. Geological Survey sea otter count suggest the population has grown to 3,054 animals from 2,711 in 2010 — about 2 percent a year.
“It appears that the high pup counts from the last few years might be translating into higher numbers of juveniles and adults in the center of the range,” said Brian Hatfield, the USGS biologist who coordinates the annual census. “This makes sense if there are significantly more sea urchin prey available to them in those areas. However, our long-term census data suggests the elevated numbers of otters seen during this survey along the Monterey-Big Sur coast may not persist,” Hatfield said.
But at the northern and southern end of the range, the sea otter population is declining by about 2 to 4 percent per year, numbers consistent with increased shark bite induced mortality in these same areas. The increase in white shark bites became evident after 2005 and now appears to be impacting the growth and expansion of the population at the peripheries of the range, as described in a recent publication.
“There’s much more to the story here than the main finding would suggest,” said Dr. Tim Tinker, a research ecologist who leads the USGS sea otter research program, “We are looking into various factors that may be affecting the survey results, including a boom in urchin abundance from Big Sur to Monterey that may explain the uptick in numbers in the range center, and high levels of shark bite mortality that are likely responsible for continued declines at the north and south ends of the range.”
USGS scientists have been tracking otter numbers since 1980, in part to determine whether they could be taken off the Endangered Species List. Under published recovery goals, that could happen if the population index were to exceed 3,090 for three consecutive years.
“On the surface it appears that the population is climbing towards recovery,” said Lilian Carswell, Southern Sea Otter Recovery Coordinator for USFWS, “but it’s clear the underlying trends in different regions must be taken into consideration. Full recovery of the population will ultimately require range expansion to the north and south.”
The full sea otter report is available online.