The population of southern sea otters off the coast of California has declined for the third year in a row, according to a study published Tuesday by the California Department of Fish and Wildlife and the U.S. Geological Survey.
The 2019 population is at 2,962 — 166 fewer otters than in 2018, the study said. The population has declined slightly each year since it hit a peak of 3,272 in 2016.
The population number is based on a three-year average of combined counts from the otters’ range along the central California coast and San Nicolas Island in the Channel Islands.
Senior environmental scientist Mike Harris of the California Department of Fish and Wildlife said the study showed high mortality rates are likely the biggest contributor to the population decline — and the No. 1 cause of death was great white shark bites.
“Evidence suggests the white shark population is recovering,” Harris said, adding that the sharks’ primary prey are pinnipeds such as seals.
The great white sharks don’t eat the otters, he said, but they bite the marine mammals to see if they could be food. Then the sharks swim away, and many of the otters die from those exploratory bites.
Harris said he estimates that scientists find approximately 450 otters that are stranded, sick, dead or injured each year and about half of those have been bitten by a shark. Shark bites don’t always immediately kill an otter.
“Sometimes the ultimate cause of death is bacterial infection from the wounds,” Harris said.
The mortality rate particularly impacts the number of strong, healthy adult otters, or “prime-aged” otters, Harris said. Other threats to otters include infectious diseases and pollution. Oil spills are also a concern because sea otters are so susceptible to them, he added.
Harris added that the study shows that the southern sea otter population still has a ways to go toward full recovery: The otters have yet to settle in their historic range, which expands from Oregon down to Baja California in Mexico.
The otters’ current range is from the Gaviota area to Pigeon Point north of Santa Cruz, and there’s also a population on San Nicolas Island, Harris said.
“These remarkable marine mammals continue to encounter hurdles, like shark bite mortality, that limit their ability to expand into areas where they historically thrived,” said Lilian Carswell, southern sea otter recovery coordinator for the U.S. Fish and Wildlife Service, in a California Department of Fish and Wildlife news release.
According to the news release, the areas that have seen the biggest decline are from Pigeon Point to the northern part of Monterey Bay and most areas south of Cayucos, which happen to be the areas where the most shark bites occur.
The largest sea otter population appears to be in the central part of the range between Seaside and Cayucos, the release said.
“One factor likely contributing to the positive trend in the central range is the recent increase in sea otter prey availability: sea urchins and mussels,” Harris said in the release.
Southern sea otters are listed as threatened under the federal Endangered Species Act and are a fully protected species under California law.
The 2018 population count showed that the population of sea otters had remained above 3,090 for the third year in a row, an important benchmark that triggers consideration by scientists to remove otters from the endangered species list.
“The U.S. Fish and Wildlife Service is currently reviewing the status of the southern sea otter, gathering the best available scientific and commercial data regarding the species,” the news release said.