White sharks are taking a heavy toll on California’s sea otter population.
A recent article for the journal Marine Mammal Science concluded that the Estero Bay and Pismo Beach areas are hotspots for shark bites on sea otters. The article written by four sea otter biologists noted an eightfold increase in shark bites along the sea otters’ southern range, which stretches from Cayucos to Point Conception.
“Over the past 10 to 15 years the number of shark-bitten sea otters in California has increased with shark-related injuries becoming the most frequently identified primary cause of death in the assemblage of beach-cast carcasses,” the article concluded.
White shark bites now are found on more than 50 percent of recovered otter carcasses, the study said.
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The trend is puzzling because sea otters are not considered a prey species for white sharks. The biologists concluded that the bites, while fatal, were exploratory only and the sharks did not intend to eat the otters.
The trend is also troubling because it threatens to stop the recovery of sea otters in California, said Mike Harris, a sea otter biologist in Morro Bay with the state Department of Fish and Wildlife, who co-wrote the article.
“It’s happening in the part of the otters’ range in the north and south where we need population growth,” he said. “Shark-bite mortality is essentially keeping the otter population from expanding into new habitat.”
The trend shows no sign of letting up. On Monday, Harris recovered a shark-bitten sea otter carcass from Morro Strand State Beach, not far from where a shark bit a chunk out of a woman’s surfboard on Saturday.
California sea otters range from Pigeon Point south of San Francisco in the north to Point Conception in the south. A census in 2014 put the otter population at 2,944, up five animals from the previous year.
Sea otters are making a slow recovery after being hunted to near extinction for their luxurious fur during the 18th and 19th centuries. A small colony survived in Big Sur.
In addition to shark bites, sea otters suffer from many other causes of death, including microbial toxins from polluted runoff and brain infections contracted from the feces of wild and domestic cats.
The animals have been listed since 1977 as threatened under the federal Endangered Species Act. Their population would have to exceed 3,090 for three consecutive years to have them removed from the list.
In the heart of their range, from Monterey to Cayucos, the otter population is stable and at the carrying capacity of their habitat. However, otter populations in the southern extent of their range have dropped by 3.3 percent in the past five years, according to the U.S. Geological Survey, which conducts an annual sea otter population count.
The cause of this increased shark-bite mortality is unclear, Harris said. The most common explanation is that the population of white sharks is increasing, although there is little data to corroborate that.
White sharks have been fully protected in California since 1994. The sharks feed primarily on seals, which are thriving. Northern elephant seals, sea lions and harbor seals have all experienced population increases in recent decades.
“Their main prey base is very robust and growing,” Harris said.
A 2014 study put the shark population at 2,400.
The authors of the article studied the reports on 1,870 otter carcasses collected since 1985. A sharp increase in shark bites on otters began in 2003.
In addition to Harris, the article was co-authored by Tim Tinker and Brian Hatfield with USGS and Jack Ames with the state Department of Fish and Wildlife.