Great white sharks may be taking a bite out of sea otter recovery efforts, according to a new survey by the U.S. Geological Survey.
“It’s quite perplexing. We’ve been working with shark biologists to try to understand what might be driving this pattern,” said Tim Tinker, a research wildlife biologist who leads the U.S. Geological Survey’s sea otter program.
He said that the USGS has been monitoring the deaths of southern sea otters (Enhydra lutris nereis)— a subspecies listed as threatened under the Endangered Species Act — since the 1960s. Research previously conducted through telemetry work and other monitoring efforts has shown that about half the sea otters that die in the wild will wash up on the beach eventually.
Before 2000, sea otters that washed up on the beach rarely displayed great white shark bites. But after 2003 Tinker said that more and more sea otters bitten by great white sharks (Carcharodon carcharias) began to surface. Today, some parts of the sea otters’ range have seen a 15-fold increase over the last decade while the amount of deaths from shark bites has increased eightfold overall.
“More than 50 percent of [southern sea otters] that turn up on beach have been dying from shark bites,” Tinker said.
The worst part, he said, is that the sharks seem to be biting in areas where otter populations were just starting to recover — on the northern and southern parts of their range, which stretches roughly from south of San Francisco around 300 miles down to Point Conception. Historically they ranged from Oregon down to Baja California in Mexico and while the north and south of the current range had been slowly increasing in sea otter numbers, they are now dropping again by about 2 percent per year.
One of the strange things, Tinker said, is that the sharks don’t appear to be eating the sea otters. Most of the wounds from the animals they’ve found appear to be from so-called investigatory bites.
“From the shark’s perspective this is just a harmless encounter. From the sea otter’s perspective, obviously, it’s not,” he said.
Many of the researchers aren’t sure exactly what is going on with the sharks. The increased bites could be an increase of juvenile or subadult sharks in the area or it could have something to do with ocean climate. Increased numbers of sea lions and seals in the area may also be drawing the giant predators.
What is bad for the sea otters may be good in terms of shark recovery. “Although it’s not necessarily a positive conservation story for sea otters, it might reflect a positive conservation story for white sharks,” Tinker said. “The marine ecosystem has very definitely been changing and some of these changes have been positive.”
But researchers are still uncertain what this could mean for sea otter numbers.
“On the surface it appears that the population is climbing towards recovery,” said Lilian Carswell, Southern Sea Otter Recovery Coordinator for the U.S. Fish and Wildlife Service, in a release, “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.”
|Joshua Rapp Learn is a science writer at The Wildlife Society.