Biologists dissect shark-bitten sea otters in an attempt to understand both species
The smell from a fresh sea otter carcass cuts through the Morro Bay breeze like a blade. Sea otter biologist Mike Harris and shark biologist Ralph Collier fight off the smell and lean over the specimen that was found floating just off the Morro Bay coast by a whale-watching boat.
“This is a real chomp,” Collier said, pulling back the dense hair on the southern sea otter to reveal a small incision. These deadly cuts are the result of a juvenile great white shark bite, he concludes.
Working in a tent-covered work station next to The Marine Mammal Center in Morro Bay last week Collier, Harris and sea otter biologist Colleen Young came together to try to answer important questions about why and how this otter arrived on the table in front of them. Harris and Young are both biologists with the California Department of Fish and Wildlife.
“We are trying to get some shark expertise to see whether there are things that we can be collecting that will help us understand this shift in (otter) mortality, this increase in shark-bitten sea otters,” Harris said.
50percent of otter carcasses on the Central Coast killed by sharks
It has been a question Harris has been looking into for a while, since researchers noticed a sharp spike in shark-bitten otters. From Estero Bay to Point Conception, the frequency of otters being bitten by sharks has increased eightfold since 2003, according to a study Harris co-authored. Published last year, the study said that more than 50 percent of sea otter carcasses found along the California coast were killed by sharks.
Not much has changed since last year.
In the first 26 days of June, 18 out of 34 dead otters found in California were bitten by sharks, Harris said.
“Another eight cases we suspect (were shark-bitten), but due to decomposition, it was hard to tell,” he said.
Changes in shark behavior
Two years ago, Collier and Harris linked up to see what they could learn from each other.
“Sea otters, I think, are a unique indicator of shark activity because we have no evidence of consumption and the deposition of some carcasses on the beach are giving us some sense of some seasonal patterns,” Harris said. “A few years ago, shark folks were saying the (shark) adults all leave, but we were seeing shark-bitten otters throughout the year, so that’s what started some of the conversation between the different groups.”
It’s our job to figure out why.
Ralph Collier, shark biologist
On Wednesday, Collier was looking over each of the bite marks in five otters found up and down the California coast — including one found on Avila Beach in April — in an attempt to determine the size of the shark involved. Using a formula that compares the distance between teeth marks, Collier determined that most of the white sharks responsible for sea otter deaths are juvenile sharks about 8 to 9 feet in length, though adult sharks have been known to attack otters, too.
Knowing the size of the shark and the time of year the sea otter was attacked has helped researchers learn that some juvenile sharks on the Central Coast are hanging out year-round — not entering the migratory cycle until they reach adulthood, Collier said.
And the size of prey consumed by white sharks is shifting, Collier said.
In years past, Collier believed that white sharks wouldn’t feed on marine mammals — pinnipeds such as sea lions and elephant seals — until the sharks reached 9 or 10 feet. But some of the bites Harris has found have been from sharks in the 6-foot range. Researchers believe sharks are biting sea otters because they are mistaking them for smaller pinnipeds.
No otter has ever been found in the stomach of a great white.
“Is this a shift in the white shark population where they are going to start going after these marine mammals at a smaller size? Or is it the oceanographic conditions that are causing this with other natural prey that are moving out of the area?” Collier said.
“It’s (shark biologists’) job to figure out why.”
New DNA tests
While there are still questions, Collier said it is safe to say that the sharks attacking otters are great white sharks, a species he believes is increasing in numbers here on the Central Coast. Other possible sharks, like mako or thresher sharks, either don’t swim close to shore where otters gather or don’t match the typical bite diameter of 14 inches found on many of the carcasses.
Collier and a colleague are developing a method of using shark tooth enamel left behind in the otter bites to create a DNA profile of the shark, something that has never been done before. That information will be used to help determine great white shark migratory patterns, which are largely unknown.
45 Otters seen in Estero Bay
300 Otters counted off Pismo Beach
By using instruments on tagged otters that allow researchers to measure water temperature along with bite marks, Harris also has been able to determine what the scene looked like. He has found that some of the otters are being killed at night by ambush, possibly while they sleep.
Though the southern sea otter remains an endangered species, in the Morro Bay harbor the population numbers are higher than they have been in 25 years, Harris said. At times, there are 45 sea otters swimming in the bay.
“During our spring survey, we had almost 300 otters around Pismo Beach,” Harris said. “But those numbers can shift.”
‘This process of science’
But, while the southern sea otter population seems to be making a comeback, the great white shark population might also be rebounding — inhibiting further growth for a species that is still trying to recover from the devastating effects of the fur trade in the 18th and 19th centuries.
“The range distribution has not changed. That’s what we really need to reach recovery goals,” Harris said, adding that the current range for southern sea otters extends from Gaviota to just north of Año Nuevo near San Francisco. “We need otters to reoccupy historic range and, unfortunately, the rates of shark-related mortality happening on those range peripheries is probably a limiting factor.”
Harris and Collier know the work they are doing will never change behavior, but agree that it is another important step in understanding great white shark and sea otter populations, both of which are considered important cornerstones of the marine ecosystem.
“It’s just this process of science,” Harris said. “We are just trying to understand the dynamic that’s influencing (otters) that we are charged with protecting and hopefully reaching recovery goals for.”