Sharks may be to blame for surge in Calif. sea otter deaths
SAN FRANCISCO -- A record number of sea otter bodies were found on California coastlines last year, a trend that leaves scientists and conservationists concerned for the future of the furry ocean animals.
About 304 carcasses were found in 2010, according to preliminary numbers released by the U.S. Geological Survey. Based on a spring count of 2,719 living sea otters, those bodies accounted for 11.2 percent of the population.
In 2008 and 2009, there were 237 and 232 otters found dead, respectively. Those deaths accounted for less than 9 percent of the reported population in those years.
The USGS numbers also showed that more pups and female otters were found dead last year than in previous years.
"It's a giant increase," said Steve Shimek, founder of the Otter Project, a nonprofit organization that supports sea otter conservation. "When you combine that with the fact this past year the spring count showed there were very few pups born, that bodes terribly for the future."
Scientists are trying to figure out why an increasing number of the cuddly looking sea mammals, which were once hunted to near extinction for their pelts, are being found dead on the state's shorelines. Their numbers were on the rise until 2007, then started declining.
A toxin produced by freshwater bacteria, called microcystin and commonly referred to as blue-green algae, has been confirmed responsible for the deaths of 27 sea otters over the past five years, said Tim Tinker, lead scientist on sea otter studies for the USGS Western Ecological Research Center.
"That by itself would in no way explain the high number of carcasses we have now," said Tinker, who also teaches ecology and evolutionary biology at UC Santa Cruz. He estimates the survey captures only about 40 percent of total California sea otter deaths.
Tinker believes shark bites -- most likely from great whites -- may be the more likely culprit.
About 65 to 70 otter carcasses were found to have been bitten by sharks last year, compared with about 60 for the previous year, which was also a high number, he said.
Because sea otters are not the sharks' preferred prey -- they typically dine on seals and sea lions -- the sharks seem to be tasting the sea otters before moving on, Tinker said, explaining that many of the otter bodies were found with just a single bite wound.
"From the shark's perspective, there's nothing lost," he said. "But from the sea otter's perspective, it's a lethal encounter."
While the causes of their deaths are still being studied, Tinker believes the bitten otters probably died from infection. Because otters have virtually no body fat, they are far less likely than a blubbery seal or sea lion to survive a bite wound, he said.
Unlike the otter population, the seal and sea lion populations have grown substantially in recent years, and their breeding and moulting spots have also increased along the coast, Tinker said.
This increase is particularly notable in the southern end of their breeding areas, from Morro Bay to Pismo Beach, where an increased number of sea otter bodies were also found, Tinker said. That means the sharks appear to be shifting their hunt south to take advantage of the increased prey, and the otters become the unfortunate collateral damage.
"The otter population is already so small that a relatively small number of lethal bites are having, we believe, a pretty major impact on the sea otter population," he said.
In 2008 and 2009, there were 237 and 232 otters found dead, respectively. Those deaths accounted for less than 9 percent of the reported population in those years.
The USGS numbers also showed that more pups and female otters were found dead last year than in previous years.
"It's a giant increase," said Steve Shimek, founder of the Otter Project, a nonprofit organization that supports sea otter conservation. "When you combine that with the fact this past year the spring count showed there were very few pups born, that bodes terribly for the future."
Scientists are trying to figure out why an increasing number of the cuddly looking sea mammals, which were once hunted to near extinction for their pelts, are being found dead on the state's shorelines. Their numbers were on the rise until 2007, then started declining.
A toxin produced by freshwater bacteria, called microcystin and commonly referred to as blue-green algae, has been confirmed responsible for the deaths of 27 sea otters over the past five years, said Tim Tinker, lead scientist on sea otter studies for the USGS Western Ecological Research Center.
"That by itself would in no way explain the high number of carcasses we have now," said Tinker, who also teaches ecology and evolutionary biology at UC Santa Cruz. He estimates the survey captures only about 40 percent of total California sea otter deaths.
Tinker believes shark bites -- most likely from great whites -- may be the more likely culprit.
About 65 to 70 otter carcasses were found to have been bitten by sharks last year, compared with about 60 for the previous year, which was also a high number, he said.
Because sea otters are not the sharks' preferred prey -- they typically dine on seals and sea lions -- the sharks seem to be tasting the sea otters before moving on, Tinker said, explaining that many of the otter bodies were found with just a single bite wound.
"From the shark's perspective, there's nothing lost," he said. "But from the sea otter's perspective, it's a lethal encounter."
While the causes of their deaths are still being studied, Tinker believes the bitten otters probably died from infection. Because otters have virtually no body fat, they are far less likely than a blubbery seal or sea lion to survive a bite wound, he said.
Unlike the otter population, the seal and sea lion populations have grown substantially in recent years, and their breeding and moulting spots have also increased along the coast, Tinker said.
This increase is particularly notable in the southern end of their breeding areas, from Morro Bay to Pismo Beach, where an increased number of sea otter bodies were also found, Tinker said. That means the sharks appear to be shifting their hunt south to take advantage of the increased prey, and the otters become the unfortunate collateral damage.
"The otter population is already so small that a relatively small number of lethal bites are having, we believe, a pretty major impact on the sea otter population," he said.
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