ANCHORAGE — Alaska’s endangered Steller sea lions could have trouble recovering because so many juveniles are being eaten by killer whales and sharks, according to findings of a six-year study published this week.
Researchers at Oregon State University and the Alaska Sealife Center starting tracked 36 juvenile Steller sea lions in 2005. By November, 12 had died, a death rate that’s not exceptional, OSU marine mammal expert Markus Horning said Thursday.
“What is different is the number of animals that have apparently died by predation,” he said.
Eleven sea lions were confirmed dead by predators, and the other may have been but could not be verified by data collected.
Predators last month killed two more young sea lions in the study, Horning said.
The western population of Steller sea lions covers an area that starts near Cordova and runs west to the Aleutian Islands. According to the National Oceanic and Atmospheric Administration, the western population declined by 75 percent between 1976 and 1990. It decreased another 40 percent between 1991 and 2000, according to the agency’s Office of Protected Resources website.
Federal managers seeking to protect sea lions have placed restrictions on commercial fishing in the Aleutians where possible competition for food has been an issue. That drew opposition and eventually a lawsuit from the state of Alaska and commercial fishing interests.
The new study focused on animals in Prince William Sound and the Kenai Fjords in southcentral Alaska and used satellite tags specifically designed for the project, Horning said.
Researchers surgically implanted tags into the animals’ gut cavity. The devices record data, including temperature readings, but transmit it only when the sea lion dies and the buoyant tag reaches the ocean’s surface or washes onto a beach.
A gradual cooling recorded by the devices would indicate an animal died of starvation, disease or some other non-violent death. A sudden temperature drop and an immediate sensing of light or water would indicate dismemberment by a predator, Horning said.
Ten juvenile sea lions in the study died in their first year after weaning, when they were 10 to 24 months old. Three died in the following year, and one died at age 4. That shows the risk of mortality declines with age, Horning said.
Previous models have suggested falling birth rates as a reason why Steller sea lions have not recovered. Those models, however, essentially present a hypothesis unconfirmed by hard data, Horning said. Instead, it’s possible that high levels of predation are the predominant factor preventing the recovery of the species, he said.
“Our model suggests that even if the birth rate would be as high as is possible, if every female out there has a pup every year, the population could still not recover unless predation were reduced,” he said.
Killer whales are generally the prime suspect in Steller sea life predation, possibly because attacks can be observed. Sharks should not be discounted, Horning said. Data from three recent sea lion deaths indicates they may have been killed by Pacific sleeper sharks, he said. Salmon sharks are another known predator and great white sharks are suspected predators, he said.
The study does not extrapolate findings to other areas such as the Aleutians. The scientists will continue to monitor the other sea lions in the study.
Jo-Ann Millesh of the Alaska Sealife Center in Seward and the University of Alaska Fairbanks was co-author of the study. It was published this week in the scientific journal PloS ONE.