Predation rates may be killing Steller sea lions, says OSU researcher

Declining sea lion populations in the waters around Alaska has generated mystery and controversy for decades, but the work of Markus Horning and Jo-Ann Mellish seeks to replace speculation with data.  

"Part of the difficulty in determining cause of death in marine mammals is that they often die at sea," said Mellish, an associate research professor at the University of Alaska Fairbanks. "While we will never completely understand the cause of the initial decline, we may be able to better understand why some portions of the population are not recovering."

In order to understand what's happening to the population of Steller sea lions in the eastern Gulf of Alaska, where the endangered species live, Horning and Mellish needed to understand how they died.      

Telemetry devices were surgically implanted in the body cavity of 36 juvenile seals.  The telemetry transmitters record and store information on the life and death of the sea lion. After that, the telemetry device floats to the

surface and transmits signals to the researchers via satellite.  

"With this data, we can distinguish between predators and other causes of mortality," said Horning, an associate professor with the Department of Fisheries and Wildlife at Oregon State University.

Alan Springer, Horning's colleague and a research professor at the University of Alaska Fairbanks, said the controversy over the sea lion decline has been raging for years with arguments over whether global warming, commercial fishing or predation is at fault.  

"In the 1970s, the Steller sea lion population in Alaska just collapsed by about 80 percent in 15 years," said Springer.  "These changes were unprecedented.  It caused a lot of controversy about what the problem was."

Horning and Mellish used the telemetry devices to understand the deaths of the 13 juveniles that died as a result of predation.  

"Data suggest that three were shark attacks, the rest, probably transient killer whales," Horning said.  

The telemetry devices leave the body as the it is torn apart by the predator, as the remains are passed through the digestive tract of the predator or as the body decomposes.

"Even with the limited number of tags we have deployed, we have learned about the most likely cause of death for specific individuals," Mellish said.  "This kind of information has never been available before."

According to Horning, predation rates may have an impact on the declining population of Steller sea lions.

"Predators have a large effect on recruitment and can reduce recruitment by up to 50 percent," Horning said. "Too few of them (female sea lions) survive to give birth to pups."

According to Horning, recruitment is "how many females survive to have pups of their own."

The results of the study are part of a continued effort to understand the population of Steller sea lions in the eastern Gulf of Alaska.  

"Markus is currently working on the next generation of implant that will be smaller. Hopefully we will be able to move into new areas of concern," Mellish said.  

According to Springer, this research provides some facts to help understand what's going on.  

"I think the research was very carefully done," Springer said. "I've been following this research for years. The results are unassailable."  

Horning had the idea for the project in 1998, and shortly thereafter, Mellish became involved.  

"He had this great idea about a new kind of animal tracking," Mellish said about Horning.  "We wrote a grant together, and shortly after I started my job in Alaska working specifically with Stellers, so it was just like all of the right pieces fell into place."

Steller sea lions do live along the West Coast from California to southeast Alaska, but they are what Horning refers to as "the eastern stock." They are increasing at a rate of 4 to 5 percent and have a  threatened status under the Endangered Species Act, whereas the Steller sea lions in the eastern Gulf of Alaska are listed as endangered.  

Horning and Mellish's research paper is available online at PLoS ONE.

McKinley Smith, staff reporter

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