NEWPORT, Ore. – A pioneering project that implants life-long monitors inside of Steller sea lions to learn more about why the number of these endangered marine mammals has been declining – and remains low in Alaska – is beginning to provide data, and the results are surprising to scientists.
Four out of five of the data sets that researchers have recovered indicate that the sea lions died from traumatic causes – most likely, attack from transient killer whales.
This comes as a surprise to many scientists and resource managers who previously thought that recent sea lion population trends are largely attributable to depressed birth rates, a loss of fecundity, or poor nutrition, according to Markus Horning, a pinniped specialist with the Marine Mammal Institute at Oregon State University and principal investigator in the study.
“This obviously is a very small sample so we cannot overstate our conclusions,” Horning said, “but the fact that four out of five deceased Steller sea lions that we received data from met with a sudden, traumatic death is well beyond what conventional thought would have predicted. It could be coincidence…or it could mean that predation is a much more important factor than has previously been acknowledged.”
Results of the study are being published in the journal Endangered Species Research. Horning also is presenting his findings this week at a meeting of the Society for Marine Mammalogy in Quebec City.
The science behind the discovery is a story within itself. The researchers worked with Wildlife Computers, Inc., in Redmond, Wash., to develop a tag that could be implanted in the body cavity of sea lions and remain there during their life span. Conventional externally applied tags rarely have the battery power to transmit data for longer than a year and are shed during the annual molt – thus information about sea lion mortality is difficult to obtain.
These new tags, however, stay within the sea lion until its death, recording temperatures for as long as eight to 10 years. When an animal dies, and either decomposes or is torn apart by predators, the tags are released and send a signal to a satellite that transmits it to Horning’s lab at OSU’s Hatfield Marine Science Center in Newport, Ore.
“We can tell whether an animal died by acute death through the temperature change rate sensed by the tags and whether the subsequent transmission of a signal is immediate or delayed,” said Horning, who is an assistant professor of fisheries and wildlife at OSU.
Horning and his collaborator, Jo-Ann Mellish, have tested cooling and decomposition rates of sea lions on animals that have died from stranding or other causes. They’ve also inserted tags within those animals to see how long it would take before the signal would transmit based on whether an animal was on the beach, was deep at sea, or was torn apart by predators.
Their protocol for inserting tags within live Steller sea lions was developed from initial deployments on non-threatened, stranded California sea lions at the Marine Mammal Center in Sausalito, Calif.
“We wanted to make sure there was no adverse impact on the animals,” Horning said, “and there wasn’t.”
Since 2005, Horning and his colleagues at the Alaska Sea Life Center in Seward have implanted tags into 27 Steller sea lions that were captured and released off the coast of Alaska. Since that time, they have received data from five animals – at least four of which appeared to die from traumatic deaths, based on the rate of tag cooling and immediate signal transmission.
Horning said the tags can precisely identify the moment an animal died from temperature data. And while they are confident in their ability to determine whether the death was caused by predation or non-traumatic causes, identifying the actual predator is admittedly a bit of guesswork, Horning says.
“There are only a couple of species that are known to target sea lions as prey,” he pointed out. “Orcas are not only common in that region of Alaska, they also have been observed preying on sea lions. Some species of sharks are known to attack sea lions, but they aren’t as common in those waters and there haven’t been any observations of predation in the study area.”
If predation of Steller sea lions is more prevalent than previously thought, Horning said, there are implications for management.
“If the proportion of sea lions killed by predation in our study was applied to population models, we estimate that more than half of the female Steller sea lions would be consumed by predators before they have a chance to reproduce,” Horning said. “We recognize that this is a very coarse estimate based on a small sample size.
“But we hope this serves as a wakeup call to begin looking more closely into the actual role of predation as a determinant in Steller sea lion populations.”