What Are We Learning?
A Complex Puzzle
Why the Steller sea lion population is not recovering is a complex puzzle that researchers are working to solve. Possibly many factors are involved: competition with fisheries for their food, low birth rates (maybe as a result of pollution), changing ocean conditions, changing ability of the environment to support animals, mortality from ship strikes, fishing gear entanglement (photo, right), movement out of the area, and high levels of predation from sharks and killer whales.
The first generation Life History Transmitter, LHX1, helps answer what role predation plays in a unique way. It takes body temperature measurements throughout the life of the animal to determine how it died. Dr. Jo-Ann Mellish and Dr. Markus Horning captured 36 young Steller sea lions and implanted them with two LHX1 tags each from 2005 - 2011. Over the course of seven years, LHX1 tags reported sixteen deaths. Check out the map showing the location of these mortality events.
What's Going On? Predation!
In ten of the sixteen deaths, the LHX1 tag sensed a very quick drop from the normal sea lion body temperature to the same temperature as the surrounding seawater. The tag also sensed light and air right away. From these results scientists concluded that the tags quickly came out of the sea lion’s body when a predator ate it. The likely cause of death in these ten juvenile Steller sea lions was from transient killer whales (photo, left). Transient killer whales regularly eat marine mammals such as sea lions. (Check out Alaska Crime Scene Investigations to discover how scientists used the body temperature to solve this puzzle.) In three more cases, at least one of the two tags was swallowed by the predator: as before the tags sensed an immediate temperature drop, but only sensed light and air several days later. Dr. Horning believes this is because these tags were swallowed by a cold-blooded predator. The tags only sensed light and air after they were passed or spit out by the predator. The likely cause of death in these three sea lions was from attacks by sharks.
One of the sixteen deaths was more complicated. The temperature drop was more gradual, and the LHX1 tags did not see light or sense air for several weeks. This is what scientists would expect to see in the case of a death by other causes, like starvation or disease. However, Dr. Horning concluded that a large predator killed this animal and tore it into pieces. This is because the tag cooled quicker than it would have cooled in a whole, intact body, and the tag came out of the body about two weeks quicker than expected.
Two Deaths a Mystery
Even with advanced technology two deaths still remain a mystery. Scientists only received a small amount of data, which was not enough to determine a cause of death.
Fourteen out of Sixteen
Deaths Likely from Predators
Researchers conclude that at least fourteen of sixteen sea lions that died were very likely the victims of a predator, eleven most likely transient killer whales, and three most likely from sharks.
Tags Working A-OK
From tests in ten carcasses (dead sea lion bodies, photo-left) and from the actual research study, scientists received data from 46 out of 52 LHX1 tags. From this Dr. Horning concludes that the team can expect to receive data from 98 out of every 100 animals with dual LHX tags that die. This successful data recovery rate allows him to do research on fewer animals to get a scientifically significant result. (See FAQs).
Researchers have known for a long time that roughly seven out of ten of young Steller sea lions die before they reach age five. (Our results reflect a similar percentage and support this older data.) However, what is very new and quite surprising is the information on how these animals are dying. “While we have always known that killer whales and sharks ate Steller sea lions, we now have strong evidence that the majority of deaths in juvenile Steller sea lions result from predation,” says Dr. Horning. “This is an exciting new finding that could have significant implications for conservation and management.” It could take until 2018 or later to get information about the cause of death in mature adult Steller sea lions as the study animals age.
How much of a problem is predation for the recovery of the Steller sea lion?
This depends on the number of Steller sea lion deaths compared to the number of births. The second generation of the Life History Transmitter (LHX2) will help researchers find out how many pups female Steller sea lions have over their lifetime. This allow scientists to determine exactly how much of a problem high levels of predation are for the recovery of the Steller sea lion.