Researcher Suspects Metals Are Cause of Sea Lion Decline

Something is slowly killing the Western Steller sea lion at an alarming rate and a Texas A&M University researcher believes that increased metal concentrations in the available food sources could be the reason.

The researcher, Robert Taylor, Ph.D., of the Veterinary Integrated Biosciences Department at Texas A&M in College Station, Texas, specializes in studying heavy metal behavior in biological systems. He and his co-workers believe metals — specifically mercury — may be the culprit in the declining number of Western Steller sea lions. Their work on the subject has been published in Marine Pollution Bulletin.

The Steller sea lion has attracted scientific attention in recent decades due to significant, unexplained declines in their number over a large portion of their range in Alaska , Taylor said.

"The overall [Steller sea-lion] population has decreased by approximately 80 percent in the late twentieth century," says Taylor.

Surprisingly, however, only the Western sea lion population has seen a decline. The Eastern population has actually showed a marginal increase, but the researchers point out that the increase is lower than what is expected for the species.

"The most important observation is that there are significant differences in [some] metal concentrations between pups of the Western and Eastern populations," says Taylor. "In particular, the element mercury stands out … its concentrations are much higher in several tissue types in the declining Western population."

"Pup production and survival are key factors in population changes and it's possible that elevated contaminant levels in Western pups are contributing to the stresses that affect the overall health of this population," explains Taylor.

Talking about the higher mercury content, Taylor says, "I think the most likely scenario is that there could be different food sources for the two populations, with the western population relying upon a food source that is higher in mercury."

Taylor says that the Western population may be directly and indirectly affected by commercial fishing. "[Commercial fishing] could impact the seals directly if they were caught in nets, but it might impact them indirectly if it removed their favored prey and caused them to eat something else. If this alternate food source is something with a somewhat higher mercury level then the seals and their pups end up with higher mercury in their bodies," he says.

"If mercury levels get high enough, an animal's behavior can be impacted. You don't have to kill an animal with poison in order to affect the overall population. All you have to do is adversely affect its breeding or pup-rearing behavior and then population numbers drop."

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