In cold blood: Investigating sea lion deaths

CC BY 2.0 Jon. D. Anderson

The mystery of Steller sea lion deaths has baffled scientists. About 10 years ago, their populations in Alaska plummeted by 80 percent - but scientists couldn't observe any factors that would trigger these deaths. In fact, sea lions looked healthy and reproduced successfully, so why were they disappearing?

A new study from Oregon State University and the Alaska SeaLife Center takes us one step closer to uncovering the truth. About a decade ago, Markus Horning and Jo-Ann Mellish began developing tags that can record data about temperature and light changes, and whether they are surrounded by tissue or salt water or air.

The idea was that the tags would float up after a sea lion had been ripped apart by a predator or had decomposed from a natural death. Upon reaching the surface, the tag would transmit data to a nearby satellite, and Horning and his team would receive a message that one of the creatures had died.

These tags were placed in the guts of 45 sea lions in a way that would not affect their behavior or survival. Over the course of the experiment, 17 sea lions died, 15 of which seemed to have been killed by predators.

The next step of the study was to find out who the killers were - and that's where light and temperature data became particularly useful. Horning and his team observed that the temperature around the tag dropped rapidly at the time of death, suggesting that the sea lion had been ripped apart. The tags also sensed light and air rapidly after the death - which again hinted at a sea lion being shredded.

Obvious culprits included killer whales or white sharks, but three of the tags had more mysterious data. The temperature around them dropped to levels found in deep ocean and the area around the tag stayed dark. It would take the tags 5 to 11 days to reach the surface after death was recorded - so what was going on?

Without more information, the scientists could only guess and evidence pointed to sleeper sharks.

"We know next to nothing about sleeper sharks," Horning told TreeHugger. What they do know, however, is that sleeper sharks are cold-bodied animals so that would explain the temperature drop. The sleeper sharks could be ingesting the tags along with the sea lions, and that would explain why it took so long for tags to reach the surface.

Researchers also know that sleeper sharks can get bigger than a great white shark and that their populations in Alaska are probably pretty big. Every year, between 3,000 and 15,000 sleeper sharks are caught in fishing nets - that suggests that there are many many more out there who aren't getting caught.

"Our findings also suggests that we need to be cautious whenever we decide to alter our manipulations of systems," added Horning. "For example, fishery activities will likely have an impact on marine ecosystems and on protected species, but so will reducing or stopping fisheries, especially within systems that have adjusted to a given level or intensity of fishing."

In many parts of Alaska, fishing has been banned in order to protect Steller sea lions. But Horning's study suggests that fishing bans may not be as effective as initially thought. Yes, sea lions may have more fish available, but sleeper sharks might be more common too, since they aren't being killed in fishing nets. This information that can be used to inform future conservation strategies.

"Our data will be useful in future studies that may look at how the risk of predation may influence the decisions sea lions make about where to hunt for food," said Horning. "This is important, so that we can more effectively protect these endangered sea lions, promote their recovery, manage fisheries, and fisheries interactions in a changing ocean ecosystem."

In cold blood: Investigating sea lion deaths
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