Salmon sharks are closely related to mako and great white sharks, and one of the few warm-blooded fish species. Photo: Kristine Sowl, U.S. Fish and Wildlife ServiceWhile studying Chinook salmon in the Bering Sea, researchers have found themselves in the wake of an unlikely killer.Andrew Seitz is a researcher at the University of Alaska Fairbanks, who has spent the past several years studying Chinook salmon. He said the first sign of foul play came from satellite tags used in his research this winter. The tags gather behavior and migration data for the salmon, taking temperature and depth readings every two minutes — then relaying them to researchers by satellite later on.Seitz said those temperature readings were what alerted him to the fact that something was, well, fishy.“[The] temperature went from between 45 to 55 F, and it jumped up to 65 to 80 degrees in a matter of a couple minutes. And there’s no water temperature that warm in the Bering Sea,” he said.Seitz suspected immediately what had happened — his tags, and the salmon they accompanied, were in the belly of a warm-blooded predator.But, he explained, the mystery didn’t end there. Marine mammals, much like humans, have a body temperature between 98 and 100 degrees Fahrenheit — meaning the salmon weren’t the victims of an arctic seal or sea lion.“Which leaves just one suspect,” said Seitz.”The salmon shark.”Salmon sharks, which are closely related to mako and great white sharks, are one of the few fish able to keep their body temperature warmer than their surroundings — allowing them to pursue their favorite prey into even the icy waters of the Bering Sea.Still, Seitz said he was surprised to find the sharks in Northern waters during the winter, adding that the predators could be a factor in Alaska’s low king salmon returns.“It’s too early to even speculate on whether salmon sharks are actually having population level effects. But its certainly worth considering and the impact of salmon shark predation should certainly enter the conversation about what is controlling or influencing abundance of Chinook salmon in the Bering Sea,” he said.Seitz plans to further investigate the impacts of predation on king salmon by deploying more satellite tags from a Japanese research vessel this summer.