Muscle Warmers

Study explores how salmon sharks brave the cold.

It turns out that salmon sharks, the North Pacific cousin of the mako (see "Salmon Shark Slugfest" in the November 2005 SWS), have something in common with warm-blooded mammals, according to Canadian scientists.

The researchers tested three salmon sharks caught in the Gulf of Alaska. After measuring temperatures from different parts on the sharks' bodies, the scientists found that some muscles only work when much warmer than the surrounding water, which helps explain why a salmon shark never stops moving; otherwise its muscles start to cool down, causing the fish to sink. The constant motion generates significant body heat, which warms the muscles and allows the fish to survive the extremely cold waters of the North Pacific.

"Other fish can cool themselves down and still function all right," Douglas Syme, one of the authors of the study, told reporters. "The salmon shark can't do that. It's like us, it has to stay warm all the time."