Tagging of Pelagic Predators

The salmon shark is a unique animal. This shark has a very broad geographic and vertical distribution, ranging across the North Pacific from the cold subarctic waters of the Bering Sea to warm, tropical waters as far south as Hawaii (22ºN), and diving to depths in excess of 1000 m. Across all of these diverse environments and habitats, salmon sharks are highly active and successful predators. It is well known to be a highly active shark capable of swimming at great speeds. Indeed it is frequently observed breaching from the water when in pursuit of salmon in Alaska. Of particular interest is their ability to use the very cold and productive waters of the Bering Sea and Gulf of Alaska. Some salmon sharks even remain in subarctic throughout the cold, dark northern winter, essentially living in water that is a few degrees above freezing for months at a time. Only a handful of species of sharks, such as the spiny dogfish and Pacific sleeper shark, utilize these cold waters and none of them are nearly as active or wide ranging as the salmon shark.

How do they do this? How are they able to use such a diverse array of habitats, especially the cold waters of the subarctic North Pacific? As anybody who’s been swimming in cold water knows, it takes a lot of energy to maintain your body temperature and still remain active. The same general issues basically apply to salmon sharks. Salmon sharks, however, have some unique adaptations that allow them to deal with these thermal issues. All sharks in the family Lamnidae (e.g. salmon, porbeagle, mako, white sharks) have some degree of anatomical and physiological specialization associated with a capacity for heat conservation and endothermy. Salmon sharks possess counter-current heat exchangers (retia mirabilia) that allow them to retain the metabolic heat generated by highly aerobic tissues in the muscles, viscera, kidney, and brain. This enables the sharks to maintain body temperatures that are elevated as much as 21.2ºC (about 70ºF) above ambient water temperature. In addition, salmon shark cardiac physiology is also specialized for utilizing cold habitats, which allows their hearts to provide their active, warm tissues with enough oxygen to effectively forage in cold environments. So in essence, through these adaptations, these sharks are able to generate and conserve body heat, allowing them to be more active and effective across a wide range of temperatures. These unique characteristics allow the salmon shark to be highly active predators across their broad geographic and vertical range, and likely underlie their ability to use the highly productive boreal and polar waters of the north Pacific.

This last summer, as part of ongoing research we tagged several salmon sharks with SPOT tags in Prince William Sound, Alaska. The tracks of three sharks currently transmitting their positions highlight the remarkable ability of salmon sharks to use a huge range of habitats. Over the last several weeks, if you were following the sharks on the TOPP or GTOPP websites, you would have noticed a shark (1710023) entered the Bering Sea. This is only the second shark that we have tracked into the Bering Sea, and the first to cross the international date line (180ºW). This shark spent several weeks near St.George Island, one of the Pribiloff Islands, in the Bering Sea, not too far from the ice edge. Needless to say, this shark is in some very cold water (1-2ºC) and the simple fact that it is there at all demonstrates how its endothermic capabilities allow it to utilize this relatively extreme habitat.

Another shark (1710014) is in the deep oceanic waters of the North Pacific approximately 1200 miles west of Santa Barbara California. It has just entered the warm clear, waters of the subtropical gyre and is in water temperatures of around 18-20ºC. The third shark is currently in the southern California Bight, just off San Diego, and chances are if it was finning at the surface you may be able to see it from land. Very few, if any, other species of marine fish or elasmobranch are able to move across such a variable oceanscape and utilize such a diverse array of environments, and this is just one of the reasons why salmon sharks are such a fascinating shark to study.