Nothing is more emblematic of the ocean's hidden mysteries than a shark's dorsal fin on the horizon. Unfortunately, following sharks beneath the sea surface has been technically daunting, and until recently, we’ve known very little about how these top predators use the ocean realm. Today however, scientists are rapidly mapping the shark's habitat, with the help of advances in electronic tag technology. The dorsal fin on the surface -- which once evoked the unknown -- now helps provide information that may help conserve some shark species.

In a misty Alaskan fjord, the tiny wake from a shark's dorsal fin sinks back into the sea as the animal dives and slips from view. But overhead -- in space -- Argos satellites have picked up the signals that are transmitted from the tag at the tip of the shark's dorsal fin. By monitoring these radio signals, Argos satellite will locate the salmon shark and send the positional data to the TOPP website allowing scientists to trace its long journey across latitudes, from subarctic to subtropical waters.

The technology behind the satellite tags is impressive, affording researchers the unprecedented ability to track salmon sharks (makos, blue sharks and threshers) throughout their range. These tracking studies are also leading to a better understanding of the specialized physiology that affords the salmon shark its enormous habitat, spanning a wide range of oceanic habitats.

Since 1999, TOPP researchers have traveled to Prince William Sound, where a cousin of the white shark thrives on a rich diet of abundant salmon, squid and herring. Against a backdrop of glaciers, mist and snow-flanked volcanic peaks, Dr. Barbara Block's team of TOPP shark taggers have deployed 51 satellite transmitting tags on the 6-8 foot sharks.

After three years, TOPP researchers amassed a rich cache of data from salmon sharks that over wintered in Alaskan waters and that migrated south into the subtropics, presumably to pup. This data includes over 13,000 days of long-term positional data collected by using SPOT (smart position or temperature transmitting tags) tags, and as much as 5,000 days of high-resolution diving behavior and temperature information from PAT (pop-up archival) tags.

In a paper appearing in the October 7 issue of Science Block, Ph.D. student Kevin Weng, physiologists Pedro Castilho, Jeffery Morriseette, Ana M. Landeira-Fernandez, and NOAA scientists David Holts, Robert J. Schallert and ADFG biologist Kenneth Goldman, report on the results of their salmon shark research, comparing the data from this species with that collected from blue sharks. Comparing tracks and the cardiac physiology of these shark species, the scientists describe how these different species use their North Pacific habitats, and how physiological differences in the sharks’ hearts are potentially correlated with differences in the thermal niches they occupy.

The tags made it possible to record the sharks’ journeys, answering some of the questions about where they go and why. The authors found that female salmon sharks often make migratory journeys from the forage-rich Alaskan Fjords to the oligotrophic subtropical gyre. Along the course of these journeys, the sharks encountered temperatures that ranged from 36 to 75 degrees Fahrenheit (2 C to 24 C), a large thermal range. Some sharks even overwintered in the cold Alaskan winters where they spent long periods in frigid water temperatures. Scientists found that in contrast, blue sharks prefer warmer waters.

The study also reports on the remarkable physiology that enables the endothermic sharks to make a living in such a boreal habitat. While the ability of lamnid sharks to thermoregulate keeps their swimming muscles, viscera, brain and eyes warm, this specialization does not extend to their hearts, which must pump blood that is chilled as it passes through the animal's gills.

For a salmon shark, this means that the heart must not only be able to endure a rapid drop in temperature as the animal dives through the thermocline into cooler waters, but must also be able to beat; i.e. contract and relax, effectively, in the near freezing temperatures of Prince William Sound in the winter. The scientists hypothesize that the over expression in sharks of two key proteins responsible for moving the calcium in and out of the cardiac cells, make it possible for lamnids' hearts to perform well in cold water. Studies in the lab have shown particularly high expression rates of these proteins in salmon sharks.

The early and significant results of TOPP data presented in Science represent a landmark for the project. This study demonstrates the power of the technology TOPP employs, and the effectiveness of the program's broad, interdisciplinary approach towards revealing hidden worlds and advancing our understanding of the mysterious animals that inhabit them. For Block, Weng and their TOPP colleagues, it is only a start. -- Diane Richards