Electronic tags broadcasting from the dorsal fins of salmonsharks reveal that these top predators migrate from the glacial watersof Alaska to the warm seas off Hawaii, according to a new study in thejournal Science. The salmon shark's ability to survive such a broadrange of thermal conditions is attributed to high levels of specializedproteins that keep its heart muscle cells beating at very lowtemperatures, say the study's authors.
"Sharks are decliningglobally, yet the movements and habitats of most species are unknown,"says Stanford University biologist Barbara Block, chief scientist ofthe study, which included researchers from the National MarineFisheries Service (NMFS), Monterey Bay Aquarium and the AlaskaDepartment of Fish and Game. The research was conducted as part of theCensus of Marine Life, an international network of scientists seekingto understand the abundance and diversity of life in the oceans.
Dual tagging technology
Writingin the Oct. 7 issue of Science, Block, Stanford doctoral student KevinWeng and their colleagues compared the migration and unique cardiacphysiology of the salmon shark—a close relative of the white shark—withthe warmer, subtropical migration and physiology of the blue shark,another large predator.
In the study, the scientists used twokinds of satellite tagging technology. One, known as the SmartPosition-Only Tag (SPOT), tracks individual sharks in real time as theymigrate across the sea. The SPOT tag is attached to the tip of thedorsal fin, and when the shark surfaces, the exposed tag transmits theanimal's position to orbiting satellites. Scientists can then downloadthe data and follow the shark's day-to-day movements on their computersback home. In fact, this technology allows anyone with Internet accessto track the sharks' daily migration in near-real time on a webbrowser: http://www.toppcensus.org.
"I like to check on theposition of the sharks with my coffee every morning," says Block, theCharles and Elizabeth Prothro Professor in Marine Sciences atStanford's Hopkins Marine Station.
The second technology, calledthe Pop-Up Satellite Archival Tag (PAT), records water pressure,temperature and light while the animal swims. Unlike SPOTs, whichremain fastened to the fin, PATs detach from the animal on apre-programmed date, then float to the surface and begin transmittingdata to orbiting satellites. Combining the two technologies allowedBlock and her colleagues to generate a rich dataset that includedhighly accurate details about each shark's position and divingbehavior, as well as surrounding oceanographic conditions.
"Wecombined radio uplinking SPOTs that give us accurate positions anddata-logging PATs that store information for the duration of theprogrammed mission," Block explains. "By using two types of tags, we'reable to accumulate a larger dataset on the sharks' habitat andpreferences with a greater accuracy than we've been able to do before."
Totag salmon sharks, Block and her colleagues traveled to Prince WilliamSound in the Gulf of Alaska during the summer. There, in calm fjords,they encountered acrobatic sharks leaping after pink, coho and othervarieties of salmon returning to spawn. From 2002 to 2004, thescientists tagged 51 female salmon sharks. For comparison, co-authorDavid Holts, a biologist with NMFS, led the researchers to the SouthernCalifornia coast, where 31 blue sharks were outfitted with SPOT and PATtags.
During the three-year study, the scientists obtainedtracking data from 48 of the 51 salmon sharks. Twenty-one weredouble-tagged with SPOT and PAT instruments, while 27 carried one tagor the other. Six sharks were recaptured during the study, includingthree outfitted with pop-up tags containing full archival records ofthe sharks' minute-by-minute behavior. The longest distance traveled byan individual shark was 11,321 miles over 640 days—equivalent totraveling nearly halfway around the Earth.
All told, researcherscollected more than 13,000 days of positional tracking data from thesalmon sharks. "These data reveal a striking seasonal migration fromsubarctic to temperate and subtropical regions, presumably to forage orgive birth to their young," says Weng, lead author of the study. Incontrast, the satellite data showed that blue sharks remain in warmerPacific waters off the continental United States and Mexico.
"Theability to follow many individuals for a year, and in some cases fortwo or more years, is virtually unprecedented," Weng notes.
Todate, the researchers have used combined tagging on two other sharkspecies, makos and common threshers. Block predicts that widespread useof this technique will provide much needed information about how sharksuse the oceans. "From these data we can map areas of high use,visualize shark migration corridors and identify species-specific hotspots where shark populations may benefit from increased protection,"she adds.
Warm bodies, cold hearts
The researchersalso discovered that while some salmon sharks remain in the NorthPacific for the entire year, others travel thousands of miles south tothe subtropical waters of the Hawaiian Islands. During these longmigrations, they encounter water temperatures ranging from 36 to 75 F(2 to 24 C). While in the cold subarctic, the sharks ranged betweennear-surface waters and depths as great as 450 feet.
"Sometimesin winter the surface waters, which are less salty, were so cold thatthe sharks spent more time in warmer, saltier waters below," Blocknotes. "When I glimpsed the sharks' radio positions from these frigidseas, I often wondered what it would be like to overwinter in the wildsof an Alaska fjord chasing herring in constant darkness."
As theymove farther south, the sharks spend more time in deeper water,sometimes diving about a half-mile below sea level to avoid warmertemperatures at the surface.
The capacity to summer in theplankton-rich seas of the temperate north or to overwinter in chillyAlaskan waters sets salmon sharks apart from other laminids, a familythat includes mako, white and porbeagle sharks. All laminids share theunusual ability of maintaining an internal body temperature that's upto 70 F warmer than the surrounding water. But the capacity to elevateinternal body temperature does not extend to the heart, which isconstantly flooded with blood that cools to ambient water temperaturesas it passes through the gills to pick up oxygen.
As part of theScience study, the researchers sought to understand why the salmonshark's heart continues to function when it is ice cold. "The sharkheart slows down in the cold, just as our own heart would," Block says."But what sets it apart is where our heart would simply stop, thesalmon shark keeps on ticking."
Laboratory analysis of hearttissue from six salmon sharks revealed high levels of specializedproteins that control the release and uptake of calcium ions, which areresponsible for maintaining rhythmic cardiac contractions. Theresearchers discovered that the rate of calcium uptake by theseproteins was about 10 times faster in salmon shark hearts than in bluesharks, which inhabit much warmer water. This finding may explain theability of salmon sharks to maintain their heartbeat and supply theirwarm, active bodies with blood even as the heart cools to 35 F.
"This is clearly a unique shark species—the warmest of all gill-breathers in the ocean," Block observes.
Thesalmon shark project is part of the Tagging of Pacific Pelagics (TOPP)initiative, a research program affiliated with the Census of MarineLife. TOPP scientists use electronic tags to study the migrations of 22species of marine animals throughout the North Pacific.
"Manysharks are threatened by fishing around the world, and biologicalknowledge is urgently needed to design management strategies," theauthors wrote. "Satellite tracking technologies can be used to rapidlymap shark habitats worldwide, which is critical to their futureprotection."
Other co-authors of the Science study are Stanfordphysiologists Pedro Castilho, Jeffery Morrissette and AnaLandiera-Fernandez; Robert Schallert, a technician at the Monterey BayAquarium; and shark biologist Kenneth Goldman of the Alaska Departmentof Fish and Game. The study was funded by the Office of Naval Research,the National Science Foundation and the Gordon and Betty Moore, Davidand Lucile Packard, and Alfred P. Sloan foundations.
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