No privacy for traveling great whites
State tags send data in study of shark migration
In a press conference yesterday, Energy and Environmental Affairs (EEA) Secretary Ian Bowles and Greg Skomal, a Division of Marine Fisheries senior biologist, reviewed the data accumulated from three electronic tags placed in great white sharks in waters off Cape Cod last September.
Mr. Skomal, an Island resident, spearheaded the tracking project. His team tagged five great whites near Chatham. It was the first successful tagging of the species in the Atlantic Ocean, using electronic satellite technology.
The first tag - which was placed on a 12-foot shark - surfaced on January 15, 50 miles east of Jacksonville, Florida, and began transmitting data. Two weeks later, the second tag - which was placed on a 10-foot shark - surfaced on February 4, 30 miles north of where the first tag appeared. A third tag appeared off the coast of Florida, 80 miles south of the first two tags, on Monday, March 1.
The tags, which collect and record water temperature, depth and light levels to help scientists determine where a shark travels, transmit data for several days using satellite-based technology. Using light-level data archived by the tags, biologists were able to recreate the most probable movements of the sharks, shown on the accompanying chart.
Over the past several weeks, Mr. Skomal, who heads DMF's shark research program, analyzed data transmitted by the tags, looking for information about how deep and how far the sharks traveled, information that leads to better understanding of migratory behavior.
The preliminary data indicate that the sharks remained in southern New England waters until late September before moving west and then south along the East Coast of the U.S. While off Massachusetts, the sharks demonstrated daily movement from the surface to depths below 100 feet.
During their southern migration, the white sharks remained on the continental shelf and continued to rise and descend through the water column, on an almost daily basis. By mid to late October, they were off the coast of North Carolina. The first tags to surface indicate that two of the sharks arrived off the coast of northern Florida by early December, and remained in this region until their tags popped loose, as they are designed to do.
Although the sharks moved between the surface and the bottom, they spent more than 90 percent of their time at depths less than 150 feet. One of the sharks exhibited a deviation from this pattern when it entered the Gulf Stream off North Carolina for 10 days in early November and exhibited daily dives to depths as great as 1,500 feet.
The analysis of the first two tags show the sharks to which they were attached demonstrated seasonal residency off southern New England and northern Florida, two highly productive areas that likely provide ample feeding opportunities. The purpose of the deep diving behavior exhibited by one of the sharks remains a mystery, but such behavior is typical of white sharks off the Pacific Coast. With two additional tags due to pop up in the coming months, the scientists say they hope that more insights into the life history and ecology of these sharks and their movements in the North Atlantic are forthcoming.
Finding great whites can be difficult. Getting close enough to hit one with a harpoon in order to embed a tag requires equal measures of luck and skill. Bill Chaprales, a commercial harpoon fisherman from Cape Cod, helped the biologists develop the techniques they needed to locate and get close to the fish and use a harpoon to plant a tag that would remain in a free-swimming fish.
If all goes well, researchers will have new information about the movements of a species that scientists have rarely tracked in the Atlantic. The biology of great whites in the Atlantic is poorly understood, particularly in terms of seasonal movement.