Saturday, September 19, 2009

Scientist thrilled by presence of Great White sharks

The great white sharks sighted off Orleans and Chatham last week sent bathers scrambling from the waters but one man was running in the other direction; Dr. Greg Skomal.

Skomal is a marine biologist with the Division of Marine Fisheries, based on Martha’s Vineyard who specializes in sharks. He enlisted harpoon ace Bill Chaprales of Marstons Mills, captain of the EZYDUZIT to use his skills to help imbed high tech tags in five of the sharks.

They did that a week and a half ago, tagging an eight-footer and 10-footer on Saturday Sept. 5, and three more on Monday Sept. 7, including a 15-footer. A spotter crew located the sharks from a plane piloted by George Breen.

These five great white sharks are making history as the first to carry the electronic pop-up tags in the North Atlantic. They have been tagged in the Pacific.

“It’s a species that not only is poorly understood in the Atlantic but one for which information is essential to get a good management plan in place,” Skomal said. “I’m excited to get five tags in place.”

After attending a conference last week Skomal is back on Martha’s Vineyard and he’d love to add a few more sharks to the database. The stormy weather prevented any more tagging last week but so far the sharks have been shy.

“We haven’t been able to locate any others,” he admitted. “We had northeast winds for four days and could get out and our pilot couldn’t see anything because of all the rain. When we were out last week we saw at least two or three other sharks that were not tagged so we knew at that time they were still out there. The question is are they still there?”

Swimmers and lifeguards may be asking that too. They general consensus is that they were drawn in by the plump and tasty seals. Seal numbers are up over recent decades and gray seals are year round residents of Chatham. The teeth of great white sharks are serrated, designed for ripping blubbery flesh.

“September still has plenty of warm water and optimal conditions for them from the environmental perspective,” Skomal noted. “The shark that was around Naushon Island in 2004 came in late September. We’ve had sightings extend into November.”

In any case the five sharks should provide some fascinating data. The tags contain a microcomputer and will remain on the sharks and the first will pop in mid-January, if all goes well, and then pop off, float to the surface and transmit data. It won’t provide real time information on what the shark is up to, instead it as retrospective on where it’s been.

“Pop up tags have been used in the Pacific and Indian Ocean with fantastic results,” Skomal said. “Think of them as a data logger put on the fish that comes off and transmits data to a satellite. It’s complex because you’re dealing with salt water, mircotechnology, batteries and such. It logs information on depth, temperature and ambient light levels.”

Pacific great whites have been documented swimming from California to Hawaii and diving as deep as 3,000 feet.

Unfortunately GPS doesn’t work because the fish are under water. That makes calculating the sharks’ former locations murky. The light data is critical for that. It will allow scientist to estimate day length and hopefully determine the longitude and latitude. Longer days as winter rolls on will indicate the shark was further south. Sunrise and sunsets can give the longitude.

“That method of geographical location is very problematic if the shark doesn’t go far,” Skomal admitted. “It’s not good on a fine scale.”

But if the sharks travel to South America for instance, it’ll be very informative.

“We don’t have a really good sense of their migration pattern,” Skomal said. “All we have is distribution data based on sightings that are fishery dependent, those caught by fishermen, caught in gear, sighted by chance, and that amounts to 4 to 500 data points a year.”

Now, for the first time, individual sharks can be tracked.

“The tags were attached using an intramuscular dart,” Skomal explained. “It’s important to be precise to do it properly because you don’t want to hurt the fish. We used a harpoon-tagging pole to place the tag with pinpoint precision. It’s worked for us with fish the size of blue fin tuna and basking sharks.”

Skomal is the author of the 278-page “Shark Handbook”, available from the Cider Mill Press in Maine. It presents the science of sharks for the general reader covering many species and general biology.