CHATHAM – A number of white sharks cruising around Cape waters are sporting not just one but two tags, which is expected to add significantly to the body of knowledge about the area's most notorious summer visitors.
Along with acoustic tags that record a shark's movement when it passes by receivers deployed in waters throughout the region, Dr. Greg Skomal of the Massachusetts Division of Marine Fisheries has been deploying PSAT, or pop-up satellite archival tags, that collect fine-scale data such as water depth and temperature. So far this summer he's tagged six white sharks with PSAT tags, including several that had acoustic tags from previous years.
Three of the tags were provided by the Canadian Department of Fisheries and Oceans, and three were provided by Dr. James Sulikowski, a shark researcher with the University of New England in Biddeford, Me. What's unusual about the tags provided by Sulikowski is that they are designed to provide short-term data, popping off the shark and floating to the surface to transmit information via satellite after just 28 days.
The short-term satellite tags provide a finer level of data, at more frequent intervals, than the typical PSAT tags, which collect information over a longer period, from six months to a year.
“You can imagine all the really fine-scale information you can get” with the short-term tags, Sulikowski said in a telephone interview Monday.
The “high resolution” data provided by the short-term satellite tags “gives us a kind of snapshot of the temperature and depths of these animals when they're around Cape Cod in the summer,” particularly during times when researchers can't observe sharks, such as at night, said Skomal. Combined with data from the acoustic receivers, this will provide a much more detailed picture of shark behavior in Cape waters, he said.
“For all the seals and sharks there are, we don't see a lot of attacks, so are we missing something?” Skomal said. “Are there not a lot of attacks or are they happening when we're not around? And is there a pattern? We're only out two days a week, let's not forget that.” The data from the satellite tags will tell researchers whether the sharks are remaining in shallow water or slipping offshore into deeper water, all correlated with time and water temperature.
Sulikowski provided Skomal with 15 of the short-term PSAT tags, made by Canadian company Lotek Wireless. He said they cost about $2,000 each, plus the cost of satellite time, about half the cost of the longer-term PSAT tags. Funding comes from the university, the National Marine Fisheries Service and private donors, he said. He's used the tags previously on porbeagle sharks, a relative of white sharks, and, working with the Cape Cod Commercial Fishermen's Alliance, on skates.
PSAT tags pop off the shark at a pre-programmed time, float to the surface and transmit data to a satellite. Band width and battery power limit the amount of a data a long-term PSAT can transmit, said Skomal, often providing just a summary of data.
“That's great if you're looking at broad-scale movement of a shark from Cape Cod to Florida,” he said. The short-term tags, however, can transmit more data. Given that the tags can collect data points every 10 seconds, the high resolution of the information is extremely valuable, especially when correlated with the acoustic movement tracking.
“Think how that would help me around Cape Cod,” Skomal said.
Sometimes PSTA tags find their way to shore. Sulikowski said his name and number are affixed to the tags he provided to Skomal, and if he gets one back, he can extract even more data than allowed through satellite transmission.
Sulikowski is particularly interested in correlating shark reproductive cycles with movement patterns. To that end, he's provided Skomal with a muscle biopsy dart developed by the Amherst Machine in North Amherst, with which a plug of muscle can be extracted from a shark. Analyzing hormones in the tissue will determine the reproductive stage of female sharks.
“Are these sharks pregnant, are they not pregnant, it will tell us a lot about how important that environment is,” Sulikowski said. This will add another component to the data Skomal has already collected, including identifying more than 280 individual sharks in the first three years of his five-year population study.
The tissue samples will also be used by other researchers he's collaborating with at the Woods Hole Oceanographic Institution, which is studying shark feeding ecology, and the Florida Museum of Natural History, which is looking at shark genetics, said Skomal.
“There's a lot to be learned from just a plug of muscle,” he said. “You can't do genetics from tags.”
Skomal expects to start using the biopsy dart as soon as he gets back on the water, probably on Wednesday. Weather precluded going out on researcher trips earlier this week. He said biopsies will first be taken from sharks that are already identified in the department's database.
Many of the sharks researchers have seen on this season's shark population study voyages were identified in previous years, including several that were tagged. “We've seen at least a dozen old friends,” Skomal said. The goal of the shark population study, funded by the Atlantic White Shark Conservancy, is to develop the data necessary to accurately assess the population of great white sharks in Cape waters. After three full years, researchers are both seeing and detecting on the acoustic receivers the same sharks more often. Skomal said in 2015, 30 percent of the sharks identified had already been cataloged; last year that rose to 40 percent.
“As the number of sharks we see converges with the numbers in our catalog, it means the accuracy of our population study is getting better,” Skomal said. Since 2009, more than 110 sharks have been tagged in Cape waters.
This season has started out slower than last year, with more sharks being seen off Truro and Provincetown than the usual hot spots off Monomoy and Nauset. Skomal said cooler water temperatures may be responsible, “and that makes a difference.”
Sulikowski said he spends about four days a week on the waters of the Gulf of Maine looking for porbeagle sharks. Working with Skomal, linking shark research in northern and southern New England, is “an amazing collaboration,” he said. The technology now available will allow data collection that is “going to offer a whole suite of answers to questions we've had for so long.”
This is also the Discovery Channel's Shark Week, with several programs focusing on Cape sharks and the scientists who study them. Over the weekend, the Atlantic White Shark Conservancy held its annual Shark Gala to kick off Shark Week. More than 240 tickets were sold to the nonprofit organization's largest fundraiser, raising some $110,000, said Executive Director Cynthia Wigren.