CHATHAM – Researchers can't yet conclusively say how many great white sharks are in Cape waters during peak periods from August through October, but one thing is clear: Chatham and Orleans are the hot spots.
In 2016, acoustic buoys off Chatham and Orleans had the most detections of any of the receivers deployed from Marshfield to Nantucket Sound. Similar results were seen in previous years.
“Orleans and Chatham remain the top two areas for detecting acoustically tagged white sharks,” said Dr. Greg Skomal of the state division of marine fisheries.
In the third year of Skomal's five-year white shark population study, funded by the Atlantic White Shark Conservancy, 146 individual animals were identified. Of that total, 89 hadn't been seen before, while 57 were previously identified. Those numbers have steadily increased in each of the study's three years, Skomal said.
Last summer's numbers were up roughly 40 percent and 60 percent of those individuals were new to the study, he said.
“That tells us that ones we did not identify previously are still coming to the area,” he said. Since the study began in 2014, a total of 257 individual white sharks have been identified.
DMF's John Chisholm makes identifies individual sharks by characteristic markings on the animals through the use of videos taken on the population study's twice-weekly research voyages. He compiles them into a database that is available to view on the agency's website.
But Skomal cautioned that the numbers represent raw data and do not indicate how many sharks may be in Cape waters at any one time. But there is enough data now to begin to refine that information, and he was planning to get together this week with a UMass doctoral student who will be modeling the data to extrapolate the size of the population.
The data collected from the acoustic receivers will help refine the information to determine residency, or how many of the apex predators are around at any given time.
“Do they stay for weeks, do they stay for months, do they stay for hours or days?” Skomal said.
One thing has remained consistent since white sharks began showing up in 2009, drawn by the increasing population of gray seals: sharks generally start to arrive in June, the numbers increase in July and max out in August through October.
“The seasonality hasn't changed much,” Skomal said. Numbers drop off in November and the sharks are, for the most part, gone by December. The last detection this year was in mid December, he said.
The acoustic receivers only detect the 85 great white sharks with which Skomal has attached acoustic tags, only a fraction of the individuals identified. Altogether more than 100 sharks have been tagged off the Cape, the remaining 15 with satellite tags. Skomal said the acoustic tags provide data about localized movement of the sharks, while the satellite tags show broader movement of the animals beyond local waters.
There are acoustic receivers deployed as far north as Cape Ann and as far south as Nantucket. A receiver off Marshfield was the farthest north a shark was detected, Skomal said. A shark was also detected on an acoustic buoy in the new artificial reef off Harwich.
Cynthia Wigren, president of the Atlantic White Shark Conservancy, said an acoustic receiver inside Chatham Harbor, just south of the north cut, received a single detection in October. During one of the twice-weekly research trips last summer, white shark James was seen swimming through the north cut, but apparently did not swim far enough into Pleasant Bay for his tag to register on the acoustic receiver.
Skomal said he hopes to plug a hole in the acoustic receiver network along the Cape's east side by getting one placed between Coast Guard Beach in Eastham and buoys off Truro. Wigren said the Conservancy has an acoustic receiver for the area but wasn't able to get it hauled out to the location last summer.
The data on shark numbers and movements was presented at the season's first meeting of a working group of Cape beach managers Monday.
“It's significant information for our beach managers,” Wigren said, helping them determine how to handle and anticipate the presence of white sharks in local waters. For the coming season, beach managers were comfortable relying on tools rolled out last year to educate the public about the presence of sharks, including signs, a safety video, a flag warning system and the Conservancy's Skarktivity smartphone app.
“Our plans are really to continue what we were doing last year,” said Dan Tobin, director of Chatham's Park and Recreation Department. Patrols both on and off shore at Lighthouse Beach will be back in July, he said.
New technologies for monitoring beaches and detecting sharks are also being investigated. Data from the existing acoustic receivers must be manually downloaded; the Holy Grail of shark monitoring is real-time detection buoys, which provide immediate notifications when a shark with an acoustic tag swims by. At least two prototypes didn't work out, but a third is now being prepped for deployment in June in the so-called Shark Cove area east of Monomoy. Harbormaster Stuart Smith said the new prototype real-time receiver will be placed in a buoy made by the Gilman Corporation, which makes aids to navigation for the Coast Guard and NOAA; the company created a buoy especially for the real-time shark detection equipment, powered by a solar array. Wigren said detection equipment in a water-tight compartment will use the cellular network to send data to onshore receivers, and eventually could be incorporated into the Sharktivity app. At $15,000, however, the real-time acoustic buoy, even if it works, is not likely to replace the existing network of acoustic receivers.
There's also interest in running a trial study of technology that involves a type of balloon mounted with a high-definition camera that can be used to monitor the waters off popular beaches. Unlike drones, Wigren said, the balloons can stay out over the water longer without having to recharge batteries.
“We see this as interesting and promising technology, to be able to have a steady view of an area and look at it for white shark behavior, since we can't be on the water all the time,” she said. Tobin said he's interested to see how that technology functions in the Cape's environment.
“We hope that may have applications for beach managers down the road,” he said.
The Sharktivity app continues to be downloaded and is now available on the Android platform as well for iPhones, Wigren said. A new function of the app now being developed will allow users to track individual sharks, she added.