A new study of the distribution of the endangered great white shark in Canadian waters says an underwater detection network suggests the population remains stable but is not growing.
That runs counter to worries the ocean’s greatest predators are increasingly prowling the region — perceptions fuelled by a suspected attack last August on a woman in the waters off Nova Scotia’s Cape Breton Island and cellphone videos the same month depicting a shark chewing a seal carcass.
Shark-tracking apps have also become popular, as the Ocearch group has operated in the region for several seasons tagging animals and allowing the public to follow the creatures online as they migrate into the northwest Atlantic from July to November.
However, work by a consortium of leading great white shark experts studying the animal’s behaviour says sightings in Canada aren’t translating into increased detection by underwater acoustic networks that pick up signals from tagged animals.
The collaborative study published last month in the Canadian Journal of Fisheries and Aquatic Sciences says that when the increased number of tagged sharks and the bigger array of detection systems are taken into account, the number of great whites in Canadian waters appears to be holding steady.
It says while there have been theories of increased numbers of great whites on the basis of sightings, “we found limited corroborating evidence.”
“There was no systematic increase in the proportion of the tagged population visiting Canadian waters, which has remained relatively constant during the years where an appreciable number of animals had been tagged (2016 onward),” says the study.
The document is co-authored by Heather Bowlby, the lead researcher at the federal government’s Canadian Atlantic Shark Research Laboratory, Megan Winton of the Atlantic White Shark Conservancy in North Chatham, Mass., and Gregory Skomal of the Massachusetts Division of Marine Fisheries.
The vast majority of the sharks were tagged off Cape Cod between 2009 and 2021, with about three per cent of the sharks tagged in Canadian waters in 2018 and 2019.
Of the total 227 tagged sharks, only about a quarter make the annual trip to Atlantic Canadian waters, according to the study of the past decade’s migrations.
Bowlby said in a recent telephone interview that when the scientists accounted for increases in monitoring, they found a “consistent proportion of the total number (of sharks) tagged” were showing up on the acoustic networks that receive their signals.
For example, the data shows that in the Bay of Fundy in 2016, the 70 acoustic receivers deployed in the area detected three great whites, while four years later, with three times as many receivers, nine great whites were detected — even though more sharks had been tagged. Over the last five years, the study found, between 11 and 19 per cent of acoustically tagged sharks were detected in Canadian waters.
Bowlby said the paper’s main purpose is “build the foundation” to describe the critical habitat for the great white sharks in the region.
She said observations on the shark’s behaviour — collected from satellite tags that can track the shark’s depths — has raised important questions about prevailing views that temperature and other environmental aspects of the ocean are the only factors in the animals’ location.
She noted the tags show the sharks were diving to depths of about 50 metres in coastal locations during the summer months, and seemed to undertake this behaviour regardless of temperature ranges of the water.
The data also indicated that most of the sharks coming into Canadian waters from Cape Cod are younger, swimming long distances to hunt prey that includes seals.
Bowlby has a nuanced message on how swimmers and other recreational users of Nova Scotia’s waters should react to the presence of the sharks.
She said that since the research shows no “appreciable” increase in the sharks’ abundance in Canada, recreational users of the Atlantic region’s beaches are not at greater risk. However, Bowlby says “the great white is a powerful marine predator, and a bit of prudence is warranted sometimes.”
Paul D’Eon, the director of the Nova Scotia Lifeguard Service, said in an interview Monday that even in the late 1970s, when he was beginning his 48-year career with the service, he heard fishermen tell anecdotes of catching great white sharks. He’s come to believe little has changed over the decades.
“I think the risk is extremely low,” he said. “It’s more reasonable to be injured in a drive to the beach than in being attacked there by a shark.”
Nonetheless, the lifeguard service has a policy on shark sightings where water is cleared for a minimum of two hours following a sighting at a patrolled beach.
—Michael Tutton, The Canadian Press