Climate change, more seals and effective conservation in the United States are possible explanations for the apparent increase in great white sharks in Atlantic Canada, according to a newly-published paper in the Canadian Journal of Fisheries and Aquatic Sciences.
The peer-reviewed report speculates on why more of the apex predators are being seen in the summer months, especially off Nova Scotia.
The most intriguing hypothesis is the great white shark range has shifted northward in summer, into an area where they were rarely seen in the past.
“A northward range expansion could be related to multiple factors, including warming Canadian waters due to climate change, population recovery, and/or increased regional prey abundance,” the authors state.
Or maybe, they’ve been here all along and we never noticed.
“A large, highly mobile, predatory shark may have been historically abundant in Canadian waters, yet considered ‘rare’ simply due to our inability to observe them,” the paper states.
It says it found records of 60 great white shark “observations” in Atlantic Canada between 1872 and 2016. There were 27 sightings, 26 caught in nets and seven observations were inferred from teeth in gear and wounds on seals and porpoises.
What the tagging shows
The report is based primarily on satellite tracking data from Florida-based Ocearch, which staged heavily self-promoted and highly-publicized tagging events off Nova Scotia in 2018 and 2019.
Over the two-year period, 17 great white sharks were captured — most at Ironbound Island near Lunenburg, N.S., and some near Scatarie Island off Cape Breton. Holes were drilled through their dorsal fin and fitted with a satellite-transmitting tag.
All six great white sharks tagged off Nova Scotia in 2018 returned in 2019.
Because the satellite tracking data is not precise, hotspots for occurrence were estimated based on modelling.
The hotspots occurred on the southeastern coast of Nova Scotia and the Bay of Fundy. A secondary hotspot occurred in waters off southern Newfoundland that include the Grand Banks.
Since 2013, Ocearch has also tagged 18 great white sharks in U.S. waters. Half of them have been later seen in Atlantic Canada.
“The frequency of U.S.-tagged sharks entering Canadian waters, and the successful targeted capture and tagging of multiple white sharks off Nova Scotia over two consecutive years, indicate seasonal, inter-annual presence of white sharks in Canadian waters and higher regional frequency and abundance than previously thought,” the report states.
The authors suggest great white sharks may move north in summer months because temperatures off the United States are getting too warm and Canadian waters are now just warm enough.
“An increase in Atlantic Canada white shark sightings in recent years may therefore be the result of white sharks seeking cooler northern waters during the warm summer months,” the report states.
They may also be attracted by more abundant prey as grey seal populations explode.
“It is therefore possible that with greater prey availability, white sharks are experiencing a similar increase in fecundity and survival rates. An increase in shark sightings in Atlantic Canada due to an increase in the local seal population would mirror that observed in Massachusetts,” the report states.
Canada’s Department of Fisheries and Oceans also tagged a great white shark in Nova Scotia, a young male off Port Mouton in 2018.
It was the first great white shark tagged in Canada.
It and a female tagged off Cape Cod spent the summer of 2018 off Nova Scotia.
The tracking device showed what appeared to be a search pattern to intercept grey seals moving from the huge colony on Sable Island to areas where seals come ashore in Nova Scotia and elsewhere on the Eastern Seaboard.
It is part of a government effort to identify where the endangered predator lives — its “critical habitat” — when in Canada.
The DFO scientist leading that project, Heather Bowlby, told CBC News in 2019 there are likely very few great white sharks coming north.
To put the numbers in perspective, it took DFO three days to find the great white off Port Mouton and three hours to find 15 off Cape Cod.
The DFO research was not affiliated with Ocearch and the data it generated does not appear to have been used in the Fisheries Journal article, which was led by authors from the University of Windsor in Ontario.