Over the past decade, great white sharks have been appearing far more often in Atlantic Canadian waters. What is encouraging these formidable predators to travel so far north?

This summer, one seasonal visitor seemed especially fond of the waters off Nova Scotia, Canada.

Only months earlier, the traveller had been swimming around Florida. When spring gave way to summer, however, he made his way north.

His preference for Nova Scotia's cold coastline over the Sunshine State attracted attention because he was not an ordinary holidaymaker. He was a great white shark measuring nearly 14ft (4.2m), known to researchers as “Contender”.

Contender was far from the only one. Other great whites from the Atlantic population have also been travelling northward and appearing off Canada's eastern provinces. Shark numbers have recovered in parts of the US Atlantic coast as seal populations have grown, while colder, deeper northern waters are increasingly becoming an important habitat for the species.

As shark encounters become more common, so does the possibility of unwanted contact with people using the same waters. In September, a diver was bitten by a great white near Quebec's Gaspé Coast. He had been diving with two others in poor visibility. His injuries were not considered life-threatening, but the event worried local diving and watersports businesses, leading some operators to cancel planned dives.

Why, then, are great whites expanding their range farther north?

Great white sharks are not newcomers to Canada's Atlantic coastline. The species generally favours cool water, within limits. They are among the few sharks able to keep their bodies noticeably warmer than the water around them, giving them an advantage while hunting. Their warmed muscles can react more quickly in cold conditions, although maintaining that body heat also demands extra energy.

Seals are one of their preferred foods, and seal populations have risen along both US and Canadian shorelines in recent decades. One possible reason is that warming seas linked to climate change are drawing more fish species north. Surface waters around Nova Scotia are thought to have warmed by roughly 1.5C (2.7F) over the past 30 years, while temperatures on the seafloor have increased by about 3C (5.4F).

More fish can support more seals, and more seals can attract more great white sharks.

Alanna Canaran, a dive instructor and ocean science educator in Halifax, Nova Scotia, first relocated from Toronto in 2010. “At that time in Nova Scotia, seeing a white shark was pretty much unheard of,” she says. She worked elsewhere during much of the 2010s before returning to the province in 2020. In just over a decade, she found the situation had changed considerably.

“There are days when you are in the water and it just feels very sharky. You can sense that energy. In summer, as the water gets warmer, visibility often drops and it turns a bit green or hazy – so I am sure they have been around.”

Some people she knows have experienced much closer sightings. One of Canaran's diving partners was recently at Birchie Head, a popular site near St Margaret's Bay. “She was looking down at rocks, lobsters and other things, then suddenly looked up and saw the underside of a great white shark swimming directly over her.”

Canaran's friend alerted the other diver, who eventually looked up and spotted the shark above them. “They stayed close to the bottom and slowly made their way toward shore, getting out near the side of a cliff because they were obviously frightened.”

Great whites do not deliberately hunt humans, but they may come closer to inspect activity underwater, especially splashing and noise. Research has shown that sharks can mistake a wetsuited swimmer or the silhouette of a surfboard for a seal. On rare occasions, an attack may follow. More commonly, the animal remains at a distance and simply watches.

In another incident this summer, recorded by the Canadian Shark Attack Registry, a diver reported that a great white had “bumped” their fins while coming closer to investigate. Scuba divers releasing noisy streams of bubbles may look less seal-like than snorkellers in wetsuits, but sharks are naturally inquisitive animals.

“I include shark information in my training courses and guided dives,” Canaran says. “I also make clear that the chance of an attack is very, very, very low, although it is increasing.”

One reason sightings may be rising, Canaran says, is that more people are using the waters where sharks are now spending time. Nova Scotia's population has recently exceeded one million, and Halifax now has around 400,000 residents. That means many more people boating, diving and spending time along the coast.

“Even in 2020, when I came back, there were only a couple of dozen scuba divers going out regularly,” she says. “Now there are definitely a couple of hundred, as well as more people on boats. More eyes on the water naturally means more sightings.”

Marine biologist and shark researcher Neil Hammerschlag, who records sharks while operating cage-diving trips, has also observed an increase in great white activity.

“Last year was definitely the most sharks we had seen, and this year we already seem to be seeing them earlier than before,” he says. “I do not necessarily think the population itself is larger. I think individual sharks are visiting Atlantic Canada more frequently than they once did.”

According to Hammerschlag, Nova Scotia's waters remain cold but are well within the range that great whites can tolerate. The abundance of food provides another strong reason to visit. “Nova Scotia offers a lot of prey. It is a healthy marine ecosystem with plenty of large animals for them to eat, including seals and tuna,” he says.

“Individual sharks learn, remember and build mental maps. They explore an area, recognise an opportunity, remember it and return.”

His team has been working to understand the regional population: which sharks return year after year, how many are males or females, and what sizes they are. “Last year, we identified 120 separate great whites over 55 days,” he says.

The frequency of encounters was very different from one animal to another. A handful of sharks were recorded four or five times, while two were seen during roughly 30% of expeditions. Most, however, appeared only once.

“We probably saw 80% of them one time and never again. That suggests they are moving through the area.”

As sightings and interactions have increased, scientists and instructors have tried to reassure Canadians that the change is not entirely unexpected.

Healthier food supplies may eventually support more sharks, although great white populations grow much more slowly than seal populations. “If there are many sharks around places such as Cape Cod or New York, and competition becomes high, they may begin spreading into the northern and southern edges of their range,” Canaran says.

Before the Quebec incident, Hammerschlag said that many Canadians seemed excited by the prospect of seeing a great white close to home. Yet they may also be thinking more carefully about how they use the ocean. “I think people are a little more thoughtful when they enter the water,” he says.

Nova Scotia's cool water may also reduce risk in some ways. “Most people are not staying in it for very long, and they are certainly not going very deep. A great white cannot enter water that is only two feet deep.”

That greater awareness may be useful, Hammerschlag says. “That is exactly what is needed: not fear, but mindfulness. It can help protect people while also improving understanding and conservation of sharks.”

Neither Canaran nor Hammerschlag believes the recent attack should stop people from going into the sea. But they say people should recognise that sharks are part of the marine environment. Canaran also notes that Nova Scotia's Indigenous communities, including the Mi'kmaq, have long understood that great whites could share these waters.

“Indigenous people travelled in canoes and hunted seals with harpoons, so it is remarkable to think that a great white shark could have been nearby as well,” she says. Indigenous fishing communities, she adds, developed their own ways of keeping sharks at a safer distance.

“They would attach spruce roots and sea grass beneath the boat, which could confuse the sharks,” she says. “It made them think the water was shallower than it really was, so they would avoid the boats.”

Writing after the September attack, Canaran said: “Last week I was teaching an open-water diving course in the ocean and woke up to many messages about what happened in Quebec. Did it stop me from diving? No. Did it make me more cautious that day? Yes. Sharks were on my mind, so I chose to teach in a very sheltered little cove with lots of eelgrass.”

The period with the highest shark activity is now ending, and fewer great whites are expected in the colder months between October and March. Canaran and Hammerschlag offer similar advice for reducing the already low likelihood of an encounter.

“Avoid places where sharks' preferred prey, such as seals, are present, particularly when the water is murky and a shark may find it harder to tell its natural prey from a person,” Hammerschlag says.

Canaran says she is more careful than before, but she will not give up diving. “If I only dived when visibility was perfect, I would probably manage only five dives a year. I am going to keep diving.”

The bigger question is whether the sharks will return in comparable numbers next year, and whether Canadians will adapt to a new coastal reality that includes great whites near shore. “It is about learning again how to live alongside them. We have done it before, and I am confident we can do it again.”