A big jump in the number of great white sharks seen off Massachusetts has given rise to a couple of compelling mysteries.

Greg Skomal, senior marine fisheries scientist for the state government, tags great whites along the coast. He collaborates with researchers in Halifax who tag blue sharks and other species, and collect data from Skomal's tags when they're in Canadian waters.

Skomal is still collecting this year's data, so he can't comment on it yet. But from mid-June to October in 2014, his team identified 68 individual white sharks off the eastern shore of Cape Cod.

In the summer of 2015, that number increased to 141 individuals.

Many questions yet to be answered

There are still many questions left to answer, such as what proportion of the total existing population is being spotted off the coastline.

"Is it 10 per cent, is it five per cent, is it 50 per cent?" he said. "We could be seeing just a fraction of what's out here."

Skomal believes the population is rebounding after their numbers were diminished as a result of the expansion of commercial fisheries during the last century.

Greg Skomal of the Massachusetts Division of Marine Fisheries

Greg Skomal, a senior marine fisheries scientist for the Massachusetts government, worked with the Atlantic White Shark Conservancy to tag the seventh white shark of the season off Monomoy National Wildlife Refuge on July 18. (Atlantic White Shark Conservancy/Facebook)

Great whites are still considered endangered. But their increasing numbers around Cape Cod have resulted in frequent headlines including a recent one in the Cape Cod Times about a six-year-old boy who hooked a great white while fishing, and posters at beaches warning swimmers to watch out.

In 2012, a swimmer was injured by a shark off Cape Cod, but Skomal said humans are not a preferred menu item for these animals.

He believes the sharks are drawn to the Cape by grey seals that have proliferated on Nova Scotia's Sable Island, and are now spreading out and establishing new colonies.

Why aren't the great whites visiting Sable Island? 

Fred Whoriskey, executive director of Dalhousie University's Ocean Tracking Network, said one of the mysteries still to be solved in Canadian waters is why none of the great whites tagged in Massachusetts have been recorded near Sable Island.

Scientists long thought the hundreds of thousands of grey seals there were an irresistible draw.

Asked if it's only a matter of time before we have to worry about great whites around Nova Scotia's shores and beaches, Whoriskey told Mainstreet's host Bob Murphy that it's not likely given how cold the water is for people.

"I was out to Crystal Crescent over the weekend, and while there were many people at the beach, there were none in the water. So I don't think we're in any immediate danger," he said.

"But I do think it's going to be fantastic to have these animals back."

Despite the public's fears, shark attacks are exceedingly rare: According to Florida's Museum of Natural History, there have only been 1,301 confirmed unprovoked shark attacks off the shores of the U.S. since 1580 — or about three a year.

Whoriskey thinks more great whites may help rein in the population of grey seals — something fishermen have complained about for years — and bring a new equilibrium to the ecosystem off our coast.

With files from CBC's Mainstreet