Early Wednesday morning, a group of Dalhousie University researchers and students ventured out on choppy waters, steaming 17 kilometres off Nova Scotia’s Sambro Island.
The study, performed through Dalhousie University’s Ocean Tracking Network, is attempting to gather information on one of the most common, yet least understood, top predators in Nova Scotia waters.
Little information is available on Canadian blue shark populations, so one purpose of the study is to determine how many blue sharks patrol the waters off Nova Scotia.
“We know for sure that regionally, here, a lot of the Atlantic Canadian fisheries are catching blue sharks in high numbers as bycatch. We want to make sure that juvenile populations are not being affected by this,” said Brendal Davis, an instructor and laboratory manager at Dalhousie University.
The bycatch threat is just one problem facing blue shark populations.
The illegal shark fin trade results in the death of countless animals every year.
“[Blue sharks] are considered the most common species in the international fin trade, that is the shark fin trade," said Davis. "They're quite abundant, although globally their population is considered to be declining.”
Unregulated shark slaughter is bad news for sharks, but also for ecosystem health.
“This is all about the top predators in the ocean ecosystems. Where they are going, we will know where the ocean is going. They respond the first to changes,” said Fred Whoriskey, head of the blue shark project and executive director of Dalhousie’s tracking network.
“They drive the interrelations among the species. When you have a healthy population of top sharks, then you don't have the population movements in other species you don’t want. For example, there has been a lot of controversy in the fishing industry about grey seals. One of the most effective ways of dealing with grey seals is making sure that the things that eat grey seals [have a healthy population].”
How to catch a shark
To catch a shark, one must think like a shark — and sharks think with their stomachs.
The Dalhousie researchers chummed the waters with a stinky mixture of fish and squid in a perforated bucket. A shark’s keen sense of smell allows it to detect a single drop of blood in the water up to half a kilometre away.
About 30 minutes after the chum went into the water, the first blue shark was hooked. After a brief fight the shark was hauled into the boat. where researchers grabbed it by the tail and turned it onto its back. This technique puts the shark into a trance-like state called tonic immobility.
The team worked quickly to place a housing that sprays salt water into the shark's mouth so oxygenated water would flow over its gills.
Davis made a small incision a few inches long, placed the acoustic tracker inside the shark’s body and quickly stitched the wound. The shark was then safely released back into the water — all in under 10 minutes and without too much stress to the animal.
The team's goal was to tag six sharks, but the entire project will see 40 sharks tagged over a two-year period
Acoustic receivers, anchored to the ocean floor and placed on oil and gas infrastructure at sea will pick up the signals from the transmitters implanted in the sharks.
The world's most extensive array of acoustic receivers is located off the coast of Halifax, spanning a distance of 185 kilometres offshore. There are also receivers located in the Cabot Strait and in the Bay of Fundy between Nova Scotia and New Brunswick.
In addition to information about the animal’s size and species gathered when the shark is tagged, researchers are able to track a shark’s movements for life, every time it swims near a receiver.
This gives scientists information on shark movements and about where they feed, where they’re located season to season, and even where blue shark pups are hatched and raised.