Greenland Sharks Undeterred by Shark-Repelling Hooks
To the shark’s detriment, its vacuum-like feeding technique allows it to bypass the repelling effect of magnetic fishing hooks.
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Even by deep-sea standards, Greenland sharks are weird. The Arctic-dwelling fish live for centuries, the longest lifespan of any animal with a spine. Where and when they breed are mysteries. Many have wormy parasites dangling from their eyes. But just like other sharks, they’re frequently hooked as by-catch, contributing to the 50 million sharks unintentionally caught around the world every year. Helping these quirky sharks avoid the hook, however, is proving challenging.
The sharks are falling victim to the turbot fishery off Baffin Island, Nunavut. This fishery uses longlines with hundreds of wickedly sharp hooks baited with squid to catch flat, meter-long turbot that hang out on the seafloor. In 2009, changing ice conditions forced the fishers to switch from a winter fishery to a summer one—precisely when the sharks spend more time close to shore. That year, shark by-catch skyrocketed—fishers snagged 4.3 tonnes of Greenland shark for every tonne of turbot. Losing sharks to by-catch is bad for the sharks, but it’s also bad for the fishers, who lose their gear and miss out on valuable turbot.
In the early 2010s, a promising technology, called the selective magnetic and repellent-treated (SMART) hook, was developed by SharkDefense, a group of researchers based in New Jersey. The hook creates a magnetic field, which interferes with some sharks’ electrosensory organs and may keep them from coming too close. Previous studies on the SMART hook showed it repelled spiny dogfish and bonnethead sharks from other longline fisheries, so Scott Grant, a biologist at Memorial University in St. John’s, Newfoundland, thought it might work for the Greenland sharks, which rely on their ability to sense electric fields to navigate and hunt in deep, dark waters.
But Grant’s study was discouraging. He and his colleagues found that the SMART hooks snagged the three-meter sharks just as often as conventional fishing gear. The researchers even found some wild sharks with two or three SMART hooks in their jaws. But the scientists were unsure why the SMART hooks failed to keep the sharks away.
Grant and his team turned to videos of Greenland sharks feeding to understand why. They discovered that, like a handful of other shark species, Greenland sharks feed by sucking prey into their mouths, rather than by ramming or biting their prey. They also found that a Greenland shark can vacuum up food from as far as 30 centimeters away. The SMART hook used for turbot is relatively small, and the magnetic field it produces may be too weak to deter sharks from that distance, Grant says.
Neil Hammerschlag, a shark ecologist at the University of Miami, says it’s important to test shark repelling technologies. “Figuring out what works is important, but just as important is figuring out what doesn’t work,” he says.
Protecting species such as Greenland sharks, Hammerschlag says, may require strategies other than fancy technology. Shifting the fishery to avoid the by-catch species could make more sense. For example, Hammerschlag says, studying environmental variables such as temperature or depth could reveal spots where the target species is found without the by-catch species.
With the Canadian government’s recent pledge to invest CAN $2.5-million in growing Nunavut’s fisheries, including the turbot fishery, Grant is racing to figure out what will keep these wonderfully weird sharks off the hook.