New England Aquarium scientists search for ways to reduce the accidental capture of sharks.

Jennifer Weeks

With the arrival of beach season, new reports are trickling in about people injured or killed by sharks. Since late April, several have died from shark attacks near San Diego and on Mexico’s Pacific coast. But marine biologists emphasize that sharks are in much more danger from humans than the other way around.

Fishermen hunt sharks worldwide for their meat and fins, which are prized in Asian countries. Now researchers at the New England Aquarium (NEAQ) in Boston are finding that bycatch—accidental harvesting by fishermen targeting other species—is also a serious threat. Sharks grow and reproduce slowly, so they are slow to recover from over-harvesting. Last month, the International Union for the Conservation of Nature reported that, thanks to targeted fishing and bycatch, 16 out of 21 species of open-ocean sharks and rays were at heightened risk of extinction.

Longline fishing gear snags a scalloped hammerhead shark (Sphyrna lewini), one of many shark species accidentally captured by fishermen. (Courtesy John Mandelman, New England Aquarium)

Bycatch of sea turtles, birds, and marine mammals such as dolphins has been studied more widely than the accidental take of sharks. Two NEAQ scientists, Tim Werner and John Mandelman, contributed to a recent study that aimed to fill the gap. The authors found that pelagic sharks (species living in the open oceans) are frequently hooked by longline fishing boats–commercial operations that set out lines 45 miles long on average, rigged with thousands of baited hooks. The lines stay in the water for up to 24 hours, so species such as sharks that breathe by moving through water can be hurt or killed if they are hooked.

“Most fishing methods are inherently indiscriminate,” says Mandelman, a shark biologist. “Longlining isn’t as hard on ocean habitats as trawling [dragging large nets across the seafloor], but it can have significant impacts.”

Bycatch bypass

Mandelman and other NEAQ scientists are studying the US Atlantic pelagic longline fishery, where sharks and rays have accounted for 25 percent of the overall catch in recent years. One question they would like to answer is why mortality rates as a result of being hooked on a longline vary widely among species. For example, in one data set from the southeast US, 3 percent of tiger sharks died after capture, compared to two-thirds of silky sharks. “Just because we throw them back alive doesn’t mean they’re going to survive,” says Mandelman. “Some species are much more physiologically resilient than others.” Knowing which species are especially vulnerable could help researchers focus their efforts to lower the rate of shark bycatch.

Many types of repellent have been proposed to keep sharks away from fishing boats, both to reduce bycatch and to keep sharks from preying on hooked fish and damaging fishing gear. Unlike most bony fish, sharks can sense electricity in seawater, so one option is to drive them away with magnetic materials that will overstimulate the electrochemical sensors they use to detect prey. Some researchers have found that hanging ingots of electropositive metals (alloys of substances like palladium, which react with seawater to produce an electrical field) overboard can repel sharks. However, in NEAQ trials, spiny and smooth dogfish (small sharks) were not affected by either magnets or electropositive metals.

Other experiments have used semiochemicals–chemicals produced by living creatures that can either attract or drive away other organisms. SharkDefense, a commercial repellent designed for use in marine rescue operations, contains substances derived from decayed shark tissue.

“There are a lot of different strategies, but nothing defined yet,” says Mandelman. But, he points out, changing fishing practices can also reduce bycatch–for example, setting longlines at depths below areas where sharks feed and shortening the time the lines are left out in the water. “It’s important to incorporate fishermen’s views,” he says. “Reducing bycatch benefits them too, and they have a lot of strategies, although they may not be shared widely between different fisheries.”

According to Greg Skomal, an aquatic biologist with the Massachusetts Division of Marine Fisheries, shark bycatch is a major challenge for managing stocks at the international level. “Nobody has tried systematically to summarize what we know about it until now, so we really need this kind of review,” he says. “If you can’t quantify bycatch, you can’t do a stock assessment to figure out how many sharks can be taken without hurting the population. So the bycatch issue underlies everything else.”