This video shows how a Turtle Excluder Device works, via YouTube

Technology extremely similar to the Turtle Excluder Device has been used to reduce dogfish bycatch in the hake trawl fishery. Other technological solutions to reducing shark bycatch include the use of nylon leaders, which are easier for sharks to bite through than those made of wire, allowing sharks to easily free themselves when caught. Using differently shaped hooks called circle hooks doesn’t reduce the number of sharks which get caught, but it does lead to a higher chance of survival for sharks that get caught.


Targeting the Shark Sensorium

Much of the recent research into shark bycatch reduction technology has focused on the unusual sensory abilities of these animals. Sharks and their relatives can detect electromagnetic fields, which can be exploited to keep sharks away from baited fishing hooks.


“Laboratory and field experiments have demonstrated that magnets and electropositive metals can have a repellent effect on sharks,” says Aurelie Cosandey-Godin, a postdoctoral researcher at Dalhousie University. Thus, applying magnets or electropositive metals to fishing hooks can help keep sharks away from the gear without affecting how many of the target species are caught.

A magnet being brought near a shark

Cosandey-Godin explains how it works:

In our field experiment, we used electropositive metal pieces made of 76% neodymium and 23% praseodymium (Nd/Pr), but others have tested magnets composed of different mixture of neodymium, iron and boron. Electropositive metals and magnets usually include some composite of rare-earth metals. The metals either oxidize and/or react with seawater, creating small electrical fields that can be detected by the shark. Since bony fish such as tunas and swordfish do not have electroreceptors, these methods have the potential to reduce shark bycatch rates without affecting target catch rates.


While the repellent effect of magnets and electropositive metals on sharks is promising, results seems to vary widely by shark species.

These tools reduced bycatch of sandbar sharks by more than two-thirds, significantly reduced scalloped hammerhead bycatch, and increased the time it took for spiny dogfish to approach bait. However, magnets and electropositive metals had very little effect on reducing catch rates of blue sharks, a major bycatch species in several Atlantic fisheries, and can sometimes even increase blue shark catch. “Some fishermen felt that the magnets were attracting sharks and renamed them ‘mako magnets,’” said Cosandey-Godin.


Example of electropositive metal pieces used in a study of the Canadian pelagic longline swordfish fishery showing how they quickly dissolve in seawater. From left to right: new to 24h in seawater. Courtesy Dr. Aurelie Cosandey-Godin


Even in cases where magnets and electropositive metals have been shown to successfully reduce shark bycatch, what works in a research setting won’t necessarily work on a commercial fishing vessel. For one, the financial and logistical cost of deploying them can be prohibitively high. Rare earth metals can be expensive, attaching them to thousands of fishing hooks takes a lot of time, and they don’t last particularly long before they degrade in seawater. Turtle excluder devices were much more effective at reducing bycatch and much less expensive, but fishers were still so opposed to them that they blockaded Gulf of Mexico port cities and even rammed Coast Guard vessels.

Nevertheless, using electromagnetic fields to repel sharks from fishing gear is promising. In some cases, at least, this technology is likely to be part of the solution to an environmental problem that is only starting to get the research and advocacy attention it deserves.


Lead image: Finetooth shark, via Wikimedia