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Magnets as blue shark deterrents?

Overfishing is having a huge impact on one-third of elasmobranch (shark and ray) species. Sharks and rays play a vital role in their ecosystems, meaning that reducing the catch of sharks and rays across fisheries not targeting them is imperative for their conservation.

Blue sharks (Prionace glauca) are the most commonly caught species of shark globally, with estimates of 20 million sharks caught annually (IUCN). Most blue sharks are caught as bycatch from tuna and billfish (e.g. swordfish) fisheries, and although in many cases the entire animal is processed, there are instances where the sharks hold no commercial value for fishers. As blue shark populations are declining globally, it is essential to find ways to reduce bycatch- particularly in the North Atlantic, where declines of up to 50% over three generations have been observed (Rigby et al., 2019).

So how do we reduce the bycatch of sharks and rays and not the target species?

Sharks have six sensory systems, shared with most other fish (vision, smell, taste, hearing, touch and lateral line). However, elasmobranchs can also detect electromagnetic fields. This means that electromagnetic fields could potentially deter sharks from fishing gear while not impacting the catch of other fish species.

To see if using electromagnetic fields as a deterrent from fishing vessels, scientists have conducted studies on shark responses to magnets which, so far, have had mixed results (e.g. Lucas and Berggren, 2022; Favaro and Côté, 2015; Hart and Collin, 2015; Jordan et al., 2013). Results varied depending on the species assessed, the type of magnet used and other variables (including how much food the sharks have had access to beforehand and the presence or absence of other sharks).

Using a ferrite magnet (the type you would find on your fridge) Newcastle University, Marine Research and Conservation Foundation (MARECO), Celtic Deep and Blue Shark Snorkel are collaborating on the first behavioural experiment to see how blue sharks respond- building on the growing body of literature studying shark responses to magnets. By deploying pelagic BRUVS (baited remote underwater video systems) to film how the blue shark approaches a baited container. In half of the deployments, there is only bait in the containers and the other half contains both bait and ferrite magnet blocks.

The videos are filmed in stereo, using 2 GoPro cameras allowing us to calculate the 3-D position of the sharks, measure their length and how close they approach the bait. Using this information, we can tell if the sharks are deterred, attracted, or unaffected by the magnets in the presence of bait.

If the magnets can deter blue sharks, they could be implemented in fisheries as a cost effective bycatch reduction technique. If they do not deter blue sharks, the study closes off a research gap and provides evidence that alternative bycatch mitigation methods would be necessary.

We have just finished the pilot trials of the study, so keep an eye out for updates! Click here to see an example clip from one of our deployments.

Blog by Sol Lucas- a PhD student in the Marine MEGAfauna lab at Newcastle University, investigating low-cost technologies to reduce marine megafauna bycatch.

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