The effects of neodymium-iron-boron permanent magnets on the behaviour of the small spotted catshark (Scyliorhinus canicula) and the thornback skate (Raja clavata)

Elasmobranchs (sharks, skates, and rays) are frequently captured as bycatch on a wide variety of fishing gears, such as pelagic longlines and hook-and-line fisheries, and therefore many species have experienced severe population declines. To reduce elasmobranch bycatch, scientists have begun exploring the effectiveness and potential application of elasmobranch-specific repellents, such as permanent magnets and electropositive metals. For the present study, the behavioural responses of captive small spotted catsharks (Scyliorhinus canicula) and thornback skates (Raja clavata) were observed in response to neodymium-iron-boron (Nd2Fe14B) permanent magnets. Results demonstrate that both R. clavata and S. canicula; (1) significantly avoided the Nd2Fe14B magnets more often in comparison to the control and procedural control and (2) significantly fed from the control and procedural control more often in comparison to the Nd2Fe14B magnets. Data also suggests a relationship between water temperature and the avoidance distance by R. clavata, with closer approaches prior to avoidance occurring in association with water temperatures of ≤12 °C. Additionally, the tail beat frequency associated with the avoidance behaviour of S. canicula was significantly slower (≤9 beats/10 s) in water temperatures of ≤12 °C. The findings from this study agree with previous electrosensory repellent studies, in that elasmobranchs detect and are deterred by permanent magnets however, the present study also demonstrated that there is a correlation between avoidance speed and distance with water temperature. These findings suggest that water temperature may be correlated to magnetic repellent effectiveness and thus warrants further experimentation.