Behavioural response of benthic elasmobranchs to a neodymium magnet under controlled laboratory conditions

Grew M, Raoult V, Gaston TF (2024) Behavioural response of benthic elasmobranchs to a neodymium magnet under controlled laboratory conditions. Fisheries Research 271:106926.

Global elasmobranch population decline can be largely attributed to anthropogenic impacts such as commercial fishing. As such, reducing bycatch of these animals is a key management objective in many fisheries. Magnetic deterrents such as permanent and rare-earth magnets can deter both pelagic and benthic elasmobranch species. However, the literature is inconsistent and shows varied levels of effectiveness depending on the species and the deterrent. To broaden our understanding of species-specific differences in deterrent effectiveness, this study assessed the efficacy of a rare-earth magnet to deter four benthic elasmobranchs under laboratory conditions: Port Jackson shark (Heterodontus portusjacksoni; n = 10), epaulette shark (Hemiscyllium ocellatum; n = 8), eastern fiddler ray (Trygonorrhina fasciata; n = 10), and the blue spotted mask ray (Neotrygon kuhlii; n = 7). After evaluating the behavioural response of the four study species in the presence of a N52 neodymium magnet and a control in 207 trials, the neodymium magnet did not effectively prevent individuals from entering an experimental compartment. The magnet only had a minor deterrent effect, slightly reducing the proportion of successful attempts at entry through the door and over the neodymium magnet (control: 0.94 ± 0.23; neodymium magnet: 0.74 ± 0.3; mean ± standard deviation). We hypothesised this was a result of species-specific biological and behavioural factors that reduce the effectiveness of magnets as deterrents for these species. Our results suggest fisheries management moves away from trialling magnets as elasmobranch deterrents due to their inconsistent effectiveness, and rather investigate other devices such as those using electrical fields that show greater potential.