Assessment of permanent magnets and electropositive metals to reduce the line-based capture of Galapagos sharks, Carcharhinus galapagensis
Robbins, W.D., Peddemors, V.M. and Kennelly, S.J., 2011. Assessment of permanent magnets and electropositive metals to reduce the line-based capture of Galapagos sharks, Carcharhinus galapagensis. Fisheries Research, 109(1): 100–106.
Sharks possess anterior electrosensory pores (ampullae of Lorenzini), which allow them to detect very weak electromagnetic fields. Powerful magnetic fields may overwhelm this sense, and repel sharks, even in the presence of an attractant. For fishers targeting fish in proximity to Critically Endangered shark stocks such as the grey nurse shark in NSW, the application of such repellents in conjunction with baits may prevent the accidental capture of protected sharks. Using underwater video and hookless gear, we tested seven rare earth magnet configurations, two ferrite magnet configurations and two rare earth electropositive metals as means to reduce the rate at which a non-endangered test species (the Galapagos shark, Carcharhinus galapagensis) attacked baited lines. Configurations of three 50 mm diameter rare earth magnet discs showed the most potential, with a vertical configuration of magnets alongside the bait reducing attacks by 50%, and a stacked configuration of the same magnets above the bait also producing significantly more aborted investigations of the bait prior to it being taken. No other magnetic or electropositive metal configuration produced significant reductions in attack rates, time taken to strike, or number of prior investigations. Our study showed that the overriding factor determining Galapagos shark behaviours towards baits was conspecific density. The number of sharks present increased as trials progressed, with a corresponding decrease in their time to attack baits. This effect was particularly apparent when three or more animals were present. These higher shark densities diminished the effectiveness of our experiments as individuals engaged in non-selective “mob” rushes towards the closest bait. Although our results showed that social interactions between sharks outweighed individual responses to bait protection devices, magnetic deterrents have significant potential for reducing shark bycatch for species that occur in lower densities, or which interact less vigorously with conspecifics.