Reactionary behaviour of free-ranging Galapagos sharks (Carcharhinus galapagensis) to an electronic shark deterrent
Robbins, W.D. and Peddemors, V.M., 2011. Reactionary behaviour of free-ranging Galapagos sharks (Carcharhinus galapagensis) to an electronic shark deterrent. Presentation given at the 3rd Annual Oceania Chondrichthyan Society Conference, 13 – 15 September 2011, Gold Coast, Queensland, Australia.
Sharks possess electrosensory pores around their noses (ampullae of Lorenzini), which allow them to detect very weak magnetic fields. Powerful electromagnetic fields may overwhelm this sense, and repel sharks, even in the presence of an attractant. Electronic shark deterrents (ESDs) are commercially-available units designed to decrease the risk of shark attack when worn by divers, as well as offering fishers protection of their catch when fishing with such a unit deployed from the boat. However, very little is known about how such units alter the behaviour of sharks. Here we investigated the behavioural changes of Galapagos sharks (Carcharhinus galapagensis) around two types of food attractant, berley and bait, in the presence of a commercially-available ESD. Berley trials, using a canister exuding fish odours, but not permitting feeding attracted high densities of sharks, with up to 19 individuals present per trial. Significant decreases in both shark abundance and number of canister interactions were seen when the ESD was activated within 2 m of the canister, while no effect was observed when the ESD was deployed 10 m above the canister. Midwater bait trials using a whole Australian sardine capable of being eaten by the sharks were rapidly depredated (<7 sec), with only a minimal delay when the ESD was activated. The number of non-depredating bait investigations remained unchanged with ESD activation, suggesting little behavioural change within the ESD electromagnetic field. Our findings show that electronic shark deterrents will dissuade, although not completely deter, casual interest from sharks, even when in high densities. However, they have little effect on preventing sharks from entering the deterrent's field when targeting a tangible food item.