Assessment of the effectiveness of deterrents for elasmobranch by-catch in pelagic longlines

Elasmobranchs play an essential ecological role in marine ecosystems but are currently among the most threatened animal groups globally. In the Mediterranean Sea, the conservation status of sharks and rays is particularly critical, underscoring the urgent need for management measures. Many are caught unintentionally, often forming the main bycatch in commercial fisheries. Research over the past decade has focused on mitigation measures aimed at reducing interactions with fishing gear, including the use of bycatch deterrents such as electric devices. However, the effectiveness of these measures has so far been variable, highlighting the need for more field-based evidence. This study evaluates the effectiveness of an electric deterrent in mitigating the interactions of elasmobranchs in pelagic longlines. Field surveys were conducted in the Pelagie Archipelago (Strait of Sicily) using experimental longlines equipped with electric deterrents and underwater cameras to monitor behavioural interactions. The longlines featured alternating branchlines with hooks, active electric deterrents, and inactive deterrents to account for confounding operational effects. Video analyses showed that the pelagic stingray (Pteroplatytrygon violacea) and the sandbar shark (Carcharhinus plumbeus) interacted most frequently with the gear. Behavioural analyses indicated that bait interactions were significantly lower in the presence of active electric deterrents, accompanied by a significant increase in withdrawal and a decrease in the proportion of time allocated to feeding. In conclusion, this field study indicates that electric deterrents are an effective bycatch mitigation tool. Although further trials are needed to disentangle the electric stimulus from other sensory cues, this technology significantly reduces the risk of accidental capture for both the observed species, offering a promising solution for reconciling commercial fishing with the conservation of vulnerable species.