Null effects of decomposing shark tissue on baited-hook catches of elasmobranchs

Abstract

The effects of decomposing shark tissue on catches of benthic longlines targeting various carcharhinids were assessed to inform possible use as a semiochemical shark deterrent. During 15 nights fishing, four benthic longlines (each comprising 18–30 hooks baited with mullet, Mugil cephalus) were deployed to 12–56 m overnight for 12–21 h off eastern Australia. Two of the longlines had 2.0–3.0 kg of decomposing shark tissue placed into porous cylindrical canisters (520 × 105 mm polyvinyl chloride) secured to the mainline mostly between every three hooks (15–20 m apart), while the other two longlines had empty canisters. In total, 150 fish were caught, comprising 14 species of elasmobranchs and especially tiger sharks, Galeocerdo cuvier (31% of total). The decomposing shark tissue in the canisters had no effects on catches of any species or groups, with variability among most attributed to fishing depth (positive relationship) and also soak time (negative) for carcharhinids and G. cuvier. Irrespective of the contents of the canisters and the lack of any semiochemical effects, there was some evidence of fewer sharks caught on adjacent hooks as moonlight increased, and possibly because of a visual response. There was no depredation of any decomposing shark tissue in the canisters, but three juvenile hooked sharks were substantially depredated, and presumably by larger individuals. Most (∼70%) of the remaining hooked sharks survived. While this study showed no repelling effects of decomposing shark tissue, the conclusions are restricted to the experimental conditions, including the source of tissue and the distance between hooks, which might be used as upper limit in any future work assessing for effects.