SMART drumlines at Reunion Island do not attract bull sharks Carcharhinus leucas into nearshore waters: Evidence from acoustic monitoring

摘要

Following a series of shark attacks, local authorities in Reunion Island developed an experimental shark control programme using innovative fishing gear, namely Shark Management Alert in Real Time (SMART) drumlines (SDL). From January to November 2014, four SDL were deployed 24 h per day, four days per week to target bull (Carcharhinus leucas) and tiger (Galeocerdo cuvier) sharks and to test the fishing efficacy of the SDL. Presence and residence time of 19 acoustically tagged bull and 19 tiger sharks, which had been tagged up to two years before the SDL deployment, were modelled against different SDL configurations, which included bait type and presence of bait or catches on the hooks, as well as environmental parameters before, during and after SLD deployment. There was insufficient acoustic data from the tiger sharks for any analyses. Bull sharks spent less time in nearshore waters when drumlines were deployed, and their presence was influenced by sea surface temperatures (SST), rainfall and time of day. There was no difference in the number of bull sharks detected in the SDL deployment area compared with surrounding sites. As SDL catch rates were only poorly correlated with presence of tagged sharks, their efficacy in catching sharks present in the area could not be accurately determined. Overall, the results show that SDL, deployed without chum and baited with small, whole, low-fat fish cannot be considered as "shark magnets", which could attract dangerous bull sharks inshore where they would pose a threat to the safety of surfers and other sea users. The detection of tagged bull sharks moving into and out of the SDL fishing area indicates that these fishing devices do not provide an impenetrable barrier to the passage of this potentially dangerous species.