A novel experimental approach to investigate the potential for behavioural change in sharks in the context of depredation

Abstract

Baited video systems have been widely used to assess the relative abundance and diversity of sharks in locations around the world, however they provide limited information on behaviour. We developed and pilot tested a novel experimental approach to investigate whether repeated deployments of baited video systems in the same location could generate quantitative data on shark behavioural patterns, in the context of shark depredation (where sharks consume hooked fish). Specifically, we sought to test whether repeated exposure to boats and food in the same location would lead to a change in the arrival time and first feeding time of sharks, over a short timescale. We used the Ningaloo Marine Park (NMP) in Western Australia, a location where higher shark depredation rates have been identified in consistently fished areas, as a case study. A modified Baited Remote Underwater Video (BRUV) system was repeatedly deployed at two fished sites and two sites within a no-take marine reserve in the NMP, over six consecutive days, to mimic repeated recreational fishing and the availability of hooked fish for sharks to depredate. This approach was designed to investigate and disentangle the potential role of changes in behaviour versus variation in shark abundance, as a mechanism for how and why shark depredation can occur. Here, we report preliminary results from this methodological approach, where time of arrival and time of first feeding declined markedly in the fished site over 6 days of BRUV deployments, compared to the control site in the no-take marine reserve. A greater number of individuals from four carcharhinid species were observed at the fished site, compared to only three individuals from two species in the no-take marine reserve. The preliminary results from pilot testing of this novel experimental approach suggest that, with further modifications to identify individual sharks, and a greater spatial and temporal replication of sampling, it may be possible to identify behavioural changes occurring in sharks in the context of shark depredation. Understanding this mechanism can bring important benefits for fishers and managers, as it can lead to modifications in fishing methods designed at reducing the occurrence of behavioural changes in sharks, and thus mitigating shark depredation.