Standard Talk (15 mins) Australian Society for Fish Biology Conference 2022

Testing the effectiveness of deterrent devices in reducing shark depredation (#71)

Gary Jackson 1 , Peter Coulson 2 , Ainslie Denham 1 , Alex Hesp 1
  1. Department of Primary Industries & Regional Development, Hillarys, WA, Australia
  2. Institute for Marine and Antarctic Studies , University of Tasmania, Hobart, TAS., Australia

Shark depredation is a major issue for many commercial and recreational fishing operations around northern Australia. A recent FRDC workshop highlighted the issue’s complexity, the lack of dedicated research on depredation and its impacts and discussed potential options for mitigation. Following on from technological developments to reduce risk to water users from shark bite, deterrent devices designed to protect fish-catch are increasingly being developed and marketed to mitigate shark depredation. However, as with bite-protection devices, the effectiveness of specific deterrent technologies is often poorly demonstrated and the claimed efficacy is typically not based on independent and scientific testing.  We tested three deterrent devices (electrical, magnetic, acoustic) while line fishing for demersal scalefish in the Pilbara region, Western Australia. We conducted an a priori power analysis to develop a sampling design that would determine the devices to be “effective” if they resulted in a 50% reduction in the number of fish depredated. Field testing involved rod-and-line fishing during daylight in separate one-hour fishing sessions, in which one of the four deterrent treatments (three devices and no device i.e. control) were deployed in randomised order. Testing occurred in water depths mostly between 20-50 m, with 1-5 km separating each site. Video from underwater cameras mounted on the fishing lines was used to (1) identify the species of fish caught and sharks involved, (2) examine the behaviour of sharks when approaching hooked fish and (3) determine the length of time for sharks to initiate depredation. While acknowledging the inherent challenges in running experimental trials in the marine environment, we offer results of this study as a starting point for developing guidelines that could be used to test similar deterrent devices that will developed in the future such that commercial and recreational fishers will have confidence in their adoption as effective mitigation measures.