Physical and psychological deterrence strategies to mitigate odontocete by-catch and depredation in pelagic longline fisheries: progress report

Note: also published as a conference paper WCPFC‐SC9‐2013/EB-WP-10

Odontocete by - catch and depredation in pelagic longline fisheries is globally widespread and may have negative impacts on the conservation and welfare of the odontocetes involved and on the economic viability of the fisheries involved. This study attempted to develop two differently designed devices that would prevent odontocetes from depredating caught fish, thus putting themselves at risk of becoming by – caught when doing so. This was achieved using physical deterrence (i.e. by shrouding the fish with a barrier) and psychological deterrence (i.e. utilising prior negative experiences of temporary entanglement in fishing gear) strategies. Both devices were designed to fit directly to a branchline some distance from the hook, then descent towards and shroud the caught fish using a line tension trigger mechanism. Contrary to expectations, incidences of by-catch and depredation were frustratingly rare during the sea trials, suggesting their occurrence varies in time and space. All incidences occurred on control branchlines that were not fitted with a deterrent device, suggesting the potential of this technology to deter depredating odontocetes should not be discounted.
The presence of the devices on branchlines had negligible effect on fish catch rates, size and survival, were physically robust and relatively easy to integrate into fishing operations. Although this study provides interesting insights into the development and impact of this technology, a considerably larger data set that more thoroughly and accurately depicts the efficacy of these devices is needed. Future development should also focus on minimising the cost of implementation, in order to ensure voluntary and widespread uptake.