Draft - Mitigating operational interactions between odontocetes and the longline fishing industry: a preliminary global review of the problem and of potential solutions

Context and need: Operational interactions between odontocetes and the longline industry is a global problem. The odontocete populations involved are at risk of population decline due to the incidence of by-catch mortality. The longline fisheries involved are at risk of becoming economically unviable due to the incidence of catch depredation. Identifying and developing mitigation strategies is a priority for ensuring the future sustainability of odontocete populations and longline fisheries.
Approach and methods: This review begins by defining depredation and by-catch, then outlines the history of longlining and describes the fishing gear and practices used. The available published literature is then summarised with a view to describing the trends in and focus of the literature. This information was used to identify the odontocete species that depredate from and become by-catch on longlines, and where these events occur. The review concludes by detailing the mitigation methods that have been or may be trialled in the future.
Results: By-catch of odontocetes was found to occur globally and in many longline fisheries. There are only a few reports of the level of loss, although the level of this phenomenon remains unclear. Of the few cases reported, by-catch ranged between 0.002 and 0.231 individual caught per set. At least 13 odontocete species are involved, although the lack of information about population size and life history characteristics make it impossible to determine if this is sustainable. Depredation by odontocetes occurs in most longline fisheries and may lead to significant economic losses, with one fishery reporting fleet-wide daily losses of between US$928 and US$5,480 in the mid-2000s. Since then, considerable effort has been committed to solving this problem and potential solutions have included acoustic and physical tools. Acoustic mitigation tools have proven difficult to develop to assess. In contrast, recent innovations in physical depredation mitigation devices (PDMDs) have yielded promising results, although they have received less attention to date.
Synthesis and applications: The issue of (i) catch depredation by odontocetes from longline hooks and (ii) by-catch of depredating odontocetes on longlines is a global problem that requires immediate attention. Mitigation strategies should include the development and implementation of PDMDs, but should also be inclusive of a suite of other tools. The experience of fishers and their enthusiasm to be involved in developing mitigation tools should not be underestimated. Governments, research institutions, fisheries and funding bodies that are associated with this problem are encouraged to participate and invest in international collaborations that are focused on finding globally applicable solutions.