Cost-effective mitigation strategies to reduce bycatch threats to cetaceans identified using return-on-investment analysis

Globally, fisheries bycatch threatens the survival of many whale and dolphin species. Strategies for reducing bycatch can be expensive. Management is inclined to prioritize investment in actions that are cheap, but these may not be the most effective. Here, we show the utility of an economic tool, return-on-investment, to inform costeffective reduction of cetacean bycatch. We use a case study of Australian fisheries to prioritize three different management actions for dolphins and whales – spatial closures, acoustic deterrents and gear modifications - for trawl, net and line fisheries. The return-oninvestment analysis highlighted greater expected benefits for a lower investment using a costeffectiveness approach that explicitly considers costs to fisheries, compared to traditional conservation approaches where the primary goal is to save whale and dolphin species. We use the results to identify the most highly-ranked management strategies across Australia to abate dolphin and whale bycatch in fisheries at a fine spatial resolution. Although trawl net modifications were the cheapest strategy overall, there were many locations where spatial closures were the most cost-effectivesolution, despite their high costs to fisheries, due to their effectiveness in reducing all fisheries interactions. This study demonstrates a new spatial approach to mitigating fisheries bycatch. The method can be used to reduce bycatch threats to mobile marine species across diverse fisheries at relevant spatial scales, improving conservation outcomes.