Unintended effects of single-species fisheries management

Ecosystem-based management is widely recognized as the path to achieve sustainability of ecosystem services. Tuna Fisheries Management Organizations have incorporated an ecosystem approach into their mandate, but their decision-making process essentially relies on individual stock assessments. This study investigates possible unintended consequences of management measures that primarily focus on single target species. In 2016, the Indian Ocean Tuna Commission (IOTC) adopted a plan for rebuilding yellowfin tuna stock. We examined the impacts that this measure might have had on the fishing strategy of purse seine fleets and on silky shark mortality, their main elasmobranch bycatch. The economic dimension of this possible ecological impact was also explored. Logbook and observer data from the French fleet, coupled with IOTC data from Spain, Seychelles and Mauritius, were used. After the implementation of the measure, an increase on the number of fish aggregating device (FAD) sets and an expansion of the fishing effort were observed. These resulted in a 35% increase on silky shark bycatch for the French fleet and a 18% increase for all fleets combined. Based on the estimated catches, the mean forgone consumptive value of silky shark bycatch was evaluated at US$ 1.6 million. Taking the conservation value into account, the social cost of this forgone ecosystem service could increase up to USD 14 million. This work is a first exploration into to the socioeconomic dimension of trade-offs between the use of FADs in tuna purse seine fisheries and shark bycatch and can be applied to other FAD-associated species.