Ecosystem effects of bluefin tuna Thunnus thynnus thynnus aquaculture in the NW Mediterranean Sea

In recent decades the eastern stock of Atlantic bluefin tuna Thunnus thynnus thynnus has declined, driven by excessive catches partially mediated by the growth of the capture-based aquaculture of bluefin in the Mediterranean. This study addresses the potential direct and indirect food-web effects on trophic linkages in the ecosystem through the removal of both small pelagic fish species and wild bluefin tuna for aquaculture operations in the Mediterranean. A mass-balanced model representing the southern Catalan Sea during the 1990s was modified to include a bluefin tuna farm partially supplied from fish captured in the area modeled. Six scenarios were developed to simulate possible changes to the capture-based aquaculture operations and possible impacts to the ecosystem. The addition of one bluefin tuna farm in the Catalan Sea did not produce substantial effects in the ecosystem. Simulation scenarios that include the level of production already present in a similar ecosystem in the Murcia region resulted in large fluctuations of both biomass and yield for bluefin tuna, as well as for many species in the modeled ecosystem. Increases in biomass of lower trophic level functional groups were observed with reductions in biomass from higher trophic level predators. These outcomes demonstrate that removal of biomass at top and intermediate trophic levels can have direct and indirect outcomes on the structure of the ecosystem due to the complexity of the food web. Our results suggest, in the case of the Western Mediterranean Sea, increasing bluefin tuna farming activities will likely contribute towards further degradation of an already highly exploited ecosystem.