Interactions between bluefish Pomatomus saltatrix (L.) and coastal sea-cage farms in the Mediterranean Sea

Abstract

Coastal sea-cage farms aggregate large concentrations of pelagic and demersal fish. The large numbers of cultured fish and aggregated wild fish often attract a range of marine mammal predators which may break into cages and attack the cultured fish. To date, predation by a finfish species within sea-cages has not been documented. In the Mediterranean Sea, the bluefish Pomatomus saltatrix (L.) aggregates around sea-cage farms and enters into cages to predate on the cultured fish. We obtained information about the effects of bluefish predation on aquaculture production through a questionnaire that was completed by fish farmers in Spain, Italy, Malta, Turkey, Greece and Cyprus. In addition, we identified the abundance, size and stomach contents of bluefish aggregated around three fish farms on the coast of Spain through visual counts, and from captured bluefish both inside and outside of the sea-cages. Bluefish occurred around fish farms in Spain, Italy, Malta and Turkey. Farmers in SE Spain reported its presence only inside seabream (Sparus aurata) cages, while in Turkey bluefish were reported from inside seabass (Dicentrarchus labrax) and seabream cages. Greatest aggregated biomass of bluefish reached 1049 and 3191 kg at the Altea and Guardamar farms, respectively, with abundance peaking at 4500 individuals at both farms. Size structures differed markedly between farms, with smaller individuals aggregating at Altea. Stomach content analysis revealed that bluefish on the outside of sea-cages consumed pelagic species such as Sardinella aurita and Trachurus mediterraneus, while they predated on seabream once they incurred into cages, often consuming only the tails of many fish. The interaction of bluefish with sea-cage aquaculture is, at present, a problem of local concern restricted to some areas of the Mediterranean Sea, but its widespread distribution suggests this piscivore may be a problematic predator in other regions.