engleski

Interaction of semi-offshore tuna farms with marine biodiversity

Tuna farms play an important role in wild fish populations in several ways. Fish cages, which present artificial elements in coastal ecosystems, are considered large fish aggregation devices (FADs) because numerous wild fish species are aggregated in large numbers throughout the year. The cages provide shelter, where the unused portion of the food that passes through the cages adds to the attraction. Tuna farms may also impact egg production and potential recruitment of Bluefin tuna in the Adriatic Sea because they are supplied with juvenile fish and practice prolonged farming period of 18 to 32 months, which overlaps with the period of fish first maturity. To assess the attraction of tuna farms to wild fish populations and the spawning activity of caged farmed fish, species richness and abundance of wild fish were estimated by visual census, while reproductive output and egg production of farmed tuna were estimated by gonadal histology and intensive ichthyoplankton collection with genetic identification of tuna eggs at a farm off the southwest coast of Brač Island, Croatia. On average, 1200 fishes from 12 taxa and 7 families were recorded per transect of 11 250 m3 in the immediate vicinity of the tuna farm during the three-year study period. Both species richness and total fish abundance were significantly higher near the fish farm than in the control areas in all study years. Gonadosomatic index values for farmed tuna indicate that the maturation period peaks in May, followed by the onset of spawning in June. Barcoding of tuna-like eggs collected near farms during the spawning season confirmed the presence of Thunnus thynnus individuals. In conclusion, tuna farms can serve as functional marine protected areas if no fishing conditions are practiced within the leasehold areas, while the phenomenon of eggs escape by tuna spawning in the cages could have positive effects on the recruitment of Bluefin tuna in the Adriatic Sea

capture-based aquaculture ; wild fish ; spawning ; barcoding ; diversity

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