Assessing the fish assemblage associated with FADs (Fish Aggregating Devices) in the southern Tyrrhenian Sea using two different professional fishing gears

Abstract

Fish Aggregating Devices (FADs) are used worldwide by fishermen to improve fishery yields, based on the finding that juveniles and adults of both pelagic and demersal fish species are attracted to floating objects. FADs fisheries are usually exploited using purse seine nets. In Sicilian seas, two different purse seines are used: (a) classic purse seine and (b) a transformed beach seine known as raustina. In this study we compared these two fishing gears in order to determine the best one for assessing fish diversity and to evaluate their impact on the species caught. We sampled the fishes associated with FADs from July to November 2003. Each haul was monitored by a SCUBA diver who observed the species escaping from the different types of net. Results showed that both gears catch the same fish species but significant differences in abundance were detected between gears. In particular, the purse seine caught a higher number of Thunnus thynnus than the beach seine (respectively: 8.2±1.1 vs 0.9±0.2), which in turn proved more efficient in catching C. crysos (31.4±3.7 vs 8.9±1.5 caught by purse seine). SCUBA observations revealed that T. thynnus escapes capture more easily from the beach seine than from the purse seine, due to their behaviour of swimming at a depth beneath that reached by the beach seine. On the other hand, the smallest specimens of C. crysos pass more easily through purse seine mesh, which is larger than beach seine mesh. Both gears are equally useful for assessing biodiversity associated with FADs, although they give different results for fish abundance. As the catches of FADs fishery target species (i.e. Coryphaena hippurus) were similar for the two gears, the modified beach seine could be proposed as a management tool to reduce the catches of young T. thynnus in accordance with European legislation.