Movements of grey mullet Liza aurata and Chelon labrosus associated with coastal fish farms in the western Mediterranean Sea

Abstract

Grey mullet occur in abundance around sea bream and sea bass farms where they forage on waste fish feed, a behaviour that could modify their natural movement pattern and distribution. In this study, we used visual census to record grey mullet aggregations at fish farms in the western Mediterranean Sea. We also mapped the movements of 2 species (Liza aurata and Chelon labrosus) between farms and adjacent coastal fishing areas, using acoustic telemetry. Grey mullet were frequently observed in the vicinity of the aquaculture cages and represented an important abundance and biomass at the farms. The presence and swimming depth of the tagged mugilids at any of the farms were neither significantly related to the time of the day nor the feeding period, except for C. labrosus, which showed a tendency towards deeper waters (~15 m) during feeding periods. Some of the tagged fish stayed in the vicinity of the farms for longer periods and also moved frequently to other farms and nearby commercial fishing areas. Other tagged fish remained at the release location for shorter periods, before they moved out of the study area or possibly were caught by local fishermen. This is the first study using acoustic tagging in wild fish around Mediterranean fish farms that demonstrates that offshore aquaculture farms and local fishing grounds in the western Mediterranean Sea are connected through movements of wild fish. These farms attract and affect large numbers of commercially important fish species; probably causing ecological changes not only in the immediate proximity of farms but also several kilometres away from the farms.