AIS data, a mine of information on trawling fleet mobility in the Mediterranean Sea

In the Mediterranean Sea, fishing vessels often operates throughout the geographical subdivisions adopted for statistical data collection (Geographical Sub-Areas; GSAs), causing a potential mismatch between catches site and reporting site. This paper provides a quantitative assessment of the fluxes of fishing activity of bottom trawlers across the Mediterranean Sea, by analyzing the Automatic Identification System (AIS) data broadcasted in 2017. Fishing activity was analyzed from three perspectives: fishing site, port of arrival and registration site of the vessel. For each GSA, a "fidelity score" was calculated to quantify the proportion of fishing time spent in the home GSA; an "intrusion score" was computed to quantify the effort deployed by vessels registered elsewhere. Major vessel fluxes were detected between GSAs, and fleets were classified based on their mobility. Areas showing fleet overlaps were identified and those characterized by the largest overlaps were selected as case studies. The lowest fidelity scores were observed in the central Mediterranean (GSAs 11.2, 15, 16 and 18), while the highest intrusion score was recorded in the southern Mediterranean and around Crete. The vessels responsible of the largest fluxes were those registered in GSAs 16, 18, and 6. The case studies included: GSAs 23, where several fleets exploited narrow slope areas; GSA 13, where multiple fleets overlapped in a relatively wide area; and GSA 17, where two fleets overlapped in a wide platform area. Mobility was distinguished in short-range - involving platform areas of contiguous GSAs - and long-range - involving slope areas of non-contiguous GSAs.