The effect of bottom trawl fishery on biomass variations of demersal chondrichthyes in the eastern Mediterranean

Chondrichthyan species are considered highly susceptible to fishing activities and as "indicator species" for overfishing and ecosystem stability. In European waters, chondrichthyes have become a focus of conservation actions due to the abundance declines of several species. In the present study, fishery independent data obtained from the Mediterranean International Trawl Surveys (MEDITS) in the Aegean and E. Ionian Seas (eastern Mediterranean) were coupled with bottom trawl fishing effort data obtained from vessel monitoring systems (VMS) in order to evaluate the effect of fishing on biomass variations of demersal chondrichthyan species. The study focused on commonly captured species such as Raja clavata, Galeus melastomus, Scyliorhinus canicula and Squalus spp, as well as to the total chondrichthyan biomass. The effect of fishing effort on biomass indices, expressed in terms of kg per square km of swept area, was examined by means of Generalized Additive Model techniques, using also the sampling position, year and depth as co-variates. Overall findings suggested a biomass decline of chondrichthyan species with increasing fishing effort. Nevertheless, it was also found that certain species (e.g G. melastomus in both areas and S. canicula in the Aegean Sea) can withstand fishing pressure and it is likely that discard survival rates, depth preferences and fisheries exploitation patterns, are the main drivers explaining variations regarding the impact of fishing on the different stocks.