Modeling discards in Trawling Mediterranean Northern Alboran Sea Fishery

Abstract

Target and Bycatch species metrics estimated from fishery-dependent data were explored to assess their use in governance of habitat conservation in respect to fisheries. Fishing data collected by onboard observers in otter-trawl boats between 2011 and 2012 at monthly sampling frequency in the Alboran Sea (Western Mediterranean) were used to build maps of sensitivity to fishing stress. Maps were drawn by means kriging interpolation techniques of biomass and abundance (Catch Per Unit of Effort, CPUE) in kilogram and number per fishing hour of blue whiting (Micromesistius poutassou), European hake (Merluccius merluccius), and red mullets (Mullus barbatus and Mullus surmuletus) target species, seabreams (Pagellus acarne, Pagellus bogaraveo, and Pagellus erythrinus), and mackerels (Trachurus mediterraneus, Trachurus trachurus, and Trachurus picturatus) bycatch species and Bogue (Boops boops) bycatch discarded species. Modelling discards by means Generalised Additive Models (GAMs) use environmental (sea surface temperature and chlorophyll-a from satellite data and NAO climatic index); spatial (latitude, longitude, depth and port) and temporal (season, haul duration, moon phase), as well as technical (boat length and power) explanatory variables. The main causes of discards, for both target and bycatch species, are associated to the seasonality of the recruitment and the changes on the spatial distribution of habitat preferences along their ontogeny. Environmental variables did not reveal significant effects, showing that operational oceanography standard products must be not enough to assess discards, and therefore products providing information on specific ecological processes to discards must be designed with this purpose. In Bycatch species, such as sea breams, mackerels and bogue, discards were also highly dependent of the port and boat (fleet/boat strategies, power, etc, and market preferences). The higher discards corresponded to these bycatch pelagic or bentho-pelagic species. Keywords: Discards, Otter-trawl fisheries, fishery conservation, operational oceanography, spatial modeling