Objective methods for defining mixed-species trawl fisheries in Spanish waters of the Gulf of Cádiz

Abstract

This paper presents the results obtained from applying an objective classification method to the daily commercial landings data of the Spanish bottom-trawl fishery off the Gulf of Cádiz for defining mixed-species fisheries or fishing trip types. Definition of these operational fisheries was based on the relatively unique and homogeneous species composition of the landings and technical characteristics of the vessels. The method was based on cluster (CA) and linear discriminant (DA) analysis techniques. The application of CA techniques to the 1993 data matrix of daily catch per unit of effort per species and active vessel resulted in the identification of 22 types of fishing trips, which were defined by the relative importance of their target and accessory species. Subsequently, different linear discriminant functions were derived for each FTT in order to classify new records. Five different fleet types were also identified through CA techniques according to the vessel’s length, GTR and HP. The distribution of active vessels by fleet type in each landing port showed a great local and regional heterogeneity of the fleets exploiting the multispecies fishery. A correspondance analysis between types of fleets and fishing trips showed a high correlation, indicating the existence of a direct relationship between the capacity of vessel mobility and the bathymetric situation of the fisheries.