Analysis of the factors affecting crustacean trawl fishery catch rates in the northern Tyrrhenian Sea (western Mediterranean)

Abstract

The aim of this study was to analyse the main factors affecting catch rates of the most important decapod crustaceans of the bottom trawl fishery in the northern Tyrrhenian Sea, western Mediterranean. Catch per unit effort (CPUE) data (kg/fishing day/boat) of deep water rose shrimp, Parapenaeus longirostris, Norway lobster, Nephrops norvegicus, and red shrimps, Aristaemorpha foliacea and Aristeus antennatus, were collected monthly in the years 1991–1999 at the auction of Porto Santo Stefano, where these species are traditionally exploited by trawlers using traditional trawl nets. The application of cluster analysis to the entire data set allowed the identification of three main “metiers” or fishing tactics, corresponding to different fishing grounds, species assemblages and characteristics of vessels. Two “metiers” corresponded to fishing activity targeting the above-mentioned crustacean species. Generalised linear modelling was used to investigate the influence of factors such as month, year and vessel on catch rates of the species studied. This model explained a considerable amount of variability in the CPUE data, which substantially reflected the differences observed in the trends of monthly landings of these species. Month was an important factor explaining deviance in the CPUE for the deep water rose shrimp, but especially for the two red shrimps. This was mostly due to the temporal pattern in the catchability of these species, as well as the seasonality of the fishing effort targeting these resources. Year significantly contributed to the variation in CPUE of all the species studied, in particular for P. longirostris, confirming that the abundance of this species is subject to distinctive inter-annual fluctuations. Vessel was a significant factor determining variability in CPUE for P. longirostris and, especially, in N. norvegicus. No single boat characteristic such as length, gross registered tonnage and engine power provided a more satisfactory explanation than the vessel factor as a whole. For Norway lobster, the high variation explained by the vessel is attributable to the fact that the fishing fleet targeting this species was the most heterogeneous, involving two different “métiers” and, therefore, boats differing in size and in fishing practices.