Analysis of the landings and trawl survey time series from the Tyrrhenian Sea (NW Mediterranean)

摘要

The present study aimed to evaluate the variation over time of the abundance of the populations of five demersal species in the Tyrrhenian Sea (NW Mediterranean) analysing data obtained from trawl surveys and commercial landing monitoring. The influence of some environmental variables (sea surface temperature, wind speed, NAO index) and some fishing effort indices (fishing days, mean size and mean engine power of the trawl vessels) was analysed as well. The species considered for the study were Eledone cirrhosa, Merluccius merluccius, Mullus barbatus. Nephrops norvegicus and Parapenaeus longirostris. From 1991 to 2006, the mean monthly landing rates were computed from the data collected at the auction of Porto Santo Stefano, the most important fishing harbour in the area.A second data set was represented by trawl survey data: two experimental trawl surveys per year have been carried out in the framework of national and international programs on the assessment of demersal resources. The mean biomass (kg/km(2)) indices were calculated. The time series analyses were carried out by means of Min/Max Auto-correlation Factor Analysis (MAFA) and Dynamic Factor Analysis (DFA). The two techniques provided coherent results, indicating that the two most important trends in the response variables are an increase in terms of both LPUE and survey biomass index, and a fluctuating pattern, even though characterized by a general increasing tendency. What drove these trends is less clear, even though the available evidence suggests that the reduction of fishing effort has been more important than environmental factors