Norway lobster ( Nephrops norvegicus) from the southwest and south of Portugal — estimation of the effects of changing trawl mesh size and fishing effort by length cohort analysis

Abstract

Nephrops stocks from the southwest and south of Portugal (Alentejo and Algarve) have been assessed to investigate the effects produced by increasing trawl mesh size and varying fishing effort. A length-based assessment method was applied, by sex, and the input data and parameters were those used by the 1991 International Council for the Exploration of the Sea Working Group on Nephrops Stocks. Five different trawl mesh sizes (60, 65, 70, 75 and 80 mm) and fishing effort changes from −90% to +50% relative to the current level of effort were simulated. Current trawl mesh size is equal to 55 mm, which is the minimum legal size for trawl nets directed to catch Nephrops. The main results from this study indicate benefits in long-term yield and biomass when trawl mesh size is increased; benefits for males are greater than those for females. If only trawl mesh size increases, long-term landings benefits range from 4 to 18% and from 2 to 9% for males and females, respectively. If reductions in fishing effort occur simultaneously with increases in trawl mesh size, the estimated landings and stock biomass benefits are higher than those obtained at current level of effort.