Impact and size selectivity of fishing gears used in estuarine crab fisheries

Abstract

Crustacean fisheries have expanded in the last decade due to the high economic value of the species. However, estuarine crustacean fisheries remain poorly studied compared with marine crustacean fisheries. In Portugal, the European green crab (Carcinus maenas) may become increasingly important for the fishing industry and seafood production in general, especially in the current context of overexploitation of estuarine traditional fisheries resources. The unknown ecological impact of crab fishing gear and its size-selectivity characteristics constrain gear regulations by fisheries authorities. We developed an integrated study over three years to analyse a C. maenas fishery in three Portuguese estuarine systems aiming to: 1) describe the fishing characteristics and modus operandi of the fishing gears; 2) analyse gear selectivity and catch rates; 3) analyse environmental impact of fishing gears; 4) Contribute to supporting better regulations for the fishery. Two types of crab fishing gear are used, box traps and drop nets. Daily catches of green crabs varied by gear, system, and month, with the highest catches observed in box traps during the warmest months. Selectivity studies indicate that a mesh size of 18 mm in box traps and 30 mm in drop nets would allow crabs larger than the minimum landing size (40 mm carapace width) to be targeted while maximising catch rates. However, regardless of the mesh size, the catch from both gears will need to be sorted to exclude juveniles from the catch. The bar spacing, which allows crabs to be sorted according to the minimum landing size, is 17 mm. Gears present none (drop nets) or low by-catch (220 g or 23 individuals/40 box traps), which was promptly discarded, indicating a low impact on estuarine communities. The methodology used in this study, which combines ecological impacts and technical gear issues (selectivity), can be directly applied to enforce regulations and improve sustainable exploitation of socio-economically important artisanal fisheries.