Mortality of brown-shrimp discards from the beam trawl fishery in the Tagus estuary, Portugal

Beam trawling results in large amounts of discards. Discards mortality may be influenced by some of the procedures aboard fishing vessels. In the present study, the influence of temperature of the sorting containers, of sorting time and of haul duration on the mortality of brown-shrimp discards from the beam trawl fishery in the Tagus estuary was evaluated. Experiments were conducted in the summer and in the winter of 2000, simulating on-board procedures, in different conditions. The sorting containers were either wooden, plastic or metallic, dark or light-coloured, and two sorting times (5 and 10 min) and three haul durations (10, 20 and 30 min) were considered. The light-coloured wooden and plastic containers presented the lowest temperatures, whereas the highest one was measured on the dark metallic container. For temperatures between 10 and 20 degreesC, mortality was nil; for values higher than 20 degreesC, mortality was high, reaching 96% in the temperature interval 40-50 degreesC. Discards mortality was similar for both sorting times considered. The mortality of discards increased with longer haul duration; for 10 min, mean mortality was 7%, whereas for 30 min was 75%. The use of light-coloured wooden or plastic sorting containers in summer and the avoidance of fishing activities at the hottest times of day in this season are suggested as management measures to minimise discards mortality in beam trawl fisheries. (C) 2003 Elsevier Science B.V. All rights reserved.