Effect of codend mesh size on the performance of the deep-water bottom trawl used in the red shrimp fishery in the Strait of Sicily (Mediterranean Sea)

The influence of different codend nets mounting diamond-shaped (16, 20, 24, 28 and 45 mm, mesh side) in the performance of the commercial bottom trawl presently in use in the Strait of Sicily (Central Mediterranean Sea) to catch deep-water red shrimps, Aristaeomorpha foliacea and Aristeus antennatus, is analyzed. In two surveys carried out in fishing grounds in spring and summer 1993, 93 daylight hauls (of 3 hours each) based on the covered codend (cover of 14 mm mesh side) method were carried out. The composition and quantity of the combined, retained and escaped catch for the most relevant species and mixed categories of the different gear configurations (codends + cover) were analyzed and compared by applying both univariate (ANOVA) and multivariate (clustering and multidimensional scaling) techniques. The reduction of by-catch, the economic loss and the economic efficiency were used as indexes of the retention performance of the different codends. The overall catch was 6398 t (23 kg/h); Aristaeomorpha foliacea accounts for most of the catch in number, weight (48% of the total) and value (87% of the total value). Bony fishes, cartilaginous fishes and other crustaceans are the by-catch, representing, respectively, 26%, 16% and 9% of the catch in weight, the rest belonging to cephalopods. The use of the different codends does not influence the overall performance of the gear. No saturation was detected, with the escapement ratio uninfluenced by the amount retained. Considering the retention characteristics, results indicate that the 16 mm and 20 mm mesh codends are not selective, whereas the 24/14 and 28/14 configurations produce increased escaping ability. The 45 mm mesh sieves out 79% of specimens, indicating that no masking effect was induced by the cover. Regarding economic efficiency, the 20/14 configuration provides values comparable to those obtained with the 24/14, whereas the 16/14 configuration is as efficient as the 28/14. The lowest efficiency was obtained using the 45/14 configuration. Multidimensional scaling and clustering techniques (performed on 20, 24 and 28 mm codends) allow to discriminate the escapement ability of the 28/14 gear configuration only. Present results support the introduction of the 28 mm mesh codend in the deep-water red shrimp fisheries in the view of a precautionary management of the resources.