Project MIPAF 6B4
The main objective of this project is to study the effect of inserting square mesh codend into the traditional commercial Italian bottom trawl. A comparative study was carried out to determine the selectivity and the efficiency of traditional diamond and experimental square mesh codends. Data on efficiency and selectivity of the two codends were collected during two fishing cruises, carried out between 30/08-07/09 and 22/09-06/10 2004 in the Central Adriatic Sea.
Mean selectivity curves were estimated for the two codends for which a number of individual hauls was fitted taking into account the between-haul variation of the selectivity parameters and based on pooled data. Reliable selectivity results were obtained on Merluccius merluccius (European hake), Mullus barbatus (red mullet), Parapenaeus longirostris (deep-water pink shrimp), Arnoglossus laterna (scaldfish), Illex coindettii (broadtail squid), Pagellus erythrinus (common pandora), Trachurus mediterraneus (Mediterranean horse mackerel), Trisopterus minutus capelanus (poor-cod) and Nephrops norvegicus (Norway lobster).
The 50% retention length (L50%) computed for the diamond mesh codend were always lower than those computed for the square mesh codend. Consequently also the selection factors (SF) were much lower than those obtained for square mesh codend. In other words, the diamond mesh codend always released lower numbers of individuals than the square mesh codend. The only exceptions are the selectivity results found for A. laterna, where both the L50% and SR increased in the experimental codend. This means that all the species, with the exception of flat fishes, make the best use of the square mesh opening, either because of their body shape or because of forcing the mesh to penetrate their body through. Regarding the selection ranges (SR), I. coindettii, M. merluccius and N. norvegicus showed a greater value in the square mesh codend, while M. barbatus, P. erythrinus, T. mediterraneus and T. m. capelanus showed larger values with the diamond mesh codend. In the case of P. longirostris, no differences were found in the SRs obtained.
Efficacy comparisons were done on the main commercial species. For six species the traditional codend showed a higher efficacy than the experimental codend both in terms of number of individuals and weight,. However, the differences were highly significant for A. media only. A significant difference was found in number of individuals caught but not in weight for L. vulgaris. The two codends showed the same efficacy on M. merluccius, nevertheless the square mesh codend resulted more efficient, both in number and in weight on A. laterna and T. lucerna. Worthy of mention is the low relative efficacy of the diamond mesh codend on N. norvegicus and P. longirostris particularly in weight. In the case of T. m. capelanus the catch coefficients between the two codends were higher in weight than in number of individuals. This is explained by the difference in the mean size of the specimens caught. The experimental codend proved to be generally more efficient on large sized individuals.
Regarding the Richness, the mean number of different species caught per haul by the experimental codend, it was lower than that computed for the traditional trawl.
In conclusion, comparative results confirmed a general idea that the diamond mesh codend efficacy on some Cephalopod species is high when compared with the square mesh codend, while all the other differences on main commercial fish species resulted not significant. Nevertheless, the diamond mesh codend showed a very low relative efficacy on some commercial Crustacean species.
