Square mesh codends for selective trawling: An Indian perspective

Abstract

Bycatch, which includes non-target organisms that are caught during the capture process, is a major problem faced by fisheries, world over. Among the different strategies to reduce the incidence of bycatch, modifications made to the gear to reduce juveniles are very significant. The total bycatch in Indian fisheries was estimated at 58,000 tonnes, which formed about 2% of the total landings. Trawling, though contributes significantly to the marine production of India, about 56% of the total catch of trawlers is bycatch. This bycatch often comprises of juveniles of commercially important species targeted by trawls and other gears. A large number of bycatch reduction devices were designed and field tested in the Indian scenario, among which the square mesh codends were found to be very effective. ICAR-CIFT has successfully demonstrated the utility of square mesh codends in Gujarat, Maharashtra and Kerala and use of these codends are now mandatory in these states. Though selection parameters for different codends (shape, size etc.) are available, a meta-analysis on these results is lacking in India. Trawls, target many resources with varying lengths of capture and hence an optimal mesh size to be used will be a tradeoff between different factors like the length at first sexual maturity, the economic value of the targeted species group and the size and shape consideration of the codend itself. Deriving the best mesh size for the codends hence becomes complicated. This study attempts to optimize the codend mesh size/shape for trawl resources along the Indian coast. Results from 64 selectivity studies on codends carried out along the Indian waters were used. The different species for which selection parameters were available were grouped based on their shape, length-girth relation and the average economic value for the respective species was also considered. The comparative evaluation of L50 values showed that use of square mesh codends increased the L50 by 2-28%. Most of the fishes studied, belonged to the ‘slender’ category, for which square meshes are very effective. The only data available in the ‘flat’ category was for Cynoglossus sp. The study shows that rather than a single mesh size; codend mesh sizes should vary depending on seasons to sustainably and effectively capture trawl resources along the Indian coast. The method and the results of the study and its implications are discussed in the paper.