Influence of Codend Mesh Size on Bycatch Composition of Two Trawls Operated off Veraval, Gujarat, India

Abstract

The deleterious impact of bottom trawling on benthic ecosystem is widely reported and attempts are made to reduce bycatch generated by trawling through technical and operational measures. Systematic baseline data on the quantity and type of bycatch will help in comparing the changes that have occurred to the fish community structure. This study compares the bycatch generated by two trawling systems with 15, 20, 30 and 40 mm mesh size codends off Veraval during 1991-92 and 2005- 06. Analysis of quantity of bycatch generated and changes in species composition, analysis of diversity and multivariate analysis were carried out to elucidate the changes on the community structure along the Veraval coast. The percentage of total bycatch generated in trawl systems during 1991-92 were 25.69±4.28 and 31.03±1.58 with 15 and 30 mm codend meshes respectively while the values were 67.77±3.22 and 22.15±2.44 respectively for 20 and 40mm codends during 2005-06. Acetes spp and Otolithes ruber contributed the major share of bycatch in the 15 and 30 mm codends, whereas Trichiurus lepturus and Rhopilema spp were the major species caught during 2005-06 in the 20 and 40 mm codend mesh respectively. Highest diversity was observed in the 15 mm codend as indicated by both the indices of diversity ‘S and d’. ANOSIM test revealed that the difference in the assemblage structure during the years were significant with a global ‘R’ value of 0.176. The results of the SIMPER analysis showed that the average dissimilarity between the bycatch assemblages during the years 1991-92 and 2005-06 was noticed to be 87.23%. It was evident from the studies that there are changes in the quantity and assemblage structure of the bycatch generated by different trawling systems over the years. This study also shows the utility of increasing the mesh sizes as a technical measure to reduce impacts of trawling on biodiversity.