Performance of Separator Trawl off Cochin, Southwest Coast of India

Abstract

Shrimp trawling generates large quantities of bycatch due to the non-selective nature of the gear. Behavioural differences and vertical distribution of the targeted species are among the main factors considered for designing trawls for selective trawling. This study attempts vertical separation of species in a trawl by inserting a horizontal panel, leading the upper and lower compartments to two separate codends. A total of 27 hauls were carried out and the species assemblage in the upper and lower codends were studied. Multivariate analysis was carried out to find the differences in the species assemblage structure between the two codends. Comparative analysis of the catches in the two codends has shown that the catch rate was about 25 times higher in the lower codend than in the upper codend. The catch rate of Parapenaeopsis stylifera and Metapenaeus dobsoni alone formed around 77% of the total CPUE in the lower codend. The contribution of shrimps in the upper codend was only about 15% of the total CPUE, which confirmed the importance of vertical height of the trawl mouth in the selective capture of fishes and for reducing bycatch, which comprised mostly of juveniles of commercially important fishes, in targeted shrimp trawling. Although the diversity indices did not show any significant difference between the codends, the difference in the species were very significant in the ANOSIM test (p<0.01) and the major species that were responsible for the difference in the species assemblages were found to be P. stylifera and Sardinella longiceps. As the catch of jellyfish were significantly lower in the lower codend, their catch in shrimp trawls can be considerably reduced by using nets with low vertical height.