Trends and composition of trawl bycatch and its implications on tropical fishing grounds off Goa, India

Trawl bycatch is a globally recognized issue with intensified effects in tropical waters affecting both the ecosystem function and biodiversity, as well as causing physical damage and habitat loss. The present study envisages temporal variations of bycatch with regard to the species and their biological interactions based on data obtained from commercial single-day bottom trawlers operating off Goa, west coast of India. The data revealed that bycatch constituted about 68% of the trawl catch, the remaining being target species (shrimp, flat fishes, sciaenids, squids and crabs). Approximately 89% of the species discarded into the sea comprised of juveniles of target and trash species, suggesting a major share of non-target species, leading to species loss. Out of 196 taxa observed in the trawl catch, 174 constituted bycatch with a significantly high percentage of biomass. Abundance of discarded bycatch species (crustaceans, echinoderms, teleosts) displayed distinct peaks during pre-monsoon whereas molluscs, crustaceans and teleosts dominated during post-monsoon. A conspicuous increase in abundance of molluscs during post-monsoon (October, 2012) and echinoderms during pre-monsoon (April, 2011 and April, 2013) is largely attributed to the recruitment process. Cluster analysis identified different clusters during pre-monsoon and post-monsoon season corresponding to their recruitment patterns and diverse species assemblages. Principal Component Analysis performed using three environmental parameters accounted for 84% (five components) and 86% (four components) of variance during pre-monsoon and post-monsoon, respectively. Regression analysis indicated a significant linear relationship between total catch and bycatch (R2=0.89), and between bycatch and discarded catch (R2=0.94).