Effects of bottom trawling on the ecological integrity of macrobenthos off Veraval, Gujarat

Abstract

Experimental bottom trawling was conducted from MFV Sagarkripa at five transects of water depths 15-20 m, 21-25 m, 26-30 m, 31-35 m and 36-40 m in commercial trawling grounds to assess the impact of bottom trawling on the macrobenthic fauna off Veraval coast. Trawling was conducted for 12 months in a span of 15 months (September 2005 - November 2006) excluding the trawl ban period (June to August). The groups of fauna present were polychaetes, gastropods, bivalves, scaphopods, amphipods, isopods, copepods, cumaceans, ostracods, shrimps, crabs, squilla, balanids, foraminiferans, octocorals, sipunculids, nemerteans, pogonophores, pterobranchia, brittle stars and teleost fishes. A total of 81 species of polychaetes belonging to order Errantia (36 species) and Sedentaria (45 species) were identified. The molluscs were represented by 15 species of gastropods, 13 species of bivalves and one species of scaphopod. One octocoral genus was also identified. The abundance and biomass of the fauna were recorded. The numerical density of macrofauna increased after trawling, exposing them from their natural habitat. The numerical density of macrofauna increased after trawl ban showing that trawl ban is giving some respite to the fauna for rejuvenation. The biodiversity indices were found to be significantly different before and after experimental trawling at 15-20 m and 21-25 m. The ABC curve and w-statistic of the total macrofauna showed that the fauna of the area studied were moderately or grossly stressed. It was also possible to delineate the impact on the fauna by MDS plots. The present study confirms the deleterious effects of bottom trawlers on macrobenthos off Veraval. Cataloguing the biodiversity of macrobenthos, identification of indicator species and updating the new data generated will aid in taking up steps towards conservation of macrobenthos. Suggestions are made for the promotion of ecofriendly gears and for conducting studies on appropriate un-trawled control sites for comparative assessment.