Impact of bottom trawling on epifauna off Veraval coast of India

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 epifauna off Veraval coast. Trawling was conducted for 17 months in a span of 20 months (September 2005-April 2007) excluding the trawl ban period (June to August). Altogether 41 species of gastropods, 1 species of scaphopod, 19 species of bivalves, 3 species of crab, 3 species of shrimps, 2 species of Balanus, 1 species of stomatopod, 4 species of finfishes, 2 species of brown algae and 4 species of octocorals were identified. The soft corals found were Litophyton sp. and Studeriotes sp. (Christmas tree soft coral). The gorgonians collected were young stages of Subergorgia suberosa and Juncella juncea (Whip coral). The presence of octocorals recorded in the month of October, immediately after the closed season (June to August) when the sea bottom is not heavily trawled suggests that this area is an abode of corals and a favourable site for coral reef formation. But intense trawling in the succeeding months destroys these valuable entities of ecosystem and the samples were not encountered in the subsequent months. The changes before and after trawling in biodiversity indices were significant at 15-20 m. The abundance-biomass curve showed that the rate of stress increased with water depth. The shallow depths are lightly trawled due to intermittent rocky nature of bottom and as water depth increases, the trawling intensity increases. The analysis of similarity of percentages in Simper showed that the dissimilarity of fauna before and after experimental trawling was more evident in lightly trawled area and remained masked in heavily trawled area. Suggestions are made for the promotion of eco-friendly gears and for conducting studies on appropriate un-trawled control sites for comparative assessment. Management strategies have to be adopted for the conservation and biodiversity protection of octocorals.