Trawl geometry studies using two offbottom trawl systems (OBTS) along the coastal waters off Cochin

Abstract

Trawling is an important commercial fishing method along the Indian coast, contributing nearly 51 per cent of total marine fish landings of India. Catch efficiency of trawl is determined mainly by the mouth opening of the net, which in turn affects the catchability. Trawl geometry has been shown to vary with factors such as towing speed, operational depth, warp/depth ratio, trawl door type and direction of current. Field experiments were conducted on-board MFB Matsyakumari-II to assess the trawl mouth opening of off-bottom trawl systems using acoustic sensors with reference to towing speed, operational depth, warp/depth ratio and direction of current during trawling. A 24.7 m ultrahigh molecular weight polyethylene and a 27 m high density polyethylene off-bottom trawl using 85 kg suberkrub otter boards were used for trials along the coastal waters off-Cochin. Trawls were operated at different speed war per depth ratios and depths. Effect of direction of tow in relation to current was also investigated. For HDPE trawl, the maximum total area of opening was recorded at 6 colon 1. warp per depth ratio with the towing speed of 3.8 knots. Similarly, for UHMWPE trawl, the maximum opening was recorded at the towing speed of 3.8 knots at a warp/depth ratio of 6 colon 1. Towing speed and warp per depth ratio significantly affected the mouth opening of the trawl. The mouth opening of both the trawls was found to increase linearly with speed of tow.