A Preliminary Study on Trawl Geometry: Effect of Speed and Warp Length on Mouth Opening

Abstract

With a total of 35,228 trawlers of various sizes in operation, trawling is one of the most important fishing methods in India (CMFRI, 2012a;b). For efficient trawling operations, the geometry of trawl, particularly opening of trawl mouth (horizontal and vertical) is very crucial (Kunjipalu et al., 1992). There are different methods to keep the mouth of the net open during operation and the most popular is use of shearing boards (otter boards) on each side of warp to attain horizontal opening. Floats, sinkers and at times vertical sheer devices called kites are used to achieve vertical opening of the net (Boopendranath et al., 1986). Two-boat trawling or pair trawling, which involves towing a single trawl by two identical boats is also practised. Though beam trawls were common, their numbers have now reduced considerably (Depestele, 2007). In trawling, the probability of catch is directly proportional to the swept volume and thus to the mouth opening of the trawl is very critical. Total drag and net drag is directly proportional to towing speed (Queirolo et al., 2009). Weinberg (2002) studied the effect of towing speed on efficiency of a survey bottom trawl at three different speeds i.e. 2.5, 3.0 and 3.5 kn. Fujimori (2005) investigated effect of warp length and efficiency in terms of opening and reported improved result with scope ratio of >3. The effect of speed and warp length on the mouth opening of an experimental bottom trawl is reported in the study.