Reducing variability in bottom contact and net width of a survey trawl by restraining door movement and applying a constant ratio of warp length to depth


Weinberg, Kenneth L., and Stan Kotwicki
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A field study was conducted to examine methods to reduce varying geometry of a demersal survey trawl net caused by changing depth and trawling speed and that could result in variable sampling efficiency. A reduction in varying trawl net geometry is important because variance in indices of abundance is the result of variability in sampling efficiency, as well as animal density. Trawl performance measures considered were door and wing spread and the contact of the footrope and lower bridles with the seabed. Three treatments were tested for their effects on these measures: 1) standard towing procedures, 2) door spread restrained by a restrictor line attached between the trawl warps ahead of the doors, and 3) doors similarly restricted in conjunction with a modified scope ratio. Generalized linear modeling showed that both depth and trawl speed significantly affected trawl measures in nearly all cases. The restrictor line reduced the effect of depth on spread and, to a lesser extent, on bottom contact of the footrope; however, it was ineffective at reducing the effect of trawl speed over the speed range observed. The combination of a restrictor line and modification of the scope ratio to achieve a consistent upward pull on the doors was most effective in maintaining trawl shape to our target dimensions.