Bridle efficiency of a survey trawl for flatfish: measuring the length of the bridles in contact with the bottom

A bottom trawl catch of flatfish is composed of fish that were initially in the path of the net and fish that were initially in the path of the bridles and were subsequently herded into the net path. Bridle efficiency (i.e. the proportion of fish in the bridle path herded into the net path) can be estimated by fitting a model to catch and trawl measurement data collected in a field experiment in which the bridle path is varied in width by varying bridle length. Since flatfish are herded by the direct contact or close approach of the bridles with the bottom, part of the necessary field work is estimating the length of the bridles in contact with the bottom. Previously, bottom contact of the bridles has been determined using video cameras, but this is not possible when the bridles are entirely obscured by the mud clouds generated by the trawl doors. As an alternative approach, a bridle-mounted bottom contact sensor (BCS) was developed to allow measurement of the off-bottom distance of a bridle and the percentage of time it is in contact with the bottom. For the survey trawl studied, approximately 43% of the lower bridle was sufficiently close to the bottom to presumably elicit a herding response; however, the middle of the three bridles sagged to such an extent that it touched the bottom ahead of the lower bridle and extended the length to 51%.