Measurements of distance fished during the trawl retrieval period

Observations of sampling trawl performance made during a multi-vessel groundfish survey raised concerns that the trawls continue to fish during the retrieval period, after the end of the pre-specified sampling period but before coming off bottom. A trigonometric analysis was undertaken to estimate the distance traversed along the bottom, and the trawl speed along the seabed during the retrieval period. The distances traversed were substantial and systematically increased with the depth of the tow. The effective trawl speed approached or even exceeded the towing speed specified by the sampling protocols, and varied systematically among the participating vessels. Subsequently, this analysis was compared to trawl positions and times recorded during the same period by an ultra-short baseline (split-beam) acoustic positioning system and a bottom contact sensor. No marked difference was found between the two methodological approaches. Neglect of the observed effects increases the impact of depth-related bias and inter-vessel variability on survey results while knowledge of these effects may help explain the “vessel effect” commonly observed when comparing the fishing performance of two or more vessels.