The affect of speed through the water on footrope contact of a survey trawl

The affect of speed through the water on the bottom contact of a trawl footrope was experimentally investigated for the poly Nor’eastern trawl, the bottom trawl used by the Alaska fisheries science center (AFSC) to conduct stock assessment surveys. Repetitive tows were made at speeds over the ground from 2.0 to 5.0 knots at 0.5 knot intervals, bracketing the 3.0 knot speed standardly used during the surveys. For each tow, speed through water was measured by placing on the trawl a self-recording oceanographic current meter and a commercially available speed through water sensor designed for use on trawls. Height of the center bobbins above the bottom was measured using a bottom contact sensor developed at the AFSC. At speed through water less than 2.5 knots, the center bobbins were resting on the bottom and, based on video observations, were actively rolling. At 3.0 knots, the bobbins were off the bottom by an average of 1.8 cm and were often not rolling but were sufficiently close to the bottom to leave a mud cloud. At 4.0 knots, the bobbins were off the bottom by an average of 6.7 cm and rarely left a mud cloud. At 5.0 knots, the bobbins were off the bottom by an average of 14.8 cm. However, at a speed through water greater than 4.5 knots the bottom contact sensor itself was observed to lift off the bottom and the predicted distance is likely substantially underestimated. The results of the study indicate that footrope contact, and therefore trawl efficiency, is not independent of trawl speed. This implies that reductions in the variance of survey CPUE might be achievable if towing speed was standardized to speed through water rather than speed over ground.