Effects of sea-state on the physical performance of a survey bottom trawl

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The effects of sea-state induced vessel motion on the physical performance of the Northeast Fisheries Science Center Yankee 36 survey bottom trawl, rigged with a roller footrope, were investigated. The trawl was instrumented with an acoustic net mensuration system to measure horizontal spread, a head rope mounted acoustic current meter to measure flow at the trawl-mouth, and five sled-mounted inclinometer sensors attached to the wings, quarters and center of the footrope to measure bottom-contact. A vessel motion sensor fixed at the center of the vessel measured roll and pitch. Trawl performance was observed over wave heights of 1-4. m. Vessel motion caused by waves at the sea-surface had a strong effect on trawl motion on the seabed. Vessel heading in relation to wave direction is an influential factor dictating the specific vessel motions occurring in a given sea-state. Of the measured components of vessel motion, pitch had the greatest affect on trawl performance. The speed of vertical stern motion, derived from pitch angles, rather than the magnitude of vertical stern displacement, was the most influential factor controlling variations observed in the footrope bottom-contact data and forward trawl velocity component measured by the current meter. Vertical stern motion speeds of 48. cm/s and greater caused the survey trawl to surge forward and exacerbated the loss of footrope bottom-contact. Since sea-surface conditions had a strong influence on net behavior, it may alter the catch efficiency of a survey trawl, we recommend fish behavior in response to trawl motion be investigated further. © 2011.

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Fisheries Research