A method to reduce survey bottom trawl variability

Abstract

A technique for limiting the variability of trawl doorspread is described. In addition, modifications of trawling technique which enable the doors to stay upright and maintain constant ground contact under different trawling conditions are presented. Observations were conducted using a standard Campelen 1800 otter trawl in 270 m of water. Doorspread was restricted by means of a restraining rope mounted between the warps, 150 m in front of the doors. When the restraining rope was in place, doorspread varied little and was not influenced by warp length; the doorspread was approximately 51 m for warp lengths of 600 to 800 m. With the restraining rope removed, doorspread increased from 54 to 63 m as warp length was increased over the same range. It was also demonstrated that doors would stay upright and maintain the same ground contact under varying conditions, if warp length was adjusted correctly according to depth. This was based on a requirement that warp tension and elevation angle be adjusted so that 2/3 of the door weight in water is counteracted by the upward pull of the warp at the end nearest the door.