The size selectivity of four codends were compared in the Gulf of St. Lawrence, Canada redfish fishery, including the regulated diamond mesh codend with a mesh opening of 90 mm (T0) and three experimental codends of different mesh openings (90, 100, 110 mm) in which the netting is turned 90° to the direction of tow (T90). Results for the regulated codend showed that there was little size selection, catching greater than 97% of redfish over all of the length classes observed. Considering the fished population, the smallest T90 codend would catch 30% fewer redfish under the Minimum Landing Size (MLS) of 22 cm compared with the T0 codend, but would also lose 16% of catch above 22 cm. The T90 codend with 100 mm mesh opening had the same size selectivity as the smallest T90 codend. The 110 mm T90 codend would catch 50% less redfish below MLS but lose 40% of redfish above. Overall, results show that T90 codends improve size selectivity in which large proportions of undersized fish are successfully released.
To reduce the retention of undersized fish in the redfish (Sebastes spp.) trawl fishery in the Gulf of St. Lawrence, Canada, we developed a full-scale shaking codend. The shaking codend uses a mechanical stimulating device, an elliptical-shaped piece of polyvinyl chloride canvas, attached to the posterior of a T90 codend that generates a lifting force with respect to drag, causing a ‘shaking motion’. A shaking codend could stimulate fish movement and increase contact probability, both of which could increase the escape of small redfish out of a codend, especially when combined with a codend that maintains mesh openings. The movement and fishing characteristics of a shaking codend (T90 codend with canvas) relative to a T90 codend (without canvas) were tested in a flume tank and field experiment. In the flume tank test, the shaking codend had a peak-to-peak amplitude (i.e. the distance the codend moves from the lowest to highest depth) > 24 cm higher than the T90 codend for each velocity tested (1.0–1.8 kt), higher amplitude ratio, and a higher period (1 revolution) that gradually decreased with increasing velocity. The total acceleration (m s-2) and drag forces (kgf) estimated for the shaking codend were significantly higher than the T90 codend across all flow velocities. The results from the field experiment, considered preliminary due to a small sample size, showed that the shaking codend significantly reduced the capture of small redfish (< 21 cm) and the best fit model did not need to consider contact probability which was necessary for the non-shaking T90 codend. Overall, the dynamics of the movement of the codend was described and could be potentially used as an effective technique to reduce the catch of small redfish, and perhaps in other trawl fisheries to reduce the catch of small fish.
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