Selection efficiency of encircling grids in a herring pontoon trap

Published source details Lundin M., Ovegård M., Calamnius L., Hillström L. & Lunneryd S. (2011) Selection efficiency of encircling grids in a herring pontoon trap. Fisheries Research, 111, 127-130.
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This study is summarised as evidence for the following.

	Action	Category
	Modify fishing trap/pot configuration Action Link

Modify fishing trap/pot configuration

A replicated study in 2010 in nine inshore areas in the Bothnian Sea, Sweden (Lundin et al. 2011) reported that herring Clupea harengus pontoon traps modified with two rigid size-sorting grids allowed the escape of high proportions of undersized herring. Data were not statistically tested. Across all trials, between 68–565 kg of the total weight of herring entering the traps (400–1,200 kg) was estimated to have escaped through the grids. By number, this was a reduction in catch of 17–76% (2,420–17,008 fish). The proportion of undersized herring removed from the catches (selection efficiency) was 54–72% across all trials. In addition, higher proportions of herring escaped through grids with 15 mm bar spacing (59–76%) than 14 mm grid bar spacing (17–25%). Data were collected from six deployments (17–120 h soak times) of a herring pontoon trap in May/July 2010. The trap was a single-walled cylindrical fish chamber (6 × 3 m) with small (24 mm) mesh to retain all sizes of herring. At each end of the chamber, a grid consisting of a 300 mm wide ring with 2 mm diameter stainless steel rods fitted vertically inside was fitted. The rods were placed at either 14 mm or 15 mm bar spacing (three deployments of each bar spacing). Underwater cameras monitored numbers of herring escaping through the selection grids. Full trap specifications are given in the original paper.

(Summarised by: Chris Barrett)