Size selection of large catches: using sorting grid in pelagic mackerel trawl

Published source details Kvalsvik K., Misund O.A., Engås A., Gamst K., Holst R., Galbraith D. & Vederhus H. (2002) Size selection of large catches: using sorting grid in pelagic mackerel trawl. Fisheries Research, 59, 129-148.
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This study is summarised as evidence for the following.

	Action	Category
	Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net Action Link

Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net

A replicated study in 1992–1999 of four pelagic areas in the North Sea off Norway (Kvalsvik et al. 2002) found that fish trawls fitted with rigid size-sorting escape grids (three designs) allowed a higher proportion of smaller mackerel Scomber scombrus to escape compared to codend catches. Grids of three different designs allowed 8–51% (average 20%) of mackerel of all sizes to escape relative to codend catches (grid: 134–44,000 kg, codend: 128–157,000 kg). The mackerel sorted out by grids had lower average weights (52 to 108 g less) than mackerel retained in the codend and the proportion of mackerel less than 400 g was reduced in all hauls by 4–14% by weight. Data were collected from 12 deployments during separate trials on three commercial and one research vessel from October–December in 1992, 1997, 1998 and 1999. Pelagic mackerel trawl nets were fitted with grids of varying size and construction and either 38 mm (eight tows), 40 mm (two tows) or 42 mm (two tows) bar spacing grid (see paper for specifications). Nets were deployed at 40–142 m depths for 45–140 min. Covers fitted over the grids collected fish escaping through them and a small-mesh inner net in the codend collected the fish that did not escape. Total catch weights in the covers and codend were recorded and subsamples of fish lengths measured.

(Summarised by: Leo Clarke)