The effect of grid spacing on size selectivity of shrimps in a pink shrimp trawl with a dual-grid size-sorting system

A Nordmore-style grid is required in many pink shrimp (Pandalus borealis) trawl fisheries in Europe and North America. However, the Nordmore Grid does not reduce the catch of small shrimps. To reduce catch of small shrimps, a size-sorting grid with 11 mm grid spacing was developed and has been used by some trawlers. However, we do not know if the 11 mm spacing is the optimal spacing for the fishery. We thus compared two sizes of grid spacing: 9 and 11 mm. A trouser trawl system was developed and used to compare a codend with a 9 mm spacing size-sorting grid and another identical codend with an 11 mm spacing size-sorting grid. No significant differences were observed in the catch rate of shrimp, or major bycatch species between the two grids (p=0.35). The 11 mm grid caught slightly less shrimps between 18 and 22 mm carapace lengths that correspond to carapace widths of 8.5-10.5 mm. This indicates that shrimps with 8.5-10.5 mm carapace width might have escaped through the 11 mm grid, but not the 9 mm grid. These findings provide a scientific base and practical guidance for choosing grid spacing based on the size of shrimp to be released. (C) 2012 Elsevier B.V. All rights reserved.