Understanding sorting grid and codend size selectivity of Greenland halibut (Reinhardtius hippoglossoides)

• Using the fish morphology-based FISHSELECT methodology we studied the size selectivity of Greenland halibut.
• Based on FISHSELECT we could explain results for size selection of Greenland halibut obtained during experimental fishing.
• For codend size selection the FISHSELECT results were well in line with results from experimental fishing.
• For grids results indicate a large fraction of Greenland halibut do not contact the grids with optimal angle for escapement.

We studied the size selectivity of Greenland halibut (Reinhardtius hippoglossoides) using the fish morphology-based FISHSELECT methodology, size selectivity data from two sets of sea trials carried out in the Barents Sea and the Norwegian Sea, and historical selectivity data collected for this species from 1981 onwards. When compared, the historical codend size selectivity data fitted well with the selectivity predictions from the FISHSELECT analyses. The historical grid selectivity data and the results from the two sea trials showed considerably lower L50 values than what would be expected from the morphological-based limit estimated by FISHSELECT. The size selectivity results obtained from the analysis of the two sea trials differed significantly, even though they were conducted using grid sections with similar bar spacing that previously were shown to have similar selective properties for other species. The differences were not caused by differences in the ability of the fish to contact the grid. Instead, these differences can be explained by differences in the ability of the fish to turn before they attack the grid. In earlier grid selectivity studies, the influence of angle of attack (θ) was not quantified. We show that the ability to contact the grid with a more or less optimal θ differs between individuals. This is important to consider in grid selectivity studies for flatfish species such as Greenland halibut because it can potentially influence results considerably and therefore can be a source of variability in results between cruises.