From behind bars to freedom: understanding the size selection of fish sorting grids

Rigid sorting grids are used in demersal trawl fisheries globally. However, in some fisheries, such as the Barents Sea demersal trawl fishery, they have been found to release large quantities of target-sized catch while simultaneously retaining too many undersized fish. Several studies have documented this issue but an in-depth understanding regarding the exact reasons for these results is still lacking. Thus, the present study investigated whether so-called fall-through experiments conducted in the laboratory for cod ( Gadus morhua ), haddock ( Melanogrammus aeglefinus ) and redfish ( Sebastes spp.) could shed light on this matter. Predictive models based on these results, enabled a more detailed understanding of the size selection pattern from fishing trials for these species. The models revealed that the unintended release of target-sized cod and haddock occurs because a significant fraction of the fish entering the trawl squeezes themselves between the sorting grid bars to escape. For redfish, approximately 85 % of the contribution on the selectivity curve could be attributed to redfish squeezing themselves through the grid bar spacings to escape compared to when we considered that they do not squeeze (approximately 15 %). The unintended retention of undersized cod and haddock could be explained by the fact that many fish made contact with the grid with a suboptimal body orientation. The method applied enabled us to gain detailed insight into the interaction between fish and sorting grids and could potentially be applied to gain an understanding of the performance of other selectivity devices used in trawl fisheries. • Laboratory-based experiments reproduced sea trial selectivity curves, revealing drivers of grid sorting inefficiency. • The most complex grouping of behavioural grid contact processes was required to reproduce the selectivity curves. • Predictive models showed that large fish escape trawls by squeezing through sorting grid bars. • The retention of undersized cod and haddock was explained by that many fish contacted the grid sub-optimally.