Caught on camera: quantifying fish behaviour in relation to sorting grids

Abstract This study analyses the behaviour of cod ( Gadus morhua ), haddock ( Melanogrammus aeglefinus ), saithe ( Pollachius virens ) and redfish ( Sebastes spp.) in relation to size-sorting grids from video recordings obtained during trawling in the Barents Sea. It quantifies how fish contact the grid, how frequently they do so, their escape success rate and whether they squeeze themselves through the grid bar spacings during the escape process. Fish primarily escaped by passing through the grid bars head-first or by contacting the grid with their dorsal side first. However, combined, these modes only occurred in 30%, 36%, 23% and 33% of all grid contact events for cod, haddock, saithe and redfish, respectively. Overall, approximately 35% of grid contact events of all sizes of cod, haddock and redfish and 14% of grid contact events of saithe would not lead to a length-dependent chance of passing through the grid. Haddock had the highest grid contact frequency compared to the other three species. On average, cod had to make at least three attempts to escape through the grid, while haddock, saithe and redfish needed approximately two attempts to escape. Squeezing was a significant factor in escape for all four species, as 42% of cod, 6% of haddock, 53% of saithe and 17% of redfish were estimated to do so. The quantified behaviours explain why the size selectivity of fish-sorting grids has been found to be sub-optimal.