Size selectivity of haddock and cod at fishing depth and the sea surface in the Norwegian demersal seine fishery

Square-mesh codends are mandatory north of 62° N in the demersal seine fisheries targeting gadoids in Norwegian waters. Surface escape of fish is common and associated with elevated escape mortality, largely driven by barotrauma. Selectivity in the different phases of the fishing process is however poorly quantified. We quantified size selection for haddock (Melanogrammus aeglefinus) and cod (Gadus morhua) at fishing depth and during the haul-back/surface stage. Three different gear configurations were tested: i) a small-mesh codend collecting all fish entering the seine (control); ii) a conventional 125 mm square-mesh codend; and iii) a conventional 125 mm square-mesh codend with a subsequent small-mesh diamond-mesh extension separated by an automatic codend releaser. Across 12 series of three consecutive tows each, 47,348 haddock and 7,563 cod were captured and length measured. Haddock escaped mainly at depth and showed a higher L50 at depth than at the surface (40.4 vs 36.8 cm). In contrast, cod showed much higher escape rates during the surface phase, where L50 was substantially higher than at depth (50.3 vs 40.9 cm). The codend releaser provided a practical method for partitioning selection into seabed and surface components. These results demonstrate clear species-specific differences in the timing of escape and suggest that gear designs promoting earlier escape at depth may reduce surface escape and associated mortality in demersal seine fisheries.