The effective selectivity of a composite gear for industrial fishing: a sorting grid in combination with a square mesh window

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A new gear consisting of a sorting grid in combination with a square mesh window was developed to remedy the bycatch of juvenile gadoids in the industrial fishery for Norway pout ( Trisopterus esmarki ). The purpose of square mesh window was to retain larger marketable fish otherwise sorted out by the grid. Experimental fishing with the composite gear showed improved selectivity of a commercial trawl with catch weight reductions of haddock ( Melanogrammus aeglefinus ) and whiting ( Merlangius merlangus ) of 37 and 57%. Application of these reduction percents to the historical level of industrial bycatch in the North Sea lowered on average the yearly haddock bycatch from 4.3 to 2.7% of the equivalent spawning stock biomass. For whiting the theoretical reduction was from 4.8 to 2.1%. Underlying these results is the deployment of a novel methodology to describe the effective selectivity of composite gears, where the usual descriptors of trawl selectivity, l 50% and SR, do not apply. In order to develop this methodology, fish were collected from three compartments of the gear during the experimental fishing. This experimental set up allowed us first to determine separate selectivity estimates of both the grid and the window and secondly to combine these into estimates of the effective selectivity of the composite gear.