Development of a codend concept to improve size selectivity of Nephrops (Nephrops norvegicus) in a multi-species fishery

Species-specific differences in morphology and minimum landing size will often result in high discard rates in mixed-species fisheries. This is the case in the Danish Nephrops-directed fishery and the aim of this study was to improve the size selectivity of Nephrops without increasing catch below minimum landing size (MLS) of cod (Gadus morhua) and plaice (Pleuronectes platessa). A new codend concept combining square meshes and diamond meshes in a four-panel configuration was developed and tested in the Kattegat and Skagerrak. A 5m long 70mm square-mesh panel was inserted in the lower panel of the codend to increase size selectivity of Nephrops while the remaining three panels were made of standard 90mm diamond-mesh netting to maintain selectivity of plaice and cod. The concept succeeded in significantly (p<0.0001) increasing L₅₀ of Nephrops without increasing discards of either plaice or cod. When using the new codend concept, expected numbers of Nephrops below MLS (carapace length=40mm) were reduced by approximately 37% but the expected weight of marketable catch was also reduced by 21%.The concept was adjusted and tested in a second experiment so that fishing using the new net configuration would remain commercially viable. This experiment indicated that size selectivity of Nephrops could be adjusted by changing the square-mesh panel but differences in mean selection between the gears were insignificant. We introduce a new analytical approach with increased statistical power to detect differences in selection parameters between different codends.