Approaching single-species exclusion in mixed demersal trawl fisheries

Abstract

Under a discard ban, mixed fisheries must often reduce catches of low-quota species to allow the continuation of fishing activities. This has led to the development of a range of bycatch reduction devices (BRDs) that aim to exploit morphological and behavioral differences among species to facilitate escape of unwanted catch from the fishing gear. However, the exclusion of unwanted species from the catch is often only possible with concomitant losses of other commercial catches. This is the case for the Nephrops (Nephrops norvegicus)-directed mixed demersal trawl fishery, where BRDs aiming at the reduction of catches of cod (Gadus morhua) often lead to considerable losses of other valuable species. In this study, we developed and tested a BRD aimed at exclusively reducing cod catches without affecting catches of Nephrops, flatfish and other roundfish. The design, a bottom escape window, exploits behavioral traits that set cod apart from other species. We collected absolute selectivity data using a paired gears approach and estimated the combined retention of the bottom escape window and a 90 mm diamond mesh codend. The results demonstrated a low total retention of cod (33%) in combination with high retentions of commercial catches of Nephrops (89%), haddock (Melanogrammus aeglefinus) (76%) and plaice (Pleuronectes platessa) (100%), for the populations encountered. This catch profile represents an important and novel achievement for Nephrops-directed mixed demersal fisheries. We compared the performance of this new BRD to one of the most used legal gears in this fishery (the SELTRA 270), demonstrated the new catch profile it can offer to the fishers, and discussed its management implications.