The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod, haddock and whiting

Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak).

The model examines five trawl designs, and also the scenarios of a cessation of discarding in all North Sea fisheries and in just the Nephrops fishery. The model is deterministic, and evaluates the relative differences between scenarios assuming all other variables remain constant. If discarding of cod, haddock and whiting in the North Sea fisheries were eliminated, stocks would increase by 41%, 14% and 29%, respectively, within 10 years. Eliminating discarding in the Nephrops fishery alone would increase stocks by 2%, 1% and 13%, respectively, reflecting the relative proportion of catches of these species in the Nephrops fishery. For cod and haddock, the introduction of the Nephrops trawl with a grid with a square-mesh codend was the only scenario in which a notable increase in stock number was observed. This trawl design facilitates the escape of fish of all ages/sizes from the trawl, effectively making the Nephrops fishery a single-species fishery. For whiting, stock numbers and landings increased under all scenarios, but forecasted landings were lower than if current discard patterns continued in all except the no-discards scenario. The dependency of the results on the validity of the assumptions and on the accuracy of the input data is discussed.