Separation of fish from Nephrops norvegicus into a two-tier cod-end using a selection grid

The legally sized fish by-catch is an important economic component of many Nephrops fisheries. However, the small mesh size typically used by Nephrops trawlers results in many undersized fish being discarded. A selection grid was inserted in a two-tier cod-end in an attempt to separate the target species, Nephrops, from the fish by-catch and to retain the by-catch in the upper cod-end. Good separation would allow fish size selection to be improved by using a larger mesh size in the upper cod-end. The separation properties of grids with two different bar spacings and two lower gaps were investigated. Nephrops, haddock and dab were caught in reasonable numbers. Most small Nephrops passed into the lower cod-end, with the proportion passing into the upper cod-end increasing to between ∼40 and ∼70% at carapace lengths of 50 mm. The grids with the larger bar spacing allowed more Nephrops into the lower cod-end, but there was no significant difference in separation between grids for haddock or dab. The proportion of haddock passing into the upper cod-end increased from ∼60% for 10 cm haddock to ∼85% for haddock of 20 cm or more. The proportion of dab passing into the upper cod-end increased from ∼30% for 10 cm dab to ∼75% for dab of 20 cm or more.