Can active behaviour stimulators improve fish separation from Nephrops (Nephrops norvegicus) in a horizontally divided trawl codend?

A promising design to improve selectivity in the Nephrops-directed trawl fishery is the horizontally divided trawl codend. Previous studies have succeeded in separating the majority of fish from Nephrops; however, cod (Gadus morhua), juvenile roundfish and flatfish still enter the lower compartment in relative high proportions. In this study we investigated if and to which extent it is possible to improve the vertical separation of fish from Nephrops by adding active behaviour stimulators. These stimulators are designed to exploit fish avoidance behaviour and lead them into the upper compartment while Nephrops move into the lower compartment. We tested two types of behaviour stimulators: a chain curtain at the entrance of the lower compartment at the point of separation and a set of rising float-lines inserted ahead of the point of separation. The length-dependent vertical separation of five important commercial fish species and Nephrops was analysed in comparison to the horizontally divided trawl codend with no stimulator, towed in parallel to the test trawl. The results showed that fish's vertical separation can be partially improved by the addition of stimulators, without complicating fishing operations or increasing the proportion of Nephrops that enters the upper compartment. However, the improvement was limited and none of the two active stimulators tested managed to simultaneously improve the separation of cod, juvenile roundfish and flatfish.