Understanding the size selectivity in diamond mesh codends based on flume tank experiments and fish morphology: effect of catch size and fish escape behaviour

This study quantifies potential size selection of a fish and a crustacean species in di-amond mesh codends during a fishing process. Changes in mesh geometry along the codends and at different catch weights were recorded in a flume tank and subse-quently used together with the morphology of cod (Gadus morhua) and Nephrops (N. norvegicus) to simulate potential size selection.

By assuming certain patterns of fish escape behaviour in the codend, it was demonstrated that it was possible to replicate results for size selection based on sea trials with similar codends. Results show that L50 can increase significantly with increasing catch weight at the aft end of the codend where most of the selection is known to occur.

The results document the var-iation in potential size selection along a codend during a catch-build-up. It is empha-sized that experimental studies aiming at describing the selectivity in different types of codends in a commercial situation, need to be based on catch levels representative for the commercial levels to prevent underestimation of the selectivity in the system, especially for designs to be included in the legislation.

It was additionally noted that escapement happens mostly in the area of catch accu-mulation. In response to questions, it was also noted that FISHSELECT methodology has been used. Mesh penetrations were simulated for each individual. Considering the relationship between catch weight and codend selectivity, studies with lower catches can underestimate size selection in commercial conditions