In this study, we compared the size selectivity of a T90 codend (netting
turned by 90 degrees) with that of a standard codend made of similar netting. Sea trials
were conducted in a Norway lobster directed fishery in the Kattegat-Skagerrak area, where
there is a need for improved selectivity because of a severe discard problem. The codends
were tested by fishing simultaneously with them in a twin trawl rig. Codend covers mounted
with Minisamplers were used, which made it possible to catch individuals escaping during
towing and haul-back separately. Herein we proposed a model to assess the sequential
selection during towing and haul back. This model takes into account the parameter
Ctow, which can be interpreted as the
proportion of fish that comes into contact with the codend meshes during towing and,
thereby, has a chance of escape. Compared to the standard codend, the T90
codend retained fewer Norway lobster both below and above the legal minimum landing size
(40 mm, cephalothorax length), thereby causing a reduction of commercial catch. The
difference was mainly due to a significantly higher escape rate during towing for the
T90 codend. For plaice below minimum landing size (27 cm), the
retention was slightly but significantly higher for the T90 codend
compared to the standard codend. A model developed for both codends showed that not all
plaice are able to attempt escapement during the towing process. For cod, the results
indicated an increased L50 (the length at which 50% of this species is
caught) for the T90 codend, but the effect was not statistically
significant, probably due to the limited number of cod retained during the sea trials. The
results demonstrated that, for all three species, a significant proportion did escape
during haul-back in both codends.