STUDIES ON THE USE OF VISUAL-STIMULI TO CONTROL FISH ESCAPE FROM CODENDS .2. THE EFFECT OF A BLACK TUNNEL ON THE REACTION BEHAVIOR OF FISH IN OTTER TRAWL CODENDS

Observations were made at sea to quantify penetration of meshes by fish caused by addition of a black tunnel behind three different open mesh netting configurations in the extension region of an otter trawl. A Marine Laboratory 'North Sea' 600 HP four-panel trawl was used throughout the study, A remotely controlled television system was positioned alongside the extension area of the net and observations of fish behaviour (mainly haddock and whiting) in natural lighting conditions were recorded on videotape for later analysis, The proportion of those fish entering the extension which escaped through the meshes was determined, With the black tunnel in place the behaviour of the fish was modified and despite their exhausted state fish swimming towards the codend were seen to turn and swim ahead of the tunnel. Fish approaching the tunnel tail first were seen to speed up and attempt to hold station ahead of it. In both cases fish appeared reluctant to enter the tunnel and many were observed attempting to pass through the open meshes ahead of the leading edge of the tunnel, The proportion of fish escaping from the three net configurations without a tunnel was low but increased with addition of the black tunnel. With a square mesh extension the effect of the tunnel was to raise the proportion escaping from around 18% to 77%, With a square mesh window, numbers escaping rose from 20% to 60% and, even with diamond meshes ahead of the extension, the proportion escaping rose from 5% to 40%, The results are discussed in relation to the selective efficiency of towed fishing gears.