Laboratory studies on fish vision and behaviour in relation to the principles underlying mesh penetration in trawled fishing gears
Fish behaviour, mediated primarily through the visual system, is known to be fundamental to the functioning of trawled fishing gears. Experiments were designed to examine the behavioural reactions of fish to visual stimuli associated with panels of meshes and their constituent components in order to increase our understanding of the principles underlying mesh penetration. Behavioural reactions of mackerel and haddock were observed in a large arena to a constricting funnel of two frames of meshes. Fish were seen to avoid the meshes where possible rather than pass through them. This reluctance to approach and penetrate meshes could be reduced by inducing the fish to race to a conditioned feeding stimulus, thereby reducing reaction distance and time. A greater avoidance reaction was seen to meshes which presented a stronger visual stimulus due to greater contrast with the water background. The orientation of twine also affected reaction behaviour. Fish avoided horizontally orientated parallel twines more than vertically oriented twines, while twine sloping diagonally upwards (with respect to the direction of travel of the fish) elicited a greater avoidance reaction than those sloping down. There was little difference between behavioural reactions to netting panels of square or diamond meshes. The behavioural reactions of mackerel to a tunnel of netting constricting from above and below as well as the sides, were examined. The presence of top and bottom panels increased the number of fish passing through the meshes on the sides. However, the orientation of twines on the top and bottom panels had little effect on the behaviour of the fish. The avoidance behaviour shown to meshes could also be reduced by the introduction of a strong visual stimulus or a physical block at the narrow end of a constricting funnel of two netting frames.