STUDIES ON THE USE OF VISUAL-STIMULI TO CONTROL FISH ESCAPE FROM CODENDS .1. LABORATORY STUDIES ON THE EFFECT OF A BLACK TUNNEL ON MESH PENETRATION

A school of Atlantic mackerel (Scomber scombrus) was conditioned to circulate a large swimming pool tank, passing through a constricting funnel of netting on each circuit. The funnel comprised two panels of netting (400 mm square mesh) narrowing to a central gap (0.6 m X 0.9 m). The proportions of fish passing through the meshes of the funnel were noted in three experimental conditions. When given a clear path through the funnel almost all the fish avoided the meshes and over 92% of the fish crowded through the gap. When faced with a fine mesh (35 mm) netting panel blocking the gap, all fish passed through the meshes of the funnel itself. When a black-walled open-ended tunnel was positioned beyond the opening of the gap all the fish passed through the meshes of the netting panels and none through the gap and black tunnel. These experiments confirm that fish keep clear of netting panels and are reluctant to pass through large meshes when a clearer passage is available to them. They also show that fish will pass through meshes when the alternative route is blocked or appears in some way (black tunnel effect or visual illusion) to be a less clear route. The results are discussed in relation to the role of fish behaviour in the selective efficiency of full-scale fishing gears.