Effects of the Net Contrast of a Model Codend on the Escapement of Juvenile Red Seabream Pagrus major

Abstract

Visual stimuli of nets, which affect fishing selectivity, vary by twine diameter, color, and material under different light conditions and visual geometries. In this study, two cylindrical model codends of two mesh sizes, 28 and 43 mm, were made of high-contrast, dark brown polyethylene (PE) netting twine and low contrast, light-blue polyamid (PA) monofilament twine. Each model codend was filled with juvenile seabream and set in the water channel of a light-blue circular tank under a flow speed 0.8 m/s for 30 min. Light conditions were set to relatively bright, dim, and dark. The resulting retention rates of juvenile seabream were 15-35% lower for the lowcontrast codend with PA monofilament than for the high contrast PE twine netting under bright and dim light conditions, while no difference was observed under dark conditions. The effects of mesh size and netting contrast on the retention rate were dependent on the light level, while the retention rate due to netting contrast was independent of mesh size. Therefore, low-contrast nets in the codend could help to reduce juvenile bycatch by disturbing the orderly optomotor response.

Escape response of juvenile fish in a shaking codend determined from tank experiments

“…Increased erratic responses and reduced optomotor responses caused by the shaking codend in addition to ASDs may encourage fish to approach the netting, facilitating their passage through the less-visible side netting in the codend (Kim and Whang, 2011). Furthermore, shaking codend could limit fish movements by reducing their swimming space over a shorter shaking period.…”