Behaviour analysis of undersized fish escaping through square meshes and separating grids in simulated trawling experiment

摘要

Understanding the fish escape behaviour through bycatch reduction devices is the cornerstone of their application as technical conservation measures. To evaluate square meshes and sorting grids as successful size-selective bycatch reduction devices in finfish trawls, we conducted a simulated trawling experiment to assess the effects of illumination, towing speed and mesh or grid orientation on the escape behaviour of undersized fish through square meshes and rigid grids. Juvenile masu salmon (Oncorhynchus masou) with average body length 13 cm were used as the experimental fish. A speed-controllable motor was used to tow the framed net in a circular water tank. An infrared CCD (charge-coupled devices) camera fixed to the towed net by means of a rotary connector was used to observe fish behaviour. In contrast to the active escape seen under lighted conditions, there was no active voluntary escape under dark conditions. As a result, no fish escaped through parallel and backward-sloping meshes, and only 13% of fish escaped through the forward-sloping meshes when the towing speed was increased. On the other hand, the encounter probability and passive escape through the longer openings in the backward-sloping grid increased under the dark condition, resulting in the highest escape ratio. This increased from 67 to 87% on increasing the towing speed from 1 to 1.5 knots, and shortened the swimming time before escaping from 5 to 1.5 min (the highest escape ratio and the shortest swimming time before escaping under the dark condition). Frame-by-frame analysis of some escape events demonstrated easy, passive fish escape through the grids with no change in the normal swimming direction. The results support the backward-sloping grid as an efficient bycatch-reduction device, especially in demersal trawling when placed well ahead of the codend, working in the dark at high towing speeds.