Reaction of juvenile flounder to grid separators

The reaction behaviour of juvenile Japanese flounder (Paralichthys olivaceus) to towed grids (0.5 × 0.2 m, horizontally, or vertically orientated bars at 10-mm intervals) was observed as a means of understanding fish behaviour in relation to grid selection for a beam trawl fishery in Tokyo Bay. Reaction behaviours were categorized within four patterns by grid types and illumination levels: (i) forward swimming in towed direction; (ii) swimming over the grid; (iii) sticking on the grid; and (iv) passing through the grid. The most dominant reaction pattern was forward swimming, but its ratio was higher for light than for dark conditions. Passing through the grid bars occurred most frequently with horizontal bars. Approximately 40% of tested fish passed through the grid in light conditions, approximately 30% in dark conditions. Most of these fish penetrated bar gaps head first, while a considerable proportion categorized as “forward swimming” kept swimming even though their tails or bodies had partly passed between the bars. It is concluded that penetration of flounders through bar gaps is governed by voluntary actions.