Moderate turbidity enhances schooling behaviour in fish larvae in coastal waters

We evaluated the effects of turbidity on school formation in ayu (Plecoglossus altivelis) [24.5 ± 2.2 mm standard length (Ls)], Japanese anchovy (Engraulis japonicus) (29.1 ± 3.1 mm Ls) larvae, which often live in turbid coastal waters, and yellowtail (Seriola quinqueradiata) juveniles (37.1 ± 2.5 mm Ls), which live in clear offshore waters. Fish were introduced into experimental tanks at one of five turbidity levels obtained by dissolving 0, 5, 20, 50, or 300 mg l−1 of kaolin in seawater. Their behaviour was video recorded, and the nearest neighbour distance (DNN) and separation angle (AS) were compared among turbidity levels. Mean DNN of ayu was significantly smaller at 20 and 50 mg l−1 than any other level of turbidity, as was AS at 20 mg l−1 compared with 0 mg l−1. Mean AS of anchovy was smaller at 50 mg l−1 of turbidity than any others. In contrast, mean DNN of yellowtail was larger at 300 mg l−1 than any others. These results suggest that moderate turbidities enhance schooling behaviour in ayu and Japanese anchovy larvae, whereas turbidity has an inhibitive effect on schooling of yellowtail juveniles, corresponding well to the habitat characteristics of each species.