Ibaraki Prefectural Freshwater Fisheries Research Institute, Namegata, Ibaraki 311-3512, Japan
Seasonal and nocturnal changes in the vertical distribution of ayu, Plecoglossus altivelis altivelis larvae during seaward migration in the lower Kuji River were examined in 2006 and 2007. Sampling was performed at a site, approximately 3 m deep, using 0.3-mm mesh ring nets (45 cm in diameter). Ayu larvae were caught mainly from late October to early November in 2006 and from late October to early December in 2007. In both years, nocturnal vertical larvae distributions were different in late October and from early November onward; the larvae were evenly distributed throughout the water column in late October, but mainly in the middle and bottom layers from early November onward. Body and quantity of yolk sizes of larvae were not different between layers. No marked differences were observed in water velocity within the water column. Seasonal changes in water temperature showed a gradual decrease from 24°C to 6°C from September to December, while in late October and early November, it was 15°C-16°C in both years. These results show that ayu larvae do not always drift at the same depth and that their distribution in the water column changes seasonally as the water temperature decreases.