Differences in larval and juvenile development among monthly cohorts of ayu, Plecoglossus altivelis, in the Shimanto River

The developmental processes of larval and juvenile ayu.Plecoglossus altivelis, in the estuarine and freshwater sections of the Shimanto River were compared among cohorts hatched in November, December and January. Formation of fins and ossification of the centra, which were completed by about 35 mm BL, hardly differed among the three cohorts. In specimens over 35 mm BL, body depth and head length were smaller in the November cohort than in the other cohorts, with pigmentation in the earlierhatched cohort being completed at a grater BL. The November cohort also remained at the “whitebait” stage until reaching a larger size (about 45 mm BL), before transforming to juveniles and commencing upstream migration. In contrast, the January cohort transformed to juveniles and started upstream migration at a smaller size (under 40 mm BL). The difference in transformation size among the cohorts is probably caused by ambient water temperatures existing at the beginning of upstream migration of each.