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 altive lis. in the estuarine and freshwater sections of the Shimanto River were co mpared among cohorts hatched in November, December and January Formation of fins and ossification of the centra, which were completed by about 35 mm B L, hardly differed among the three cohorts, In specimens over 35 mm BL, bod y depth and head length were smaller in the November cohort than in the oth er cohorts, with pigmentation in the earlier-hatched 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 juve niles and commencing upstream migration. In contrast, the January cohort tr ansformed to juveniles and started upstream migration at a smaller size (un der 40 mm BL). The difference in transformation size among the cohorts is p robably caused by ambient water temperatures existing at the beginning of u pstream migration of each.