Seasonal levels of serum vitellogenin and oocyte growth in the channel catfish Ictalurus punctatus

Seasonal levels of serum vitellogenin (VG) were measured in adult and juvenile (10–30 months old) female channel catfish using an enzyme-linked immunosorbent assay (ELISA). In adults sampled during a 2-year period, the highest mean (±s.e.) VG level was 30.21 ± 5.36 mg/ml in May before spawning and declined to 3.79 ± 0.89 mg/ml in June after the fish had spawned. Oocyte diameter increased from September to November, remained the same until February, then increased again until maximum mean diameter of 2356 ± 49 μm was measured in May. Vitellogenin was detected in the serum of juvenile fish that were 12 months old, but yolk granules were not visible in hematoxylin and eosin stained sections of oocytes until the fish were 18 months old. Mean serum VG levels of juvenile fish were lower than the adults, and no VG peak was observed in the juvenile fish. Vitellogenin was present in the serum throughout the year in adult fish and in juveniles older than 12 months.