Ovarian Development and Oocyte Diameter as Maturation Criteria in Pacific Herring (Clupea harengus pallasi)

Seasonal changes in ovaries of Pacific herring (Clupea harengus pallasi) are described. Vitellogenic and resting oocytes could be distinguished histologically by size during early development. Resting oocytes had a mean diameter of about 80 μm and a maximum of about 150 μm. Oocytes larger than 150 μm were vitellogenic and indicate early sexual maturation. An alternative non-histological method of determining oocyte diameter, involving physical separation of oocytes, yielded good maturity estimates, rapidly and simply.