Physiological and Histological Aspects of Late Oocyte Provisioning, Ovulation, and Fertilization in Pacific Herring (Clupea harengus pallasi)

Abstract

Late vitellogenic, ovulating, and ovulated Pacific herring oocytes were examined histologically and histochemically in order to understand these fine-scale processes and to evaluate egg maturation timing test criteria for use in resource management. Late vitellogenic events were investigated using an alkali-labile protein phosphorus (ALPP) assay of serum, liver, and gonad tissue. Serum levels were constant in males but declined in females. Ovarian levels declined (110 to 15 mg/L) during ovulation. Hepato-somatic indices were constant in males but declined in females (1.6 to 0.9%) during ovulation. The abundance of electron-lucent hepatocytes in female livers compared with males, their synthetic and secretory structures, and their reversion during ovulation suggest these cells as vitellogenin synthetic locations. During population ovulation, the gonosomatic index for females and males increased from 15–27%, and 15–23% respectively. Eggs hydrated from 67–76% during ovulation, and secondarily to 83% after fertilization. Unfertilized eggs dehydrated to 74% water. Alkaline phosphatase was localized in follicular granulosa and thecal cells suggesting these as locations of vitellogenin dephosphorylation. The test criteria; hepato-somatic index, lucent hepatocyte ratios, gonadosomatic index, and egg moisture are problematic but ovarian ALPP determination appears promising.