The annual ovarian cycle and the influence of pollution on vitellogenesis in the flounder, Pleuronectes flesus

A study of the annual ovarian cycle of the flounder, Pleuronectes flesus, was made, describing the different stages of germ cell maturation and the different phases of ovarian development. The development of oocytes in the flounder is a group synchronous process with two distinct generations of primary growth phase oocytes. One group becomes vitellogenic at the end of the summer while the other group does not surpass the primary growth phase. Oogonia, as well as seven stages of oocyte development and post‐ovulatory follicles were distinguished. The occurrence of these oocyte stages was used to define the different phases of ovarian development. Previtellogenesis and vitellogenesis were each divided into two subphases: the perinucleolus and cortical alveoli phase, and early and advanced vitellogenesis, respectively. These phases were followed by final maturation, ovulation, spawning and a post‐spawning phase. The GSI increased rapidly during vitellogenesis, to a maximum value of about 35 just before spawning, at the end of the winter, when the animals had migrated to the North Sea spawning grounds. After spawning the GSI dropped to 1.5. Fish that had lived in a polluted environment exhibited premature vitellogenesis, resulting in a high number of oocytes in yolk granule stage. This study on the effects of pollution indicates the importance of the description of the normal annual cycle of the ovary as a control in toxicological studies.