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 Bounder, Pleuronectes flesu s, was made, describing the different stages of germ cell maturation a nd the different phases of ovarian development. The development of ooc ytes in the flounder is a group synchronous process with two distinct generations of primary growth phase oocytes. One group becomes vitello genic 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 d evelopment and post-ovulatory follicles were distinguished. The occurr ence of these oocyte stages was used to define the different phases of ovarian development. Previtellogenesis and vitellogenesis were each d ivided into two subphases: the perinucleolus and cortical alveoli phas e, and early and advanced vitellogenesis, respectively. These phases w ere followed by final maturation, ovulation, spawning and a post-spawn ing phase. The GSI increased rapidly during vitellogenesis, to a maxim um value of about 35 just before spawning, at the end of the winter, w hen 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 env ironment exhibited premature vitellogenesis, resulting in a high numbe r of oocytes in yolk granule stage. This study on the effects of pollu tion indicates the importance of the description of the normal annual cycle of the ovary as a control in toxicological studies. (C) 1995 The Fisheries Society of the British Isles