Plasma concentrations of ovarian steroids in relation to oocyte final maturation and ovulation in female plaice sampled at sea

Blood plasma concentrations of free 17 β -oestradiol, free testosterone and glucuronidated testosterone were strongly positively related to the percentage of vitellogenic oocytes remaining in the ovaries of plaice Pleuronectes platessa caught at sea–being at their highest in pre-spawning (stage IV) females (i.e. those in which the oocytes were close to fully grown, but had not yet entered the stage of final maturation). In contrast, the concentrations of free and sulphated 17,20 β -P, 3α aL ,17, 20 β -P-5 β , and 3 α ,17,21-P-5 β were at their lowest in stage IV females. Free 17,20 β -P (the putative maturation-inducing steroid) became only slightly elevated (less than twofold) during spawning (i.e. in stage V and VI females with hydrated and/or ovulated eggs). Sulphated 17,20 β -P and 3 α ,17,21-P-5 β became slightly more elevated (three- to fourfold). However, sulphated 3 α , 17,20 β -P-5 β concentrations increased 30-fold and were at their highest in fish in which only 40% of vitellogenic oocytes remained in the ovaries. Sulphated 17,20 β -P, 3 α , 17, 20 β -P-5 β and 3 α ,17,21-P-5 β concentrations were significantly positively related to hyaline oocyte batch size; and sulphated 17,20 β -P and sulphated 3 α , 17,20 β -P-5 β were significantly negatively related to the degree of hydration of the hyaline oocytes. None of the steroid concentrations, however, was related to the time of capture. More ovulated females were found in the afternoon than at any other time of the day.