PLASMA-CONCENTRATIONS OF OVARIAN-STEROIDS IN RELATION TO OOCYTE FINALMATURATION AND OVULATION IN FEMALE PLAICE SAMPLED AT SEA

Blood plasma concentrations of free 17 beta-oestradiol, free testoster one and glucuronidated testosterone were strongly positively related t o the percentage of vitellogenic oocytes remaining in the ovaries of p laice Pleuronectes platessa caught at sea-being at their highest in pr e-spawning (stage IV) females (i.e. those in which the oocytes were cl ose to fully grown, but had not yet entered the stage of final maturat ion). In contrast, the concentrations of free and sulphated 17,20 beta -P, 3 alpha,17,20 beta-P-5 beta, and 3 alpha,17,21-P-5 beta were at th eir lowest in stage IV females. Free 17,20 beta-P (the putative matura tion-inducing steroid) became only slightly elevated (less than twofol d) during spawning (i.e. in stage V and VI females with hydrated and/o r ovulated eggs). Sulphated 17,20 beta-P and 3 alpha,17,21-P-5 beta be came slightly more elevated (three- to fourfold). However, sulphated 3 alpha,17,20 beta-P-5 beta concentrations increased 30-fold and were a t their highest in fish in which only 40% of vitellogenic oocytes rema ined in the ovaries. Sulphated 17,20 beta-P, 3 alpha,17,20 beta-P-5 be ta and 3 alpha,17,21-P-5 beta concentrations were significantly positi vely related to hyaline oocyte batch size; and sulphated 17,20 beta-P and sulphated 3 alpha,17,20 beta-P-5 beta were significantly negativel y related to the degree of hydration of the hyaline oocytes. None of t he 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. (C) 1998 The Fisheries Society of the British Isles.