Summary. Levels of oestradiol in plasma of adult female rainbow trout Salmogairdneri were determined during the annual reproductive cycle using a radioimmunoassay. From January to June, during previtellogenesis and the period of endogenous vitellogenesis plasma levels were low (1,3 ! 0,2 ng/mi). At the beginning of the period of exogenous vitellogenesis, the cestradiol level rose and a maximum (16,9 ! 0,2 ng/ml) was reached at the end of vitellogenesis in November, some weeks before spawning. A positive correlation (r = 0,58) was established between plasma oestradiol levels and the gonadosomatic index. From enzymatic cytochemical studies it was concluded that the granulosa cells, as well as the interstitial cells in the ovary, are involved in oestradiol production.Introduction.

Aromatase activity was determined in small discrete areas of the brain of the African catfish, Clarias gariepinus, by a radiometric assay. The fore- and midbrain were divided into eighteen 500-microns transverse sections. From these sections several punches (0.3 mg of tissue) were taken and incubated with [19-3H]-androstenedione. The aromatase activity, calculated from the release of tritium label during aromatization, is expressed in pmol mg-1 tissue hour-1. The highest activity (3.7 pmol) was detected in the preoptic region. The more caudally located area tuberalis, including the nucleus lateralis tuberis and the nucleus recessus lateralis, also showed a relatively high activity (2.5 pmol). A similar activity was found in the most rostral part of the telencephalon and the dorsal parts of the mesencephalon, i.e., the tectum opticum and torus semicircularis (2.3 pmol). A moderate aromatase activity was observed in remaining parts of the brain, except the cerebellum and hindbrain, in which aromatase activity was hardly detectable (0.1-0.3 pmol). It is concluded that a high aromatase activity is present in regions known to be involved in the regulation of reproduction. Since both the torus semicircularis and the tectum opticum display a high aromatase activity, it is suggested that also these structures are involved in reproductive processes.

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In general, female zebrafish,Brachydanio rerio, ovulate only in the presence of males. The stimulant must be pheromonal as even male holding water is capable of inducing ovulation. After ovulation the mating phase begins. During this phase the male follows the female and oviposition as well as fertilization takes place. Both the ovulation and the mating are controlled by pheromones synthesized by the gonads. Ovulation can be induced by testicular homogenates. After the lipid material has been extracted from the testicular homogenates, the remaining aqueous phase can still induce ovulation. However, when the aqueous phase is treated with the enzymeβ-glucuronidase, it loses the ability to induce ovulation. This is an indication that glucuronides, probably steroid glucuronides, are the compounds responsible.During the mating phase, ovulated female zebrafish become attractive to males. It was found that, after ovulation, ovarian extracts contain the compounds responsible for attracting males. The attractant consists of a mixture of steroid glucuronides.After incubation of the gonads with(3)H-precursors seven steroid glucuronides have been identified in the testis and five in the ovary.Under fish culture conditions the African catfish,Clarias gariepinus, can produce postivitellogenic oocytes throughout the year. However, in capitivity neither males nor females spawn. In female catfish maturation and ovulation can be induced by treatment with gonadotropins. It might be possible that, analogous to the zebrafish, some reproductive processes in the catfish have to be induced by pheromones. It has been demonstrated that pheromonal compounds released by the seminal vesicles are involved in the attraction of female conspecifics. The steroid glucuronide synthesizing capability of the testes and the seminal vesicles of the male catfish are examined, as well as that of the ovary before and after ovulation of the female catfish. Both testes and seminal vesicles appear to be capable of steroid biosynthesis but only the latter synthesizes steroid glucuronides. Six of these conjugates have been isolated and identified. In the female catfish the ovaries are capable of synthesizing seven steroid glucuronides, but only after ovulation.

., 1987. The significance of photoperiodicity, water temperature and an inherent endogenous rhythm for the production of viable eggs by the African catfish, Clark gariepinus, kept in subtropical ponds in Israel and under Israeli and Dutch hatchery conditions. Aquaculture, 63: 169-185.A comparison was made between the fecundity of female African catfish, Clarias gariepinus, transferred from their natural habitat in Northern Israel to nearby fish ponds and an indoor hatchery respectively, and conspecifics reared and kept in an indoor hatchery in The Netherlands.The results indicate that an inherent endogenous rhythm rather than the natural light periodicity determines the cyclical changes in ovarian activity, and that this internal rhythm is determined by environmental factors at an early stage of development. High water temperatures and the presence of males seem to enhance ovarisn activity and to shorten its resting period. The significance of gonadotropin secretion by the pituitary for the ovarian cycle is discussed.It is concluded that for propagation of C. gariepinus throughout the year, independent of the seasons, broodfish should be reared and kept in hatchery tanks at a water temperature of 25"C, with proper feeding, in the presence of males, and at any local light periodicity.