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The effect of methallibure and a constant 12 hours light : 12 hours dark photoperiod on the gonadal maturation of pink salmon (Oncorphynchus gorbuscha) Flynn, Michael Bernard

Abstract

This study was undertaken to try to delay gonadal maturation of pink salmon for one year beyond their normal two year life cycle. This would allow these fish to spawn in years of low or nonexistent escapement and possibly increase these "poor" year populations. Three experiments were conducted to investigate the efficacy of the antigonadotropic drug, methallibure, in inhibiting gonadal maturation in pink salmon. Gonadosomatic index, oocyte diameter, and stages of cell maturation in the testis and oocyte maturation in the ovary were measured. The first or pilot experiment involved a range of doses of methallibure (0.10 mg., 0.32 mg., and 1.0 mg./gm./2wks.) to determine the optimal dose for subsequent experiments. All doses had only a slight slowing effect on maturation. This result and possible undesirable effects of higher doses prompted the decision to use the 0.10 mg./gm. dose for subsequent experiments. The second or long-term experiment investigated the effects of methallibure and a constant 12 hours light:12 hours dark photoperiod on gonadal maturation of males and females for a period of ten months. Methallibure completely inhibited testicular maturation by preventing the transformation of primary into secondary spermatogonia. Ovarian maturation, however, was only slowed. The treated ovaries possessed oocytes in the oil globule stage while control ovaries had oocytes in the secondary yolk globule stage. Methallibure had an antithyroidal effect under natural photoperiod but not under constant 12L:12D photoperiod or at a high dose (1.0 mg./gm). Stress from kidney disease may have been operative in this effect. Methallibure also slowed the rate of increase in body weight. The constant 12L:12D photoperiod slowed gonadal maturation in both males and females. It is suggested that a specific day-length and an endogenous rhythm stimulate the initiation, maintenance, and termination of gonadal maturation and that the seasonal daylength fluctuations function as a synchronizer. The difference in effect of methallibure on males and females may be due to treatment beginning prior to the start of testicular maturation but after the start of vitellogenesis. To investigate this possibility, methallibure treatment was begun at successive intervals prior to the start of vitellogenesis in the third or sequential experiment. This treatment had no effect on ovarian maturation which suggests that the females are less sensitive to methallibure than are the males. Treatment with a higher dose started early in juvenile life may inhibit ovarian maturation. From this study, only the males could be delayed and, therefore, possibly spawn in "poor" years. However, Funk and Donaldson (1972) were able to achieve the same goal by maturing males in the year of hatching, thus making a three year program impractical. The value of a long program would be the delay of ovarian maturation since Funk et al. (1973) were unable to advance maturation of females by one year.

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