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An investigation of the induction of precocious sexual maturity in juvenile pink salmon Oncorhynchus gorbuscha Funk, James D.

Abstract

This study was undertaken to determine whether the gonads of pink salmon (Oncorhynchus gorbuscha), could be stimulated with exogenous hormones to reproductive maturity one year earlier than normal. This procedure, if successful, could be used as a method for repopulating ‘off’ year cycles of pink salmon in numerous rivers in British Columbia and Washington. In the juvenile males, complete sexual maturity was attained by September in the year of hatching with thrice-weekly treatments of 10.0 micrograms and of 1.0 micrograms salmon (Oncorhynchus tshawytscha) gonadotropin per gram body weight. Histological examination of the precociously mature testes, and comparison with testes from uninjected controls repealed that the time of onset of the mitotic division of spermatogonia to form the primary spermatocyte, and the process of active spermatogenesis were accelerated. At sexual maturity, a scattering of localizations of ∆5-3ß hydroxysteroid dehydrogenase activity was observed which corresponded to the distribution of the interstitial cells. A larger stock of pink salmon, in which injections were initiated 83 days later, developed mature testes in the same time interval as the normal-sized individuals. These gonads were four times larger, however. A small species difference in the action of the gonadotropin preparation was found when comparing its effect on the G.S.I., rate of induction of sexual maturity, and 3ß-ol dehydrogenase activity of immature Oncorhynchus tshawytscha. In the females, the yolk stage was induced first in fish treated three times a week with 1.0 µg/gram body weight salmon gonadotropin and 1.5 µg/gram body weight estradiol 17ß for 126 days. Oocytes containing yolk globules did not appear in pink salmon treated with 1.0 µg/gram body weight salmon gonadotropin alone for a further 42 days. Estradiol l7ß alone, or in combination with salmon gonadotropin at a dosage of 15 µg/gram body weight inhibited vitellogenesis. Formation of oocytes 2 mm in diameter required seven and one half months of treatment with 1.0 µg/gram body weight salmon gonadotropin and 1.5 µg/gram body weight estradiol 17ß, and nine months of injections with l.0µg/gram body weight salmon gonadotropin alone. Few large yolky oocytes were developed by any of the treatments. Large numbers of pre-ovulatory corpora atretica were observed in all of the treated fish. Little histochemically demonstrable ∆5-3ß hydroxysteroid dehydrogenase activity was present in ovaries from pink or spring salmon juveniles treated for 3 months with various dosages of salmon gonadotropin. The significance of the results in relation to the original problem are discussed.

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