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Inhibitory effects of 17α-methyltestosterone and estradiol-17β on smoltification of sterilized amago salmon (Oncorhynchus rhodurus)
383
Inhibitory effects of 1’7a-methyltestosterone and estradioL17p on smoltification of sterilized amago salmon (Oncorhynchus rhodurus) SATOSHI MIWA and YASUO INUI National Research Institute of Aquaculture,
Tamaki, Mie (Japan)
ABSTRACT In amago salmon (Oncorhynchus rhodurus) only fast-growing underyearling males smoltify in late autumn to winter, whereas the remaining males and females become sexually mature as parr in their first or second autumn. Furthermore, smoltification and maturation never occur in one individual at the same time, suggesting an inhibitory effect of sexual maturation on smoltification. In the present study, we administered 17a-methyltestosterone (MT) and estradiol-1’7P (E,) into sterilized amago parr to clarify effects of sex steroids on smoltification of amago. To avoid influences of endogenous sex steroids, amago were sterilized by a seriex of MT treatments during eyed egg to fry stage. Eight months after, apparently sterile psrr were divided into 5 groups (45 fish each) and used for the experiment. They were fed on diets containing MT or E, at concentrations of 1 and 10 mg/kg, respectively, or no hormone for the control group. The gonads of all the fish were checked at the end of the experiment and those having testes or ovaries were excluded from the data. After 60 days of treatment, the observation of the external appearance of the fii showed that most of the control fish smoltified, showing silvery body color, dorsal fin margin blackening, and no visible parr marks; In contrast, the hormone-treated fish remained parr. The effect of a high dose of MT was especially clear; parr marks became clearer and body color darkened. Occurrences of smolt, judged by body silvering and dorsal fin margin blackening, were as follows: control, 80%; MT 1 mg/kg, 0%; MT 10 mg/kg, 0%; E, 1 mg/kg, 33%; E, 10 mg/kg, 2%. MT and E, also increased condition factor. The seawater challenge test in 27.5”/00 sea water indicated that control amago and those treated with the low dose of E, had better hypoosmoregulatory ability than fiih in other hormone-treated groups. MT and E, also decreased gill Na+/K’-activated ATPase activity of amago. In addition, histological observation revealed that MT and E, decreased gill chloride cells both in size and in number compared with control animals. MT and E, thickened skin and gill epithelium, while the skin of control fish was so thin that their scales were easily detached by handling. These results clearly indicate that both MT and E, inhibited smoltification of amago, morphologically and physiologically. Inhibitory effects of MT on amago smoltification were greater than those of E,. The present experiment suggests that during sexual maturation of amago, androgens and estrogens which are synthesized in the gonads inhibit natural smoltification.
Involvement of endocrine systems in smoltification in the amago salmon, Oncorhynchus rhodurus Y. NAGAHAMA Laboratory
of Reproductive
Biology, National Institute for Basic Biology, Okazaki 444
(Japan) ABSTRACT Amago salmon, Oncorhynchus rhodurus, have a limited distribution in the Pacific coastal rivers of mainland Japan, and those held at the Gifu Prefectural Fisheries Ex-
Inhibitory effects of 1’7a-methyltestosterone and estradioL17p on smoltification of sterilized amago salmon (Oncorhynchus rhodurus) SATOSHI MIWA and YASUO INUI National Research Institute of Aquaculture,
Tamaki, Mie (Japan)
ABSTRACT In amago salmon (Oncorhynchus rhodurus) only fast-growing underyearling males smoltify in late autumn to winter, whereas the remaining males and females become sexually mature as parr in their first or second autumn. Furthermore, smoltification and maturation never occur in one individual at the same time, suggesting an inhibitory effect of sexual maturation on smoltification. In the present study, we administered 17a-methyltestosterone (MT) and estradiol-1’7P (E,) into sterilized amago parr to clarify effects of sex steroids on smoltification of amago. To avoid influences of endogenous sex steroids, amago were sterilized by a seriex of MT treatments during eyed egg to fry stage. Eight months after, apparently sterile psrr were divided into 5 groups (45 fish each) and used for the experiment. They were fed on diets containing MT or E, at concentrations of 1 and 10 mg/kg, respectively, or no hormone for the control group. The gonads of all the fish were checked at the end of the experiment and those having testes or ovaries were excluded from the data. After 60 days of treatment, the observation of the external appearance of the fii showed that most of the control fish smoltified, showing silvery body color, dorsal fin margin blackening, and no visible parr marks; In contrast, the hormone-treated fish remained parr. The effect of a high dose of MT was especially clear; parr marks became clearer and body color darkened. Occurrences of smolt, judged by body silvering and dorsal fin margin blackening, were as follows: control, 80%; MT 1 mg/kg, 0%; MT 10 mg/kg, 0%; E, 1 mg/kg, 33%; E, 10 mg/kg, 2%. MT and E, also increased condition factor. The seawater challenge test in 27.5”/00 sea water indicated that control amago and those treated with the low dose of E, had better hypoosmoregulatory ability than fiih in other hormone-treated groups. MT and E, also decreased gill Na+/K’-activated ATPase activity of amago. In addition, histological observation revealed that MT and E, decreased gill chloride cells both in size and in number compared with control animals. MT and E, thickened skin and gill epithelium, while the skin of control fish was so thin that their scales were easily detached by handling. These results clearly indicate that both MT and E, inhibited smoltification of amago, morphologically and physiologically. Inhibitory effects of MT on amago smoltification were greater than those of E,. The present experiment suggests that during sexual maturation of amago, androgens and estrogens which are synthesized in the gonads inhibit natural smoltification.
Involvement of endocrine systems in smoltification in the amago salmon, Oncorhynchus rhodurus Y. NAGAHAMA Laboratory
of Reproductive
Biology, National Institute for Basic Biology, Okazaki 444
(Japan) ABSTRACT Amago salmon, Oncorhynchus rhodurus, have a limited distribution in the Pacific coastal rivers of mainland Japan, and those held at the Gifu Prefectural Fisheries Ex-