Gametogenesis and Sex Steroid Profiles in Cultured Coho Salmon (Oncorhynchus kisutch, Walbaum)
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The gametogenesis of a 2-year-old coho salmon broodstock population cultured in a fish farm in southern Chile was studied. Gonadosomatic index (GSI), microscopic gonadal traits, and serum levels of estradiol-17b(E2), testosterone, and 17a,20b-dihydroxy-4-pregnene-3-one (17,20P) in both sexes were recorded beginning 9 months before spawning in bimonthly samplings. Maximum GSI means were reached during May, the month of spawning, with 16.8 ± 4.1% for females and 8.4 ± 0.8% for males, both values within the range described in the literature. GSI in males, however, was triple that of females during January, showing a faster rate of gonadal growth in males in early summer. Gonadal microscopy for both sexes showed stages corresponding to those described by different authors for other salmonids such as rainbow trout. The secondary vitellogenesis period was 4 to 5 months and corresponded with the short vitellogenesis model described in rainbow trout for broodstocks maturing at 2 years of age. The serum profiles of sex steroids in both sexes are consistent with those described in coho salmon and other salmonid species. In females, E2 and 17,20P show opposite profiles, reaching their maximum levels (E2: 45.13 ± 11.3 ng/ml; 17,20P: 24.47 ± 7.34 ng/ml) during vitellogenesis (March) and ovulation (May), respectively. In both sexes, testosterone concentration shows maximum levels in May (females: 61.68 ± 15.75 ng/ml; males: 107.8 ± 11.6 ng/ml), suggesting the physiological importance of this hormone during maturation, either directly or as a substrate for the synthesis of other hormones. In males, the maximum level of 17,20P (22.33 ± 4.5 ng/ml) also occurs in May during total spermiation, which confirms its role in semen production and semen fluid regulation as described in the literature about this hormone in salmonid males. On the basis of the data obtained, a reproductive pattern is proposed for 2-year-old salmon cultured in southern Chile.