Effect of the fingerling size on the precocious maturation in amago salmon, Oncorhynchus masou ishikawai

In the present study, effect of the fingerling size on precocious maturation in females was studied in amago salmon. Some female of amago salmon Onchorhyncus masou ishikawai mature precociously in their first October (12 months post fertilization), while most of the females mature in their second October (24 months post fertilization). At the commencement of the experiment, fingerlings were divided into four groups according to size and age of their mother. Fingerlings obtained from the precocious females which were spawned for the second time in their 24th month (0.1 + group) were divide into three lots, namely the large lot (L, mean +- SD, 14.3 +- 2.7g), the medium lot (M, 7.2 +- 1.2g), and the small lot (S, 3.1 +- 0.4g). In addition to these lots, fingerlings obtained from precocious females which were spawned for the first time in their 12th month (0 + group) were termed 0 + lot (0+, 13.4 +- 4.6g). Fingerlings were fed from April to October, length and weight measured monthly, and the maturity of the fish was investigated at the end of the experiment. We have obtained the results listed below; 1. The precocious maturity of the females of the L, M, S, and 0 + lot were 36.0%, 24.7%, 6.9%, and 34.8%, respectively. The bigger the initial mean size of the lot, the higher the rate of precocious maturity irrespective of their mother's age at spawning. 2. The precocious maturity of the males of the L, M, S, and 0 + lot were 86.0%, 90.0%, 72.5%, and 84.0%, respectively. Groups of fingerlings whose mean body weight were 7g or more in April had a male precocious maturity rate of greater than 84% in October. These results strongly suggest that the size of fingerlings is one of the most important factors for precocious maturation of females. However, body weight of fish in October does not seem to be a definite factor, because in October some of the immature fish were heavier than the mature fish even in the same lot. Reproductive ability of the precocious female which matured in their first October was considerably inferior to females which matured in their second October. Therefore, improving the reproductive ability of the precocious females could reduce generation time and speed improvement programs