In order to clarify the effects of dams on the life cycle of anadromous white-spotted charr Salvelinus leucomaenis, their life-history traits were studied in both below- and above-dam sections of two rivers in northern Japan. In the below-dam sections, the frequency of smolts was over 55.9%, male mature parr only being captured. In the above-dam sections, however, the frequency of smolts was under 15.2%, the maturity rate of parr being over 54.9% in both sexes. This suggests that the charr life cycle type in the below-dam sections was typically anadromous, compared with a land-locked type in the above-dam sections. The latter sections had lower parr densities compared with the below-dam sections, the fork length of young parr (aged 0+ and 1+) in the above-dam sections being greater than those in the latter, suggesting that a higher growth rate as a result of low density had lead to landlocking of the above-dam population. There was no difference between above- and below-dam sections in the frequency of over threshold size for smoltification (10 cm F.L. in autumn). However, the frequencies of smolts in the below-dam sections were higher than those in the above-dam sectins, suggesting that the inhibition of smoltification had promoted maturation as parr in the above-dam sections. The mechanisms of landlocking based on the threshold sizes of maturation and smoltification are discussed.
