Effects of riparian deforestation on habitat of Juvenile Masu Salmon (Oncorhynchus masou) in a small watershed, Northern Japan

To examine the effects of riparian deforestation on juvenile masu salmon (Oncorhynchus masou), I compared the salmon's habitat between an undisturbed watershed and a disturbed watershed where the riparian forest along the mainstem had been converted into meadow, in northern Japan. I investigated salmon abundance and habitat characteristics in the mainstems and the tributaries of the 2 watersheds, in 3 seasons: summer (Jul.∼Aug.), early winter (mid-Oct.∼mid-Dec.), and spring (mid-May∼early Jun.). As for the habitats of the 2 watersheds, it was noticeable that the maximum water temperature was higher and the cover of woody debris was less in the mainstem of the disturbed watershed, where salmon densities were generally lowest. According to multiple regression analyses, maximum temperature in summer and debris cover in early winter are the most important variables for masu salmon. Subyearling salmon densities tended to be higher in the narrower channels (tributaries and the upper reaches of the mainstem), probably because of the distribution of spawning redds. These results indicate that riparian forests are very important to masu salmon, and that small tributaries are important for spawning and as fry habitat.