Abstract
Habitat use and diet variation of immature (age 0+ and age ≧1+) Sakhalin taimen (Hucho perryi) during fall were studied in second- and third-order tributaries of A Stream, northern Hokkaido, Japan. Sakhalin taimen aged 0+ were most abundant near the shore of the second- order tributary, whereas those aged ≧1+ were most abundant in the thalweg of the third-order tributary. The size of the fish was positively correlated with water depth and cover patch size. The composition of the stomach content differed markedly between masu salmon (Oncorhynchus masou) and Sakhalin taimen. The former comprised terrestrial insects (IRI% = 66.8), whereas the latter comprised ephemeroptera (IRI% = 62.9). These results indicate that the masu salmon is a typical drift forager, whereas the Sakhalin taimen shows benthos foraging. Fish, particularly the Siberian stone loach (Noemacheilus barbatulus tori) and the Amur stickleback (Pungitius sinensis sinensis), constituted a fairly large part (IRI% = 30.6) of the diet of Sakhalin taimen aged ≧1+. Furthermore, those aged ≧1+ cannibalized individuals of their own species aged 0+. In order to preserve immature Sakhalin taimen, migration routes throughout the river system must be maintained and bank cover with a slow water velocity, undisturbed stream beds for benthic invertebrates and riparian forest for terrestrial insects are required.
