Abstract Growth and microhabitat characteristics, including distance from bank, water velocity, water depth and distance from river substrates at focal points of un-deryearlings of sympatric honmasu salmon Oncorhynchus masou, which is consid-ered as a hybrid of masu salmon O. m. masou, red-spotted masu salmon 0. m. ishikawai and biwa salmon O. m. subsp., and brown trout Salmo trutta were inves-tigated from early June to mid September in 1996 in the Toyamasawa River, dis-charging into Lake Chuzenji, central Japan. Both fork length and body weight of honmasu salmon were greater than those of brown trout throughout the study pe-riod. Honmasu salmon focal points were located further from the bank, in faster water velocity, and were deeper and further from the substrate than those of brown trout in each study month, except for distance from the bank after August. Com-parison of focal point environments for honmasu salmon and brown trout of equal size classes (fork length 5 mm intervals) showed that the former took positions fur-ther from the substrate. However, no significant differences were apparent in other environmental factors between the two species. The results indicated that minimal differences existed in habitat selection between honmasu salmon and brown trout for the same body sizes, suggesting that competition for microhabitats between un-deryearlings of honmasu salmon and brown trout was reduced by their different body sizes resulting from different hatching periods.
