Local factors affecting winter habitat use of non-native rainbow trout in a boreal stream in northern Japan

Despite its potential use for population control, the winter ecology of nonnative fishes is still poorly understood due to the difficulty of conducting field surveys. In this study, we investigated the winter habitat use of invasive rainbow trout Oncorhynchus mykiss at the cannel unit scale (i.e., pool, riffle). Twenty-four reaches were surveyed in late December 2013 along the Obicha River, a tributary of the Otofuke River, Tokachi River basin, Hokkaido, Japan. A total of 532 fish were captured, of which 96% were rainbow trout, whereas native salmonid was only a single southern Asian Dolly Varden Salvelinus curilus. Smaller rainbow trout (< 250 mm) used reaches with low velocity, whereas larger trout (250–520 mm) aggregated in specific reaches with deep pools with abundant cover and coarse substrate. A previous tributary-scale study in the same river system showed the importance of velocity and temperature, but not depth and substrate. Therefore, habitat selection would be scale-, as well as size-, dependent. This study provides useful information on capturing large mature adults in winter for effective control of nonnative salmonids.