Effect of damming on distribution of rainbow trout in Hokkaido, Japan

Abstract

Rainbow trout introduced into Hokkaido in 1920 have become widely distributed due to extensive release into many reservoirs and lakes for sport-fishing; their presence often results in reductions of native fish populations. We analyzed and predicted the relationship between the probability of occurrence of rainbow trout and the proximity of dams (or attributed reservoirs), using a database of the presence or absence of rainbow trout collected during 1960–2004 in Hokkaido to clarify the spread patterns of exotic species (e.g., rainbow trout) due to large-scale damming over a long period. Rainbow trout were abundant in streams within approximately 10 km of dams in recent years, regardless of whether the stream was up- or down-stream from the dam and after accounting for the effects of other environmental variables (e.g. elevation, population density, and survey year). A delayed increase in trout occurrence below dams as compared with above dams suggests that the occurrence below dams may be largely due to escapement of stocked populations and a continuously increasing abundance since 1970. The management of dams and reservoirs is necessary to prevent further spread of rainbow trout because they can threaten habitats of native Japanese salmonids through various mechanisms.