Interspecific competition for food resources between Dolly Varden (Salvelinus malma) and rainbow trout (Oncorhynchus mykiss) in the Satsunai River reservoir, upper Tokachi River System, Hokkaido

Abstract

To clarify interspecific competition between Dolly Varden and rainbow trout, their habitat use and stomach contents were investigated in the Satsunai River reservoir, upper Tokachi River System, Hokkaido, Japan, during 1998 2004. During low population-densities, both species were distributed in the surface layer, whereas at high densities, Dolly Varden were restricted to the bottom layer. When terrestrial insects were abundant on the surface, they were utilized by both pecies. When the former were less abundant, Dolly Varden fed mainly on bottom layer zooplankton and benthic animals, indicating a shift to specific feeding niches under conditions of high fish density and food shortage.