Surprising salmonid response to water diversion at four run-of-river hydroelectric projects in British Columbia

Run-of-river (RoR) hydroelectric power provides renewable energy with potentially less impact on fish habitat productivity than large, reservoir-storage hydroelectric projects. This is the first empirical study of resident fish response to water diversion at RoR projects in BC, where the abundance of rainbow trout (Oncorhynchus mykiss) was monitored in four small streams using a before–after control–impact design. Although flows in the diversion reaches of these projects were reduced by 63% to 70%, the total biomass of rainbow trout increased from 35% to 157% across the streams compared with the “before period” and “control reaches”. The response to water diversion differed among age classes: adult biomass increased with reduced flows in the growing season; juvenile biomass increased with reduced flows in the winter, higher stream conductivity, and increased flows in late summer; and fry biomass increased with higher stream conductivity and temperatures. One novel way to contextualize these shifts in resident salmonid productivity and differential growth and (or) survival by age class is via a size–density approach, which we use here to demonstrate how the carrying capacity of a stream for rainbow trout is affected by water diversion.