Effects of Flow Reduction on a Whitespotted Char Population in a Japanese Mountain Stream

Abstract

AbstractI evaluated the effects of long‐term (about 270 d/year) and large‐volume (83.3% at mean flow) reductions in stream flow on a population of Whitespotted Char Salvelinus leucomaenis. I compared a range of biological parameters (density, biomass, condition factor, and age composition) between populations of char in the free‐flowing (above dam) and regulated (below dam) sections of a Japanese mountain stream over a 4‐year period. The density and biomass of age‐1 and older fish was significantly lower in the regulated section than in the free‐flowing section: 32.8% reduction in density and 42.5% in biomass. The mean condition factor of age‐1 and older fish was significantly lower in the regulated section. In addition, the percentage of age‐2 and older fish in the total catch was significantly lower in the regulated section. Except for flow, the physical environment was similar in both the free‐flowing and regulated sections. Thus, the changes I observed in the regulated section were likely caused by flow reduction. I hypothesize that a decrease in food availability associated with the reduction in flow caused the decrease in density and condition factor in the regulated section. I also speculate that an increase in density in the pools due to flow reduction resulted in increased aggression among conspecifics. In turn, this results in an increase in the emigration of individuals, particularly older and larger fish, in the regulated section. The decreases in the density, condition factor, and the number of older and larger fish following the reduction in flow probably explain the decrease in biomass in the regulated section.Received November 20, 2012; accepted July 15, 2013