Reduced Streamflow Lowers Dry-Season Growth of Rainbow Trout in a Small Stream

Abstract

A wide variety of resource management activities can affect surface discharge in small streams. Often, the effects of variation in streamflow on fish survival and growth can be difficult to estimate because of possible confounding with the effects of other variables, such as water temperature and fish density. We measured the effect of streamflow on survival and growth of rainbow trout Oncorhynchus mykiss in a small stream in northwestern California by manipulating the flow entering four of eight enclosed stream sections (9.0–15.3 m long) containing one pool and 2.5–4.0 m of upstream riffle habitat. In the four manipulated experimental units, we reduced inflow by 75–80% over a 6-week period in summer 2003. Flow diversion substantially decreased water velocity in riffle–pool transition areas but did not strongly affect habitat volume or water temperature. Fish in control units grew about 8.5 times as much as those in units with reduced streamflow; however, discharge manipulation did not affect survival, which exceeded 90% in both treatments. The input of invertebrate drift to pools within control units greatly exceeded the input into units with reduced streamflow; the concentrations of drifting invertebrates differed to a lesser extent between control and manipulated units. Managers concerned about salmonid populations in small streams should consider dry-season streamflow to be a key variable affecting fish growth.