We provide evidence that the effects of sediment infiltration and deposition within fry emergence traps can lead to a potential negative bias when estimating survival to emergence (STE) of salmonid fry. Fry emergence traps were placed over 12 redds of brown trout Salmo trutta located in sections of three streams in the upper Clark Fork River system in Montana. The redds were capped in March 1989 and monitored through June 1989. During this period, sediment deposition became noticeable inside 8 of the 12 traps. During the final removal of the traps, substrate core samples were collected from a location inside and outside of each emergence trap. Substrate size gradation analysis indicated that concentrations of fine sediments (≤0.84 mm in diameter) were significantly higher in samples collected inside the traps than outside. These results suggested that the local sediment transport capacity was reduced within the emergence traps, resulting in increased deposition of fines. An understanding of the hydrology, ambient sediment concentrations, and sediment transport capabilities of river systems is important when considering the use of fry emergence traps. Without consideration for this potential confounding factor, STE estimates that involve the use of fry emergence traps may underestimate actual STE values.
Run-of-river hydropower reduces streamflow between diversion and powerhouse, potentially impacting fish. Hydropower license conditions include instream flows to protect fish, but monitoring instream flow effectiveness to protect fish is rarely reported.Monitoring a trout population before construction (baseline) and during operation of a small hydropower project with instream flows for spawning and incubation, summer rearing, and winter rearing based on a Physical Habitat Simulation System in the state of Washington indicated the instream flow protected the trout. The monitoring plan included decision points based on monitoring results. Increasing or stable population trends would trigger locking in an instream flow for the remainder of the license, while declines would trigger incremental increases in the instream flow, followed by additional monitoring and decision points.Three years of monitoring following beginning of the project indicated an apparent increase in the trout population over baseline, suggesting that the instream flow was protective, triggering finalization of instream flows. An additional year of monitoring conducted after several more years of project operation was consistent with the finding of the first 3 years of operation.
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