Mortality and injury rates for small fish passing over three diversion dam spillway models

Effects of low-head diversion dam spillways on survival and injury of fish passing downstream are poorly understood, especially for fragile, early life stages. The ubiquitous nature of diversion dams in waterways worldwide led us to assess survival and injury rates for small and large size-classes (nominal mean total lengths 25 and 50 mm, respectively) of morphologically general fishes: fathead minnow Pimephales promelas; hybrid rainbow x cutthroat trout (Oncorhynchus mykiss x O. clarkii, hereafter trout); and small razorback sucker Xyrauchen texanus. Fish were released in three laboratory spillway models: a 5.7-m-high, free-impinging overfall spillway (0.023 m(3).s(-1).m(-1)) with three stilling basin depths; a 5.4-m-high, smooth-faced ogee-shaped spillway; and the ogee-shaped spillway fitted with energy dissipation steps. Ogee-shaped spillway flows were 0.012-0.24 m(3).s(-1).m(-1). Mean survival proportion, corrected for handling mortality, was high in all models (0.97-1.0) for all species, size-classes, and flow conditions, except in the free-overfall spillway with stilling basin depth of 2.5 cm, where survival was lower (0.78-0.94). Survival rates of fathead minnows and trout were similar in the stepped and smooth spillway experiments, and across all flow levels. Survival of razorback suckers was slightly lower in the stepped than the smooth spillway but differences were likely due to poor fish condition, which may have also reduced their survival at the highest flows in ogee-shaped spillway models. The free-overfall spillway caused negligible injury to small fish if receiving pool depth was 15 cm or deeper. Injury rates in the stepped spillway were slightly higher than in smooth one, but serious injuries were rare. Although mortality and injury rates to fish after passage over the spillway models were typically low, higher cumulative rates from passage over many diversions indicates managers should use structures that reduce injury and mortality rates wherever possible.