Juvenile rainbow trout Oncorhynchus mykiss and steelhead (anadromous rainbow trout), fall (age‐0 and age‐1) and spring Chinook salmon O. tshawytscha, and American shad Alosa sapidissima were exposed to shear environments in the laboratory to establish injury–mortality thresholds based on estimates of strain rate. Fish were exposed to a submerged jet having exit velocities of 0 to 21.3 m/s, providing estimated exposure strain rates up to 1,185/s. Turbulence intensity in the area of the jet where fish were subjected to shear was minimal, varying from 3% to 6% of the estimated exposure strain rate. Injuries and mortalities increased for all species of fish at strain rates greater than 495/s. American shad were the most susceptible to injury after being subjected headfirst to a shear environment, while steelhead and rainbow trout were the most resistant. There was no apparent size‐related trend in susceptibility to high shear except that age‐0 fall Chinook salmon were more resistant to shear environments than age‐1 fall Chinook salmon. All groups of test fish exposed headfirst to high‐shear environments had higher injury–mortality rates than fish introduced tailfirst at similar strain rates. These results document the relationship between fish injury and a fluid force present at hydroelectric facilities and provide biological specifications for improving fish passage and survival.

Fish screens associated with irrigation diversion structures perform a vital function by protecting rearing and migrating fishes. Irrigation diversions in the western United States were developed in the late 1800s and early 1900s with little regard to how they might affect fish populations. Fish screens were installed on some diversions beginning in the 1930s but were often ineffective. Beginning in the 1980s a “modern‐era” fish screening program was initiated in the Yakima River basin in Washington State. A systematic phased approach was employed, with federal funding, to replace antiquated screens and to install screens where there had not previously been fish protection devices. Also during this time, the federal and state agencies responsible for protecting the fish resources developed regional criteria to guide design of these facilities. These criteria, developed by the National Marine Fisheries Service and used by the Washington Department of Fish and Wildlife, dictated physical metrics such as approach velocity (and mesh size) for fish screen facilities. Scientists at the Pacific Northwest National Laboratory (PNNL) developed methods for evaluating new fish screen facilities as they came “on line” to document whether the facilities were designed, constructed, operated, and maintained to be within the fish passage criteria. PNNL uses a combination of water velocity measurements, visual inspection, and underwater videography to determine whether fish screen sites are within the fish protection criteria. This annual evaluation schedule (most sites are evaluated three times/year) is a vital tool to ensure that the large initial capital investment (over $75 million USD) is being operated and maintained to protect fish.

Severe fluid forces are believed to be a source of injury and mortality to fish that pass through hydroelectric turbines. A process is described by which laboratory bioassays, computational fluid dynamics models, and field studies can be integrated to evaluate the significance of fluid shear stresses that occur in a turbine. Areas containing potentially lethal shear stresses were identified near the stay vanes and wicket gates, runner, and in the draft tube of a large Kaplan turbine. However, under typical operating conditions, computational models estimated that these dangerous areas comprise less than 2% of the flow path through the modeled turbine. The predicted volumes of the damaging shear stress zones did not correlate well with observed fish mortality at a field installation of this turbine, which ranged from less than 1% to nearly 12%. Possible reasons for the poor correlation are discussed. Computational modeling is necessary to develop an understanding of the role of particular fish injury mechanisms, to compare their effects with those of other sources of injury, and to minimize the trial and error previously needed to mitigate those effects. The process we describe is being used to modify the design of hydroelectric turbines to improve fish passage survival.

Four intergravel developmental phases of chinook salmon Oncorhynchus tshawytscha were dewatered experimentally in artificial redds. The redds consisted of aquaria containing a gravel mix and supplied with 4 liters of water per minute at 10 C. Cleavage eggs and embryos (the egg phases), and eleutheroembryos and pre‐emergent alevins (the alevin phases) were dewatered 20 consecutive times in 22‐day tests. The egg phases were considerably more tolerant than the alevins. Some cleavage eggs were killed by 12‐ and 16‐hour daily dewaterings, but embryos survived up to 22‐hour daily dewaterings. Embryos also tolerated extended, multiple dewaterings (over 60% survival for four consecutive 118‐hour periods) and one‐time, continuous dewatering for up to 12 consecutive days (over 80% survival). In contrast, about half the eleutheroembryos were killed by 4‐hour daily dewaterings, and nearly all pre‐emergent alevins were killed by 1‐hour daily dewaterings. Intergravel temperatures were affected by insolation and air temperature. Intergravel temperatures increased to lethal levels during dewatering of cleavage eggs in early fall, and limited their survival. Growth of egg phases from some females was retarded by dewatering, but this phenomenon was not consistent for all egg groups. The size of surviving eleutheroembryos decreased as the length of daily dewatering periods increased.

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