Submitted By Pacific Northwest National Laboratory, Richland, Washington
Fish may be subjected to injurious shear and turbulence stresses as they pass through turbines at hydropower dams on their way to the ocean. Some of these fish may not have obvious physical injuries, but may nonetheless suffer temporary loss of equilibrium or reduced ability to avoid being eaten by predatory fish below the dams. To examine the effects of exposure to shear stress on susceptibility to predation, Pacific Northwest National Laboratory (PNNL) used the shear stress test facility at the Aquatic Laboratory in Richland, Washington. The objective of this work was to determine whether shear stress values that did not result in major injuries to juvenile fish reduced the ability of shear strain-exposed fish to avoid predation. Direction and oversight of these experiments is the responsibility of the U.S. Department of Energy, Oak Ridge National Laboratory (ORNL), Idaho National Laboratory (INL), and the Advanced Hydropower Turbine System Technical Committee.
Figure 1. Steelhead smolt exposed to shear strain may not be killed or injured, but may have increased susceptibility to predation.
Marked test fish were introduced into the shear zone of the shear stress test facility through an acrylic tube (Figure 1). Control fish (with a different mark) were introduced into the tank away from the shear zone through an identical tube. Shear strain levels tested were less than those known to result in major injury. Adult rainbow trout, trained to eat juvenile fish, were allowed to enter the area of the tank where the test and control fish were evenly mixed. The predators were allowed to consume approximately half of the juvenile fish. The remaining juvenile fish were then collected and examined for marks (to determine whether they were from the stressed or control group) and injuries resulting from predatory attacks.
Figure 2. Results of the predation challenge to juvenile rainbow trout(n = 20 for each sample) after exposure to a strain rate of 688 cm/s/cm Dy=1.8 cm). The 'percent injured' is a combination of fish that were eaten and those showing evidence of an attack (e.g., teeth marks, bruises).
Previous testing revealed that juvenile rainbow trout received minor injuries at a strain rate of 852 cm/s/cm (Dy=1.8 cm), but showed no major injuries at a strain rate of 688 cm/s/cm (Dy=1.8 cm). In this experiment, juvenile rainbow trout exposed to a strain rate of 688 cm/s/cm (Dy=1.8 cm) were preyed upon at a significantly higher rate than control fish (p = 0.001, Figure 2). Fish exposed to the next lower test setting, a strain rate of 517 cm/s/cm (Dy=1.8 cm), were not preyed upon at a significantly different rate than control fish (p = 0.78).
The shear stress test facility built by PNNL has proven useful in evaluating the direct and indirect effects of shear stress on fish. Shear strain values that did not result in injury or mortality to test fish were sufficient to reduce their ability to avoid being eaten by a predatory fish. This finding illustrates the potential ecological consequences of the stresses endured by juvenile fish that are passed through turbines in hydropower dams. These tests have also further defined the shear stress thresholds that result in injury or reduced performance of fish that are exposed to shear strain. These data are another important step towards aiding engineers to design new "fish-friendly" turbines.
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