Final Publication: Ham, K.D., J.J. Anderson, and J.A. Vucelick. 2005. Effect of Multiple Turbine Passage on Juvenile Snake River Salmonid Survival. PNNL-15450. Richland, WA: Pacific Northwest National Laboratory
Executive Summary
Juvenile salmonids originating in the Snake River upstream of Lower Granite Dam must pass up to eight hydroelectric projects during their downstream migration to the Pacific Ocean. Fish may pass a project through a turbine or a spillbay or be screened into a bypass system that either collects fish into a barge or releases them downstream of the project. Previous reviews of studies of downstream passage for salmon at hydroelectric projects in the Columbia River basin found higher mean mortality at turbines than for spillways or bypass systems. The potential mechanisms of mortality during turbine passage may include pressure changes, cavitation, shear, turbulence, strike, or grinding. Observing those mechanisms is challenging in the field, but laboratory studies have demonstrated that a single exposure to shear or pressure changes similar to turbine passage conditions can result in injury for some individuals. Because fish pass several dams along their migration, individuals experience a series of passage events. If estimates of surviving the passage of a single project are applied to each passage event, then the underlying assumption is that the mortality at each project is independent of previous exposure. If individuals approaching a project were already sub-lethally stressed, higher than expected mortality rates might occur upon subsequent passage events. Our hypothesis is that fish passing more than one turbine will experience a greater than expected rate of mortality. Because measuring an incremental increase in mortality would be challenging in the field, we developed an approach to first assess whether such an increment has any potential to influence a fish population. This approach identified populations at risk and will help design laboratory or field experiments to address those risks.
In our study, we used a spreadsheet model of juvenile salmonid passage and survival in the Snake and Columbia River systems to simulate passage-route histories across a range of conditions for several runs and to assess the risk of passing multiple turbines. The Simulated Passage model (SIMPAS) was developed by the National Marine Fisheries Service to simulate fish passage proportions through turbines and other routes along with route-specific survivals as calibrated by research results and regional consensus. The model was run under current system operational guidelines for some representative flow years to evaluate what conditions lead to the greatest risk of passing multiple turbines. The model was modified to simulate a hypothetical increase in mortality as a result of passing multiple turbines. This multiple turbine effect (MTE) was simulated at various levels, from a small increase in mortality to the extreme case of complete mortality upon passing more than one turbine.
The proportion of fish passing multiple turbines in a simulation varied due to species differences and distribution of flow among passage routes, and in response to management options such as the juvenile fish transportation program. Even at the most extreme levels of MTE simulated, the number of successful downstream migrant juvenile spring Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) declined less than 8% during scenarios with transport. The number of juvenile fall Chinook salmon (O. tshawytscha) declined by 14% and 17%, respectively, in low and average flow years with transport. An MTE of 59.0% in a low-flow year or 44.5% in a high-flow year was required to reduce the number of successful downstream migrant juvenile fall Chinook salmon arriving downstream of Bonneville Dam by 10%. For all runs and flow years, fewer juvenile salmon were predicted to arrive downstream of Bonneville Dam for scenarios where transport was turned off.
By approaching the problem first with a simulation model, it was possible to determine that fall Chinook salmon in low and average flow years had the greatest potential for an MTE to reduce the number of juveniles arriving downstream of Bonneville Dam by at least 10%. Modeling also predicted that a relatively large MTE would be required to reduce the number of migrants by 10%. Because the MTE required to notably impact fall Chinook salmon is predicted to be large, a field experiment to detect whether it exists appears feasible. An experiment designed to detect impacts smaller than the threshold level could provide a statistically rigorous demonstration that the effect is too small to have an impact on the population. Conversely, the same experiment should easily detect an impact if it exceeded the threshold level.