Salmonid Fry Stranding Mortality Associated with Daily Water Level Fluctuations in Trail Bridge Reservoir, Oregon

Little information exists on the effects of reservoir fluctuations on stranding risk for early life stages of salmonids. This study focuses on the effects of hydropower-related reservoir fluctuations in Trail Bridge Reservoir, Oregon, where salmonids, including listed bull trout (Salvelinus confluentus) and spring Chinook salmon (Oncorhynchus tschawytscha), commonly rear. A distance-from-line sampling design was employed with permanently established transects to estimate stranding magnitude of juvenile salmonids during 30 surveys over three months in Spring 2006. All stranded fish observed during field surveys were mapped onto spatially rectified low elevation aerial photographs to assess patterns in stranding. Most fish were stranded in habitats with gradient <6%, typically in interstitial spaces among cobbles, and in “potholes.” Fish were stranded in similar numbers following small or large fluctuations, and no relationship was apparent between the range in fluctuation and the number of stranded fish, or between the average rate of water surface decline and the number of stranded fish. Based on the extrapolation, we estimated a total of 470 spring Chinook salmon fry and 326 brook trout (S. fontinalis) fry stranded in Trail Bridge Reservoir. No bull trout were observed stranded as a result of project operations. Our findings suggest that stranding in Trail Bridge Reservoir could be reduced, while still retaining the hydropower function of the reservoir, by management actions focused on restricting fluctuations to specific elevations during vulnerable fish migration periods, or increasing the slope of areas identified as having a high stranding risk.