Special operating conditions of equalized flow through both powerhouses were implemented at Bonneville Dam for this study. These conditions were intended to attract predators equally to the two tailrace areas for unbiased comparisons of survival among the various routes of juvenile fish passage, as well as to minimize tailrace eddies. They were also meant to provide high flows past the juvenile bypass outlet and to allow adequate attraction flows to the various fishway entrances for upstream migrant adults.
A regional drought severely reduced river flow during 1992 and may have created a worst-case scenario for salmonid survival due to heavy predation of test fish in tailrace areas. It is important to consider a wide range of test conditions before formulating conclusions regarding the safest routes for juvenile salmon passing Bonneville Dam during summer. Based on one year of study, we present the following tentative conclusions:
- Under the drought conditions of 1992, recoveries of marked subyearling chinook salmon in the estuary indicated significantly reduced survival of fish released into the First Powerhouse bypass system compared to fish released into First Powerhouse turbines or fish released downstream from the tailrace.
- Fish passing through the Second Powerhouse turbines and tailrace had significantly reduced survival compared to fish passing through the First Powerhouse turbines and tailrace.
- Fish released downstream had significantly higher survival than all other release groups.
- Tule stock subyearling chinook salmon used in this study were subjected to cold-water rearing and reduced rations to maintain a size range at release similar to that of normal summer migrants (upriver bright stock). However, test fish, particularly those from the final week of test releases, may have suffered extreme stress due to elevated Columbia River water temperatures resulting from the regional drought. While the immediate impacts of dam passage are thought to be fully expressed in mark-recovery differences among juvenile fish recovered at Jones Beach, the overall survival of test fish may have been reduced by temperature stress. This potential overall lower survival of test fish may affect comparisons among treatment groups using CWT data from adult contributions to the various ocean and river fisheries and returns to the lower river hatcheries.
- An estimated 75 to 90% of juvenile salmon summer migrants that encounter the powerhouses at Bonneville Dam pass through turbines instead of bypass systems. Passage survival has been significantly different between turbine-plus-tailrace passage at the First vs. the Second Powerhouses. Therefore, it is extremely important to identify the safest passage route over a wide range of river flows.
- Tag recovery of adults should be compiled through 1997 to assess passage survival differences adequately.
- The study should be repeated for 3 additional years to bracket a wide range of river flow and other physical conditions before making final conclusions regarding the relative survival of summer migrating subyearling chinook salmon through the various passage routes at Bonneville Dam.