Gas Bubble Trauma Incidence in Juvenile Salmonids in the Lower Columbia and Snake Rivers

Abstract

Gas bubble trauma (GBT) remains a controversial issue in the lower Columbia and Snake rivers. Despite improvements in the design of spillways at each hydroelectric project, spill continues to create total dissolved gas supersaturation (TDGS) greater than the 110% standard in these rivers. Waivers of the gas standard, 115% TDGS in the forebay and 120% TDGS in the tailrace, are granted from mid-April through August to allow for increased spill to aid juvenile salmonid migration. Because few studies are based on field-collected data, we chose to monitor uncertainty regarding the actual incidence of GBT symptoms on juvenile salmonids under various TDGS concentrations within the Columbia and Snake rivers. From 1996 through 1999, we collected smolts from above and below dams and compared their incidence of GBT symptoms with that of smolts collected by the Smolt Monitoring Program in dam bypasses. We found that fewer than 2% of salmonids displayed external symptoms of GBT, and most of those had less than 5% fin occlusion. Gas bubble trauma is more common at higher TDGS values in fish bypassed at dams than in fish collected above or below dams. However, the data for each group follow the same trend of increasing GBT with increasing TDGS and produce similar regression curves. The incidence and severity of in-river and bypass GBT symptoms are less than expected from laboratory studies.