Gas Supersaturation May Reduce the Survival of Yearling Chinook Salmon in the Lower Columbia River and Ocean Plume

Unusually high flows in the Columbia River in 2011 raised total dissolved gas (TDG) levels in the river above the 120% legal limit imposed to prevent harmful impacts to aquatic organisms. This provided a unique opportunity to evaluate the effect on smolt survival. In-river (IR) migrating juvenile yearling Chinook released at Bonneville Dam with acoustic tags during periods when TDG exceeded 120% received estimated maximum exposures of 134% TDG. Subsequent daily survival rates in the lower river and plume were reduced by 0.06 d^-1 (SE=0.01) and 0.15 d^-1 (SE=0.05) relative to IR migrant fish released when TDG < 120%. Transported smolts (T) released 10-13 km below Bonneville Dam had lower maximum exposure levels (126%) and experienced no difference in daily survival rates relative to unexposed smolts. River temperature levels and trends in turbidity and disease prevalence between releases of high and low exposure smolts were not consistent with the observed effects on survival rates. We conclude that smolts may suffer from chronic effects of elevated TDG exposure while migrating through the Columbia River and plume. Consideration should be given to measuring these survival losses in an explicit experimental framework that isolates possible confounding factors.