Effects of Total Dissolved Gas on Chum Salmon Fry Survival, Growth, Gas Bubble Disease, and Seawater Tolerance

Salmon that spawn downstream of hydroelectric dams in the Columbia River are exposed to dissolved gas created when water is spilled over the dam.  This dissolved gas, if high enough, can be harmful to pre-emergent salmon fry that are developing in the gravel.  The objective of this study was to document the effect of total dissolved gas (TDG) levels ranging up to 130% on survival and growth, gas bubble disease (GBD), and seawater adaptation in chum salmon fry of varying developmental stages. Three developmental stages of chum salmon fry (early, middle, and late life stages) were exposed to 6 levels (<100%, 105%, 115%, 120%, 125%, and 130%) of TDG.  Mortality was monitored through emergence and increased in all stages as dissolved gas concentrations exceeded 115% TDG.  The concentration resulting in 50% mortality (LC50) was 128.7% TDG (95% CI 127–130% TDG) in the early and middle stages; an LC50 was not estimated for the late stage.  About 90% of the early stage fry tolerated 120% TDG twice as long as the middle stage, and almost three times as long as the late stage.  There was no evidence that TDG level or exposure time significantly affected growth.  At 48 hours post-exposure and at emergence, there were significantly more gas bubbles in the bodies of exposed fish than in control fish, with most bubbles in the nares, gastrointestinal tracts, and yolk sacs.  X-ray images at emergence showed that fish were found to have one-half the maximum ratio of bubble to body area at 118% TDG (95% CI 116–120% TDG) in the middle and late stages, and at 120% TDG (95% CI 119–121%) in the early stage.  Seawater tolerance of post-emergent fish did not differ among TDG treatments, but early stage fish showed significantly higher ratios of chloride cell counts on gill filaments when compared to late stage fish, suggesting this stage was more tolerant to seawater at the time of emergence.  Taken together, our findings suggest that TDG concentrations above 115% resulted in the highest mortality and the most signs of GBD in each life stage, and that tolerance to elevated TDG diminished as fish approached emergence.  This information will be used to determine if variances to state water quality criteria will be necessary and protective of chum salmon fry present in the gravel below Columbia River hydroelectric dams during early spring spill operations.