Biological Effects of TDG. The biological effects of TDG on aquatic life are monitored as part of the regional effort to control TDG throughout the Columbia River. The Fish Passage Center (FPC) administers the program, which samples downstream migrant juvenile salmon and steelhead and monitors several aspects of the fish migration, including the incidence of GBT. The FPC summarized the results of the past seven years of monitoring for GBT in a recent letter (FPC, 2003), as follows. “Based on seven years of data from the biological monitoring program, the average incidence of gas bubble disease signs has been low, although the state-allowed maximum TDG due to spill was 120% in the tailrace and 115% in forebays during periods of voluntary spill. A high percentage of the spill that did occur in some years was involuntary and often resulted in dissolved gas levels above the 120% waiver. The following graphs (Figure 2-24 and Figure 2-25) depict the incidence and severity of signs of GBT in fish collected for observation over the seven years, grouped in 5% TDG levels. Increases in the incidence of signs were observed with increases in the levels of TDG. The severity of signs also increased, but not until dissolved gas levels were above the 120 to 125% level. These data suggest that TDG concentrations above 125% may have had a negative impact on survival. These high TDG measurements are a function of uncontrolled spill that occurred in the hydro system because of flow in excess of the hydraulic capacity of the project, or due to spill in excess of generation needs. They are not caused by the implementation of the Biological Opinion Spill Program. All of the information collected to-date of survival and the benefits associated with spill indicate that spill provides a significant benefit to juvenile survival at levels up to 125% in the tailrace of the dam.” This benefit to survival is based on the observations in the region that juvenile salmon passing through spillways typically have a survival rate of better then 98%, whereas the survival rate for juvenile salmon passing through powerhouses is often less than 95%. Since no mortality to salmon migrating in the river has been observed at TDG levels below 125%, there is a survival benefit. In Figure 2-24 and Figure 2-25, the GBT symptoms are classified by rank. Rank 1 is if 1 to 5% of the fin or eye is covered with bubbles; rank 2 is assigned for 6 to 25 % area covered; rank 3 for 26 to 50 % area covered; and rank 4 for greater than 50 % are...
