Monitoring native nonsalmonids for the incidence of gas bubble trauma downstream of Snake and Columbia River dams during the spring spill season, 2022

Abstract

In 2020, a new spill program was implemented to aid the downstream passage of juvenile salmonids at mainstem dams on the Snake and Columbia rivers. Under this program, the total dissolved gas (TDG) cap was increased to 125% and monitoring of native nonsalmonids for gas bubble trauma (GBT) became a requirement. The primary objective of this work was to measure the incidence and severity of GBT in native nonsalmonids resulting from increased juvenile fish passage spill and associated levels of TDG during the spring spill period. Native nonsalmonids were collected downstream from Bonneville, McNary, Ice Harbor, and Lower Granite dams and examined for the incidence and severity of GBT in 2022. Fish were collected at each location weekly (4 April to 13 June) during the spring spill period by backpack electrofishing and beach seining. Washington and Oregon state water quality agencies established minimum and target sample sizes for monitoring, but the minimum sample size of 50 fish and target sample size of 100 fish were not met in all weeks at individual projects due to high water flows and resulting low fish collections. Collected fish were examined for GBT according to the criteria and protocol established for the regional smolt monitoring program (SMP). Overall, GBT incidence and severity rankings were low and did not exceed the thresholds that would have triggered changes to the spill program. Using SMP criteria, maximum weekly GBT incidences were 1.9% downstream from Bonneville Dam, 8.6% downstream from McNary Dam, 7.7% downstream from Ice Harbor Dam, and 3.4% downstream from Lower Granite Dam. In contrast to 2021, several species showed signs of GBT in 2022, and GBT was commonly observed in fins and body locations other than the unpaired fins and eyes (i.e., SMP criteria). In general, the observed signs of GBT according to SMP criteria were not severe (i.e., ranks 1 and 2). Although TDG reached the gas cap, particularly late in the spring spill season, GBT incidence rates were not high probably due to factors such as habitat-related variability in TDG, species composition and varying tolerance to high TDG, low power to detect GBT incidence at small sample sizes, and unknown exposure history.