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

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.