Monitoring native, resident nonsalmonids for the incidence of gas bubble trauma downstream of Snake and Columbia River Dams, 2021

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, resident nonsalmonid
(NRN) fishes for gas bubble trauma (GBT) became a requirement. The primary objective of this
work was to measure the incidence and severity of GBT in NRN fishes resulting from increased
juvenile fish passage spill and associated levels of TDG during the spring spill period. A
secondary objective was to measure the incidence of GBT in incidentally collected juvenile
salmonids when NRN sample size targets were met. NRN fishes were collected downstream
from Bonneville, McNary, and Ice Harbor dams and examined for the incidence and severity of
GBT in 2021. Fish were collected at each location weekly (6 April to 17 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, and in all weeks
the minimum sample size of 50 fish was met and in most weeks the target sample size of 100
fish was met. 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, weekly GBT incidences ranged from 0 to 1.0%
downstream from Bonneville Dam, 0 to 6.2% downstream from McNary Dam, and 0 to 1.9%
downstream from Ice Harbor Dam. Except for one three-spined stickleback (Gasterosteus
aculeatus) collected downstream of Bonneville Dam, the only NRN species that showed signs of
GBT was sculpin spp. GBT was observed in sculpin in body locations other than the unpaired
fins and eyes (i.e., SMP criteria). If GBT incidence in all areas on the fish (i.e., paired fins,
unpaired fins, eyes, body) are combined, then weekly GBT incidence rates increase and range
from 0 to 4.3% downstream from Bonneville Dam, 0 to 15.4% downstream from McNary Dam,
and 0 to 4.7% downstream from Ice Harbor Dam. This illustrates the effect of using different
criteria to determine the incidence of GBT in NRN fishes. It also shows how the proportion of a
species in a sample that is more prone to show GBT can influence GBT incidence rate. On a
number of occasions, incidental catch of subyearling fall Chinook salmon were examined for
GBT downstream of Bonneville Dam but none showed any signs. The DG was generally below
120% and never reached the 125% gas cap during the spring spill season, which may be why
GBT incidence rates were so low as past research has shown that GBT signs in NRN fishes are
relatively low below this TDG level.