Nonsalmonid gas bubble trauma investigations

Abstract

From 2020 to 2024, 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 nonsalmonids for gas bubble trauma (GBT) became a requirement. The primary objective of this work and report was to measure the incidence and severity of GBT in nonsalmonids resulting from increased juvenile salmonid passage spill and associated levels of TDG during the spring spill period in 2024. Nonsalmonids were collected downstream from Bonneville, McNary, Ice Harbor, and Lower Granite dams and examined for the incidence and severity of GBT. Fish were collected at each location weekly during the spring spill period (3 April to 20 June) 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 low fish collections. Collected fish were examined for GBT according to the criteria and protocol established for the regional smolt monitoring program (SMP). In 2024, a new method of calculating GBT incidence rate was requested by the Washington Department of Ecology but the Oregon Department of Environmental Quality retained the calculation method that was used in past years. GBT incidence rates and severity (evaluated using both calculation methods) were low to moderate in most weeks. GBT rates never exceeded thresholds that would have triggered a reduction in spill. We found no strong relationship between GBT incidence and TDG due to exposure history and interspecies susceptibility to elevated TDG that could not be quantified. Sculpin (genus Cottus) was the main species collected at all locations but northern pikeminnow composed a significant portion of the catch at Ice Harbor and Lower Granite dams early in the season. After four years of field sampling, we conclude 1) the occupancy and catchability of native nonsalmonids is difficult and highly variable in dam tailraces, which in turn, often makes meeting collection targets challenging, 2) GBT incidence rate calculation methods can be modified to account for differing species abundances in the daily catch, 3) native nonsalmonids differ in their sensitivity to elevated TDG, and 4) predicting the incidence of GBT in native nonsalmonids from field measurements of TDG will remain difficult given the inherent variability in GBT incidence both within and between species.