Willacker et al. 2023 — Reservoir stratification and mercury in Smallmouth Bass along the Snake River
This study examined total mercury (THg) concentrations in Smallmouth Bass from reservoir, tailrace, and free-flowing habitats along an 863 km segment of the Snake River (22 impoundments), and assessed the role of reservoir thermal stratification in modulating Hg bioaccumulation. Across three fish size-classes, THg concentrations in reservoirs and tailraces averaged 76% higher than in free-flowing river segments. Among reservoir types, THg was highest in inconsistently stratified reservoirs (47% higher than annually stratified, 144% higher than unstratified). Tailraces below stratified reservoirs had higher THg than those below unstratified or inconsistently stratified reservoirs. The fraction of fish exceeding human consumption benchmarks was 52–80% from stratifying reservoirs and downstream tailraces, compared to 6–17% where stratification did not occur.
Key numbers
- THg in reservoir and tailrace fish: 76% higher on average than in free-flowing segments
- Inconsistently stratified reservoirs vs. annually stratified: THg 47% higher
- Inconsistently stratified reservoirs vs. unstratified: THg 144% higher
- Fish exceeding human consumption benchmark (stratifying reservoirs + tailraces): 52–80%
- Fish exceeding human consumption benchmark (no stratification): 6–17%
- MeHg as % of THg: 95.3 ± 0.9% (n=110; confirms THg ≈ MeHg in muscle)
- Method detection limit: 0.0021 mg/kg ww (THg); 0.0019 mg/kg ww (MeHg)
- Study segment: ~853 km, 31 reaches, 3 habitats (reservoir, tailrace, free-flowing), Snake River, Idaho/Oregon
Fish size classes (THg as wet-weight muscle equivalents):
- < 150 mm total length (juvenile/1-year-old)
- 150–250 mm (first reproductive)
-
250 mm (harvestable)
Mean moisture content used for ww conversion: 78.4 ± 0.03% (n=1,814).
Methods (brief)
Electrofishing by Idaho Power Company; fish euthanized with MS-222, frozen at -20°C. Skinless axial muscle analyzed for THg (2015, 2020: USGS CERL, Nippon MA-3000, EPA Method 7473; 2013: converted from MeHg via Battelle Marine Science Lab, EPA Method 1630). Certified reference materials: DORM-4, TORT-3, IAEA-476; mean recovery 101.7 ± 0.4%. Reservoir stratification classification from Federal Energy Regulatory Commission documentation and published limnological assessments. Statistical analysis: mixed models in JMP; log-transformed THg; river mile and season as random effects.
Implications
Certification: Reservoir-sourced freshwater fish (and fish from tailraces below impoundments) carry elevated MeHg risk compared to free-flowing river fish; the effect is greatest where thermal stratification promotes anoxic conditions. Snake River recreational fish consumption advisories are warranted at 52–80% exceedance of human health benchmarks in stratifying reservoir zones.
Courses: Strong demonstration of how dam operations (specifically stratification management, e.g., destratification via aerators) can be a lever to reduce fish Hg; the stratification-type comparison provides a mechanistic framework for food-chain Hg management.
App: Freshwater fish from impounded systems (especially thermally stratifying reservoirs) warrant elevated MeHg flags; this is additive to the lake-stratification signal the app already has from other sources.