Rainbow trout was the target species for monitoring, and individuals were primarily collected from three reservoirs (Mountain View Reservoir, Lake Billy Shaw, and Sheep Creek Reservoir) in May 2007, June 2009 and 2013, and July 2024. Fishing methods varied by year and site, and included the use of gill nets, fyke nets, hook and line, and electrofishing (Chasmar and Murray, 2026). Upon capture, fish were weighed and measured from the anterior-most part of the fish (head) to the tip of the longest caudal fin ray (tail). During measurement, the lobes of the caudal fin were depressed dorsoventrally to determine the longest portion of the tail for a measurement of total length. Rainbow trout were additionally collected directly from the trout farms that supply fish for stocking, to measure the background concentration of fish being introduced into the system. In 2007, 2009, and 2013, hatchery-raised rainbow trout were collected from the BCTF, and in 2024, from the DSTF.

In 2007, 2009, and 2013, fish were euthanized and frozen within 24 hours of capture. A 1-inch skinless fillet was taken from each fish just below the dorsal fin (Williams and others, 2015) and evaluated using the methods described in the “Laboratory Evaluation” section. In 2024, an attempt was made to sample muscle tissue using non-lethal techniques. After fish were collected on site, two muscle biopsy plugs were collected from each fish (one plug from each side), according to the methodology outlined in EPA (2023a). After clearing a small area of scales, biopsy muscle plugs were collected with a sterile, 8-millimeter-diameter biopsy punch to an approximate equal depth of 8 millimeters, placed in a glass vial, and immediately placed in a freezer. Biopsy methods have been approved in an independent quality assurance project plan (EPA, 2023b). Fish were then placed in a live well to recover from the biopsy and released when possible. If a fish did not survive the biopsy plug sampling, a 1-inch piece of muscle fillet was collected and frozen so the results of the fillet and plug method could be compared. Though rainbow trout were the focus of this study, muscle samples were collected from other species using these methods (fillets in 2007, 2009, and 2013; plugs in 2024) if they were encountered as bycatch. These included three fish species that are readily caught and consumed: yellow perch, smallmouth bass, and largemouth bass.

To assess Hg concentrations in rainbow trout prey species, macroinvertebrate samples were collected in 2008 using a D-frame kick-net, identified and sorted to the lowest practical taxonomic level in the field, and composited into a sample where adequate mass was present for metals analyses. Taxa collected were Pteronarcys californica (giant stoneflies), Hydropsychidae sp. (net-spinning caddisflies), and Limnephilidae sp. (northern caddisflies). Sample preparation and handling followed the methods outlined in Crawford and Luoma (1993). The number of macroinvertebrate individuals composited into a sample can be found in Chasmar and Murray (2026). In 2008, whole-body samples were also collected for Paiute sculpin, with each sample composited from 10 individual fish.

Although the three Duck Valley Reservation reservoirs were the focus sites in this study, some samples were previously collected at additional sites: Wild Horse Reservoir in 2007 and four sites on the Owyhee River in 2008 (fig. 1A). The four Owyhee River sites are abbreviated as OR-1 (farthest upstream), OR-2, OR-3, and OR-4 (farthest downstream) within this report for the reader’s convenience. For readers intimately familiar with the study area, a longer site code is also denoted in figure 1A as follows: OR-1 is equivalent to site RIZBI0A, OR-2 to DV0100, OR-3 to DV0122, and OR-4 to DV0103. A complete list of sampling location metadata is compiled in table 2.

Total Hg is analyzed in fish tissue rather than MeHg because total Hg concentrations serve as a reliable proxy for MeHg, which constitutes most Hg in fish tissue, and because total Hg laboratory analyses are less expensive (Bloom, 1992). For the purposes of comparison with the EPA MeHg criterion, total Hg in fish tissue was subsequently assumed to be equivalent to MeHg. Fish tissue samples from 2007, 2009, and 2013 were sent to the USGS M3 Research Laboratory (MRL) in Middleton, Wisconsin. Samples were analyzed according to EPA method 7473 (EPA, 2007), with Hg reported on a dry weight basis, with a minimum detection limit of 0.008 milligram per kilogram of dry weight. The USGS MRL obtained percent moisture values from the sample mass before and after freeze-drying. Hg concentration reported on a wet weight basis is calculated as:Hgww=Hgdw×(100−%moisture)100,(1)where

Samples from 2024 were analyzed in-kind by the EPA Region 9 laboratory in Corvallis, Oregon, with Hg reported on a wet weight basis. The laboratory uses EPA Method 7473 (SW-846) as the reference method for determining Hg concentration (EPA, 2007). Detailed analytical procedures, including deviations from the reference method, are documented in a laboratory standard operating procedure (SOP 535; EPA, 2023c). Analytical results are reported in milligram total Hg per kilogram of wet weight, with a method detection level of approximately 0.0125 mg/kg ww, with suitable sample mass provided. Analytical methods by the EPA Region 9 laboratory have been approved in EPA (2023b), a quality assurance project plan. Because biopsy plugs were submitted to the laboratory in 2024, the sample mass did not allow for laboratory analysis of percent moisture. The 2024 Hg results can thus not be converted to a dry weight basis.

Macroinvertebrate samples were analyzed for total Hg and other trace metals by the USGS Georgia Water Science Center Sediment-Partitioning Research Laboratory in 2008. The laboratory used acid digestion with quantification by hydride generation atomic absorption (Elrick and Horowitz, 1985). In addition to Hg, the full suite of analytes reported for macroinvertebrate samples comprises the following: copper, lead, zinc, cadmium, chromium, cobalt, nickel, phosphorus, arsenic, selenium, iron, manganese, aluminum, titanium, and percent moisture. At the time of writing, this laboratory is no longer in operation and the USGS Georgia Water Science Center is part of the South Atlantic Water Science Center.

All of the laboratories practice standard quality assurance and quality control (QAQC) procedures. Laboratory QAQC protocols include the use of certified reference materials, blanks, matrix spikes, calibration standards, and duplicates. Field replicates have also been submitted in various sampling years. Data quality objectives have been reviewed in each sampling year, and the laboratory QAQC metrics of 2007, 2009, and 2013 are discussed in detail in Williams and others (2015). The QAQC data for all sampling years are compiled in the accompanying data release to this report (Chasmar and Murray, 2026). In 2024, the EPA Region 9 laboratory used two certified reference materials in analytical runs, Lake Superior Fish Tissue (SRM-1946, certified Hg concentration of 0.433 mg/kg ww) and oyster tissue (SRM-1566b, certified Hg concentration of 0.0371 mg/kg ww), and reported recoveries of 93−98 percent (Chasmar and Murray, 2026). Laboratory duplicates in 2024 had a relative difference of 2−11 percent (Chasmar and Murray, 2026). Three field duplicates (a second sample taken from the same individual fish in the field) submitted in 2024 were within a 12 percent relative difference (Chasmar and Murray, 2026). All 2024 blanks and calibration standards passed internal laboratory QAQC checks. Method comparison results of 2024 biopsy and fillet samples are discussed in the “Results and Discussion” section.

For assessment of interannual trends in rainbow trout Hg concentrations at each Duck Valley Reservation reservoir, Hg concentrations were first normalized by fish length (by taking the ratio of Hg concentration divided by fish length) and log-10 transformed, to account for correlations between length and Hg and heteroscedasticity of the sample data. A Mann-Kendall test was performed on the normalized, transformed data to determine monotonic changes in Hg concentrations over time (Mann, 1945). The Mann-Kendall test was performed on each reservoir using the yearly average of the normalized and log-transformed data using the R function “MannKendall” (McLeod, 2022) in R version 4.5.2 (R Core Team, 2025), and a p-value was calculated from the resulting S statistic (S). The null hypothesis of “no change” is rejected if S is significantly different from zero (p<0.05).

Comparisons between species were made using measured concentrations without normalizing by length or log-10 transforming, because the actual Hg concentration is relevant to what people would be consuming from fish, and different fish species have different length distributions. Results of the measured Hg concentrations without data transformation are presented in report figures and tables to clearly compare concentrations to the EPA MeHg criterion of 0.3 mg/kg ww. Although measured concentrations are reported, the statistical analysis of site differences in rainbow trout Hg concentrations was done on length-normalized and log-10 transformed data. To test statistical differences between species and sites, an analysis of variance was performed, followed by a Tukey’s Honest Significant Difference test (Tukey, 1949). Significance was assessed at a 95-percent confidence interval (p<0.05). In the “Results and Discussion” section, “measured Hg” refers to the lab-reported measured Hg concentrations in mg/kg ww, and “normalized Hg” refers to Hg concentrations after they have been length-normalized and log-10 transformed. References to Hg concentrations hereafter refer to total Hg.

Measured Hg concentrations of rainbow trout muscle tissue by reservoir and year are summarized in table 3 and plotted in figure 3 so that Hg concentrations can be easily compared to the EPA MeHg criterion of 0.3 mg/kg ww. In 2024, one sampled rainbow trout from Sheep Creek Reservoir exceeded the EPA MeHg criterion, measuring 0.32 mg/kg ww. All other sampled rainbow trout in the three Duck Valley reservoirs (Mountain View Reservoir, Lake Billy Shaw, and Sheep Creek Reservoir) were below the EPA criterion in 2007, 2009, 2013, and 2024 (n=159).

Measured Hg was positively correlated with fish length (coefficient of determination [R²]=0.42, p<0.001; fig. 4), and fish sampled in 2024 were longer on average than those sampled in earlier years (2007, 2009, and 2013). The average concentration of measured Hg in the sampled 2024 fish is thus expected to be higher than in other years prior to normalizing the data by length. Results of the Mann-Kendall trend test on normalized (length-normalized, log-10 transformed) Hg concentrations showed a significant increasing trend in Hg concentration in Sheep Creek Reservoir rainbow trout from 2007 to 2024 (Kendall’s tau=1, S=6, p<0.05). The mean measured Hg concentration of sampled fish in Sheep Creek Reservoir increased from 0.059 mg/kg ww in 2007 to 0.177 mg/kg ww in 2024, which is still below the EPA criterion of 0.3 mg/kg ww (table 3). No significant trends were observed in the normalized Hg concentration of rainbow trout in Mountain View Reservoir or Lake Billy Shaw (p>0.49 for each reservoir).

Rainbow trout are stocked by the Sho-Pai Tribes and sourced from two fish hatcheries. The BCTF provides small fish, and was sampled in 2007, 2009, and 2013. All sampled fish from the BCTF had measured Hg concentrations below 0.05 mg/kg ww (Williams and others, 2015). The Sho-Pai Tribes began stocking the reservoirs with large, catch-ready rainbow trout from the DSTF at some point between 2013 and 2024, in addition to the smaller fish from the BCTF. The large hatchery fish from the DSTF had higher average measured Hg concentrations (0.089 mg/kg ww) than the smaller fish from the BCTF (0.011–0.029 mg/kg ww, depending on the sample year; table 3; fig. 5). We would expect larger fish to have a higher Hg burden on the basis of size correlation. However, the normalized Hg concentration of DSTF fish tissue was still statistically higher than that of BCTF fish (p<0.001; table 4). All hatchery fish, regardless of source, had Hg concentrations well below EPA’s recommended criterion of 0.3 mg/kg ww (fig. 5).

In 2024, Mountain View Reservoir rainbow trout had statistically lower normalized Hg concentrations than Lake Billy Shaw or Sheep Creek Reservoir (p<0.01; table 4), although it is possible this was a sampling artifact. In 2024, Lake Billy Shaw and Sheep Creek Reservoir had statistically higher normalized Hg concentrations than hatchery fish from the DSTF (p<0.001; table 4), but the same was not true for Mountain View Reservoir (table 4). Exact fish stocking schedules could not be confirmed at the time of sampling, but normalized Hg concentrations in rainbow trout from the DSTF and Mountain View Reservoir were statistically similar (p=0.412; table 4), indicating that recently stocked fish may have been sampled at Mountain View Reservoir in 2024. Lake Billy Shaw and Sheep Creek Reservoir normalized Hg concentrations were statistically similar to each other in 2024 (p=0.927; table 4) and higher than stocked fish from the hatchery (p<0.001; table 4).

In comparing the full data distribution of Hg in rainbow trout among reservoirs across all sample years, there is not a statistically significant difference in normalized Hg concentrations between the three Duck Valley Reservation reservoirs (table 4). The reservoirs may have similar Hg methylation rates to produce similar bioaccumulation values in rainbow trout, but additional water sampling would be necessary to characterize the Hg cycling within each reservoir. Interannual variability is also probable owing to variability in the population and fish stocking schedules across the sampling years. It is also worth noting that rainbow trout were sampled in slightly different seasons in each sample year: late spring (May 2007), early summer (June 2009 and 2013), and midsummer (July 2024). Interannual variability could be introduced by seasonal shifts in diet between the sampling months, or seasonality of MeHg production (Mills and others, 2018; Willacker and others, 2025). Thus, the within-site variability between sample years may make it difficult to discern statistically significant differences between the reservoirs when averaging the full dataset.

Measured Hg concentrations of biopsy and fillet muscle samples from the same individual fish were generally comparable (fig. 6). Eight method comparison samples collected in 2024 (biopsy versus fillet) were within a 20 percent relative difference. One sample had a relative percentage difference between 20 and 25, but the absolute difference in measured Hg was small (less than 0.02 mg/kg ww absolute difference). One sample had fillet results that differed from the biopsy result by 38 percent (Hg concentrations of 0.22 mg/kg ww versus 0.15 mg/kg ww, respectively), and it is possible that sample handling influenced the difference rather than true concentration differences (for example, if sample drying impacted percent moisture). We did not observe any directional bias in one method result always being lower or higher than the other (fig. 6). Other studies have also shown that biopsy results are representative of whole muscle fillet results (Stahl and others, 2021). Thus, results between methods can be considered equivalent, and either method can be confidently used to determine the Hg concentration of muscle tissue within a fish.

We chose to utilize the biopsy plug method in 2024 because it offers several advantages compared to the fillet method for sampling muscle tissue in fish. First, it is a non-lethal technique, theoretically allowing fish to be released unharmed after sampling. Additionally, the biopsy plug method streamlines the logistics of sample preparation, as taking biopsy plugs directly in the field eliminates the need to transport, store, and thaw large fish. However, we experienced some notable drawbacks to the biopsy plug method. The small size of the biopsy sample mass relative to a 1-inch muscle fillet limits the scope of analysis, restricting analysis to a single analyte (in this case, total Hg) and excluding analyses of percent moisture or additional metals. Non-lethal fishing methods can present sampling challenges, requiring more time in the field and incurring higher costs for specialized supplies. Taking the biopsy plug can be particularly stressful for certain species, and we experienced a high mortality rate of rainbow trout when taking biopsy plugs in 2024. Sampling took place in late July, and it is conceivable that the high air and water temperatures inhibited the fish’s ability to recover from a biopsy plug. Although the biopsy plug method has benefits, careful consideration of its drawbacks is essential for effective implementation in fish tissue sampling.

Comparisons between species were made on measured Hg concentrations without length normalization or log-10 transformation, because the measured Hg best represents what people would consume, and the species differ in their length distributions. All sampled fish were of reasonable consumption size, and the length distribution of sampled fish by species, in centimeters, was as follows: yellow perch, 24.5–28.3; smallmouth bass, 23.7–48.6; largemouth bass, 26.2–47.3; and rainbow trout, 19.0–69.0 (Chasmar and Murray, 2026). The small number of total fish caught for many sites and species limits statistical interpretation. One rainbow trout from OR-3 had a high measured Hg concentration of 0.58 mg/kg ww (table 3; fig. 7). Upon further investigation, this sample had an abnormally low water content of 43 percent moisture compared to a median of 78 percent observed in the full dataset of rainbow trout samples (Chasmar and Murray, 2026), and it is possible that the sample was allowed to dry at some point in the sample collection or lab analysis process. Because wet weight Hg calculations depend on the percent moisture of the tissue (eq. 1), the sample result from this single fish may be biased high. This measured Hg concentration (0.58 mg/kg ww) is reported and plotted in this report (table 3; fig. 7) but is flagged in the data release (Chasmar and Murray, 2026) and omitted from statistical tests.

Largemouth bass and smallmouth bass had higher measured Hg concentrations than rainbow trout (p<0.001 and p=0.007, respectively; table 4). Although only two largemouth bass were sampled, both had higher measured Hg than any other species (p<0.001; table 4) and exceeded the EPA MeHg criterion of 0.3 mg/kg ww (fig. 7). Despite being smaller than rainbow trout, smallmouth bass had higher measured concentrations of Hg, on average. Measured Hg concentrations in yellow perch were higher than in rainbow trout, although the difference was not quite significant at the 95-percent confidence interval (p=0.056; table 4). Given that sampled yellow perch were much smaller on average than sampled rainbow trout, the near-significant value indicates that Hg concentrations in yellow perch likely pose a higher risk to consumers than similarly sized rainbow trout. Yellow perch Hg concentrations were statistically similar to smallmouth bass (p=0.614; table 4). Data distributions of measured Hg in all fish species at all sampled sites are presented in figure 7.

Rainbow trout concentrations in the Duck Valley reservoirs may be lower than those of other species, owing in part to the high stocking frequency of the reservoirs. The unstocked species (yellow perch, smallmouth bass, and largemouth bass) may better represent the full bioaccumulation potential of these reservoirs. The unstocked species may also pose a greater risk for human consumption, especially for the highest trophic levels with a higher bioaccumulation potential, such as largemouth bass. Although stocking practices play a role, Trout (Salmonidae) family fish commonly have a lower Hg muscle concentration than fish in the Sunfish (Centrarchidae) and Perch (Percidae) families when looking at data across the United States from 1985 through 2005, and diet is an important factor to species-level bioaccumulation (Scudder and others, 2009). The high Hg concentrations measured in largemouth bass are not surprising, as they are a large predatory fish species that are expected to bioaccumulate a higher level of Hg, and the IDHW has an active statewide fish advisory in effect for bass caught in Idaho at the time of writing this report. Rainbow trout were also classified in a slightly lower trophic level (primarily insectivore) than yellow perch, smallmouth bass, and largemouth bass (insectivore-piscivore) within the Hells Canyon Complex in Idaho and Oregon (Richter, 2001), although many factors (such as food availability, fish size, habitat characteristics, and species populations) could affect the relative trophic position of these species in Duck Valley Reservation reservoirs.

One rainbow trout out of 15 sampled in Wild Horse Reservoir in 2007 exceeded the EPA MeHg criterion, measuring 0.321 mg/kg ww, and 2 other fish were close to this value (fig. 7). The normalized Hg concentrations of rainbow trout in Wild Horse Reservoir were not statistically different from any Duck Valley Reservation reservoir (table 4). Rainbow trout were sampled at three Owyhee River sites in 2008, but we did not statistically compare these concentrations to the reservoir sites because sample sizes were small. Additionally, the rainbow trout sampled from the river were smaller than those in the reservoirs on average, and a comparison of normalized Hg concentrations could be biased. One sampled rainbow trout at OR-3 had a measured Hg concentration of 0.58 mg/kg ww that we flagged as potentially erroneous but left in table 3 and figure 7. However, a second rainbow trout sampled at OR-3 measured 0.42 mg/kg ww, which was the highest measured Hg concentration observed in rainbow trout muscle tissue at any site. The measured Hg concentrations at OR-1 and OR-2 were low in comparison to OR-3. Only one fish sample, smallmouth bass, was taken at the farthest downstream Owyhee River site (OR-4). Although statistical determinations cannot be made from a sample size of one fish, this sample had a higher measured Hg concentration than any other smallmouth bass sampled at other sites (fig. 7). The extent of Hg exposure for anglers fishing the Owyhee River within the Duck Valley Reservation is less well characterized, as sampling in this area has been limited. It is feasible that the Owyhee River has different Hg cycling dynamics because the river has different hydrologic controls (depth, flow, and so on) and presumably different water chemistry than the reservoirs. The trophic position of rainbow trout in the Owyhee River is likely to differ from that in the reservoirs, because variations in macroinvertebrate communities will subsequently change the food web structure. Wild Horse Reservoir upstream of the Owyhee River sites is stocked with rainbow trout, and some of those fish may pass downstream over the spillway into the river, which could partially explain the lower measured Hg concentrations at OR-1 and OR-2 relative to downstream sites. Trout (Salmonidae) family species from streams in the United States sampled between 1985 and 2005 had a mean Hg concentration of 0.109 mg/kg ww, ranging from 0.014 to 0.588 mg/kg ww across 53 averaged sites (Scudder and others, 2009). The distribution of Hg in rainbow trout sampled from river or reservoir sites in this study was comparable, with a mean Hg concentration of 0.106 mg/kg ww, and a range of 0.013–0.42 mg/kg ww (excluding the flagged 0.58 mg/kg ww concentration at OR-3; Chasmar and Murray, 2026)

Measured Hg concentrations of all lower trophic level species are compiled in table 3 and figure 8. The Hg concentration in Paiute sculpin collected in 2008 ranged from 0.04 mg/kg ww in Owyhee River site OR-2 to 0.13 mg/kg ww in OR-1 (table 3; fig. 8), both below the EPA MeHg criterion of 0.3 mg/kg ww. In the Western United States and Canada, Eagles-Smith and others (2016) found that the median Paiute sculpin Hg concentration from 7 distinct sites between 1969 and 2014 was 0.092 mg/kg ww (maximum 0.236 mg/kg ww) and determined that sculpin as a generalized group had a geometric mean of 0.06 mg/kg ww. Thus, the two Paiute sculpin samples collected in the Owyhee River seem to be in a low to normal range. Sculpin are also considered to be primarily insectivorous and would be expected to have lower Hg concentrations than piscivorous fish owing to their lower trophic position.

Measured Hg in macroinvertebrates ranged from 0.004 to 0.044 mg/kg ww, depending on the site and taxa (table 3; fig. 8). Because we only have one result per site and species for the year sampled, we did not do any interspecies statistical comparisons. Individual taxa are plotted in figure 8 in reference to the EPA criterion, although this criterion does not apply to macroinvertebrates, and we would expect macroinvertebrate Hg concentrations to be much lower than those of predatory fish. We can assume most of the total Hg is in the methyl form in fish (Bloom, 1992), but the same assumption does not hold for macroinvertebrates, and the functional feeding group of the invertebrate species impacts the ratio of MeHg to total Hg (Clayden and others, 2014). Studies from other regions illustrate this variability: in streams of the Adirondack Mountains in New York, the ratio of MeHg to total Hg was only 52 percent in shredders (for example, northern caddisflies), and this value varied significantly between taxa, among sites, and seasonally (Marziali and others, 2021). Therefore, we cannot infer the MeHg available to fish without directly measuring MeHg in macroinvertebrates (as opposed to total Hg), and the measured Hg in macroinvertebrates should be considered the maximum possible concentration of MeHg (if all Hg is in the form MeHg). The measurements we have of Hg concentration in macroinvertebrates may help us to compare sites and track temporal trends in the base of the food web. The farthest downstream Owyhee River site (OR-4) had the highest concentration of total Hg in any macroinvertebrate: 0.044 mg/kg ww in northern caddisflies, which was 2.3 times the concentration of total Hg in northern caddisflies at OR-3 (0.018 mg/kg ww; table 3; fig. 8). Consistent sampling of the same taxa at multiple sites could help to explain the higher Hg concentrations seen downstream at Owyhee River sites OR-3 and OR-4 as compared to the upstream Owyhee River sites OR-1 and OR-2; present data do not allow for this analysis owing to limited sample size. If more sampling confirmed a higher Hg concentration in biota in the downstream river sites, the mechanisms for increased MeHg availability or uptake could be further explored. For example, land-use changes from upstream to downstream in the Owyhee River, such as increased agriculture and wetland flow returns, could affect MeHg production.

Dry weight concentrations of the following analytes in 2008 macroinvertebrate samples are provided in the associated data release to this report: copper, lead, zinc, cadmium, chromium, cobalt, nickel, phosphorus, arsenic, selenium, mercury, iron, manganese, aluminum, and titanium (Chasmar and Murray, 2026). The additional metals analyzed in these samples are not discussed in detail in this report, but aquatic insects are useful bioindicators of trace element contamination in streams (Cain and others, 1992). The 2008 data could provide a baseline level for the Owyhee River sites if any contamination occurs, such as new mining activity or other disturbances proximal to the Duck Valley Reservation.

Measuring Hg in fish tissue is ideal for assessing exposure risk to human populations because doing so directly measures the amount of Hg people consume via those fish. Because rainbow trout are widely harvested and consumed on the Duck Valley Reservation, they remain a priority species for evaluating human exposure to Hg. A robust dataset spanning sampling in 2007, 2009, 2013, and 2024 supports future assessment of temporal trends in Hg concentrations in rainbow trout muscle tissue in Mountain View Reservoir, Lake Billy Shaw, and Sheep Creek Reservoir. This study also collected limited samples of other commonly consumed fish species, such as yellow perch, smallmouth bass, and largemouth bass. This study’s small sample sizes for non-rainbow trout species limited statistical characterization of the Hg in these species as it relates to human health risk. However, concentrations above the EPA MeHg criterion were observed during this study (0.396 mg/kg ww and 1.033 mg/kg ww in largemouth bass), indicating that additional data could improve understanding of Hg exposure across the fishery. Fish collected from the Owyhee River (sites OR-3 and OR-4) also showed elevated Hg concentrations, with three of four individuals exceeding the EPA MeHg criterion. Increased sample sizes of fish harvested from the Owyhee River, including within the Duck Valley Reservation, would allow for more robust evaluation of Hg concentrations in riverine rainbow trout, particularly in the absence of stocking management practices.

Biopsy plugs, tested in 2024, were not a viable sampling method because they resulted in longer sampling times, higher costs, and a high rate of fish mortality. The fishing methods available to the Sho-Pai Tribes in 2024 consisted of gill nets, fyke nets, and hook and line, with gill nets being the most efficient method (albeit the most lethal). Biopsy plugs may be worth trying in future monitoring under two conditions. First, better ambient conditions (cooler air and water temperatures), which may improve the fish’s ability to recover. Ambient weather was very hot during the July 2024 sampling, which likely exacerbated the stress to fish upon receiving a biopsy punch. Second, a citizen fishing derby is a scenario in which anglers could bring fish they intended to catch-and-keep to a sampling station. In this setting, a biopsy plug may be the preferred sampling method because it leaves most of the muscle fillet tissue intact for the angler to take home and eat. Otherwise, it may be more feasible to sample with a traditional fillet method in future studies, so humane practices of fish euthanasia could be employed and small sample masses would not limit analytical capabilities.

Fish sampling can be difficult and expensive, and Hg concentrations can vary with natural population differences, stocking schedules, and the size distribution of sampled fish. Hg concentrations in fish also represent a composite of long-term exposure to Hg, and a large or rapid change in Hg sources or availability within the system may not be immediately reflected in fish tissue. Complementary data types, such as water-column Hg and MeHg, can more directly characterize the Hg and MeHg occurrence and dynamics (for example, methylation and demethylation), which influence fish tissue concentrations. Water sampling for Hg isotopes can help identify the relative percentages of Hg originating from legacy mining activity versus more regional atmospheric deposition (Baldwin and others, 2024; Lopez and others, 2024). Sampling of sediment, periphyton, and lower trophic level biota can characterize the movement of MeHg from the water into and through the food web, thus informing the MeHg available for fish, and subsequently humans. Food web information can also improve understanding of the diet and trophic position of sport fish (such as trout, bass, and perch) in these reservoirs and inform which species or size classes pose the greatest risk. Hg isotopes within biological samples can further clarify the sources of Hg entering the system (Lopez and others, 2025). Any data collected on Hg or other metals within the Reservation can also serve as baseline information if future changes occur in the system, such as new mining activity, land-use changes, or shifts in management practices.