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Henry's Fork Macroinvertebrate Monitoring, 2020 & 2021

Updated: Jan 14, 2022


  • In 2020, HFF staff were unable to collect macroinvertebrate samples at Ashton, Ora, and St. Anthony due to Covid-19. HFF staff were able to follow strict Covid-19 precautions and safely sample the five Henry’s Fork long-term monitoring sites, as well as the Ora site, in 2021.

  • In 2020, mean abundance of macroinvertebrates at Last Chance nearly doubled from 2019 and had the second highest mean macroinvertebrate abundance in the seven-year record for the location. Additionally, percent EPT (percent mayflies, stoneflies, and caddisflies) at Last Chance increased from 2019 to 2020.

  • While macroinvertebrate abundances at Last Chance had high variability between replicate samples in 2020, anglers reported exceptional hatches in the upper Henry’s Fork during the summer and fall of 2020.

  • Mean abundance of macroinvertebrates at the Ora site declined by over 50% between 2019 (pre-Ora bridge construction) and 2021 (post-Ora bridge construction) but there were no changes in percent EPT, HBI, or Shannon diversity at the site. This suggests there were likely larger environmental forces impacting macroinvertebrate abundances at Ora other than bridge construction.

  • Barring any unforeseen delays, we anticipate receiving raw macroinvertebrate data back from the lab by early June in 2022, and will be able to provide macroinvertebrate summary information prior to peak fishing season.

To learn about the who, what, when, where, why, and how we collect aquatic macroinvertebrates, check out the blog by Dr. Van Kirk, What Do Macroinvertebrates (Aquatic Insects) Tell Us About the Henry's Fork.

What is new on the Henry's Fork?


Mean abundance of macroinvertebrates in the Henry’s Fork stayed relatively consistent between 2019 and 2021, and there were no statistically significant changes in abundances at any of the sites. Although none of the changes in abundance were significant by traditional statistical standards, the increases in abundance in the upper Henry’s Fork in 2020, particularly at Last Chance which doubled from the year prior to 45,225 individuals/m^2, were consistent with angler reports of exceptional hatches in the reach throughout the summer and fall in the upper Henry’s Fork.

Mean abundance at the Ora site (a location we started sampling in 2019 to monitor aquatic conditions below Ora bridge during construction) declined by 25,918 individuals/m^2 (53%) from 2019 (pre-construction) to 2021 (post- construction). The decline in abundance is fairly dramatic but the lack of change in HBI, percent EPT, and diversity at the Ora site suggest there were likely larger environmental forces impacting abundances at Ora other than bridge construction (discussed in further detail in the Interpretation Section of this blog).


Shannon diversity index is a common metric used in ecology to describe the diversity of organisms within a system. Essentially, the maximum Shannon diversity index is achieved when there are equal abundances of individuals for each taxa present. This maximum number increases as more taxa are present. For example, a sample with 50 individuals of Taxa A and 50 individuals of Taxa B would have a higher Shannon diversity index than a sample that had 90 individuals of Taxa A and 10 individuals of Taxa B. Additionally, a sample that had 25 individuals of Taxa A, 25 individuals of Taxa B, 25 individuals of Taxa C, and 25 individuals of Taxa D, would have a higher Shannon diversity index than a sample with 50 individuals of Taxa A and 50 individuals of Taxa B.

Shannon diversity index scores showed little change for all Henry’s Fork sites from 2019-2020 and for Ashton, Ora, and St. Anthony from 2019-2021. Diversity index scores declined for Flat Rock, Last chance, and Osborne in 2021 by up to 0.88 index score (32%), and declines in Shannon diversity were statistically significant at Flat Rock and Osborne compared to scores in 2019. When considering the positive changes in HBI and percent EPT at these locations, the decline in diversity index is not necessarily a bad thing (discussed in further detail in the Interpretation Section of this blog). Overall, Shannon diversity index scores ranged between 1.8-2.9 for Henry’s Fork locations which is relatively high for aquatic macroinvertebrate communities, and is comparable to scores seen in South Fork Snake River macroinvertebrate communities.

Hilsenhoff Biotic Index

The Hilsenhoff biotic index (HBI) is a metric that classifies habitat quality using the tolerance levels to organic pollution and habitat degradation for macroinvertebrate taxa found at the site. This is done by scoring taxa from 0 (intolerant to degradation) to 10 (tolerant to degradation), then averaging the scores of macroinvertebrates in the sample. Higher HBI scores (higher abundances of species with greater tolerance to habitat degradation) indicate more degraded aquatic habitat conditions. Conversely, lower HBI scores (higher abundances of species with lower tolerance to habitat degradation) indicate better aquatic habitat conditions. Click here for a complete description of HBI.

HBI scores fluctuated between 2019 to 2021 but all sites stayed in the “Fair” to “Excellent” habitat quality classifications. Flat Rock was classified as “Excellent” all three years while the other sites downstream of Island Park dam were classified as “Excellent” to “Fair”. Ora had the highest average HBI scores out of all the sites at 5.1 (“Fair” habitat classification) between 2019-2021. St. Anthony had a statistically significant decline from a record low HBI score of 3.2 (“excellent” habitat classification) in 2019 to roughly the six-year average HBI score of 4.6 (“good” habitat classification) in 2021.

Percent EPT (mayflies, stoneflies, and caddisflies)

Percent EPT is the percent of the individuals at the site that are mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera). Percent EPT had modest fluctuations at most sites between 2019-2021. Flat Rock and Osborne locations had slight declines in percent EPT in 2020 but percent EPT increased in 2021 back to levels comparable to 2019. Last Chance had an increase in percent EPT in 2020 but percent EPT decreased in 2021 back to levels comparable to 2019. There were no statistically significant trends within sites except at St. Anthony, which declined from a record high of 63% in 2019 to 34% in 2021. Percent EPT at St. Anthony in 2021 was slightly below the six-year average of 41% for the site.

Interpretation of 2020-2021 results

Flat Rock continues to have the highest quality habitat of all of the Henry’s Fork sampling locations with “excellent” HBI habitat classification and 60-70% of macroinvertebrates comprising of mayflies, stoneflies, and caddisflies at the site. Contrary to what one might expect, the high-quality habitat at Flat Rock results in lower diversity metrics and total abundances of macroinvertebrates. Flat Rock has cold, clear, and clean water and there are limited nutrient inputs into the reach. This leads to fewer species of macroinvertebrates (particularly pollutant-tolerant species such as midges and non-insects which are often found in high abundances) and lower overall abundances of macroinvertebrates. In general, as you move downstream habitat quality decreases along a continuum. Through this continuum, diversity increases (at least, initially) and macroinvertebrate abundances increase as distance from the headwaters increases and habitat becomes more suitable for pollutant-tolerant species (warmer water temperature, and more sediment and nutrients in the system). We see these River Continuum Concept trends throughout our macroinvertebrate data.

Last Chance had a notable increase in total abundance of EPT from 2019 to 2020 with overall macroinvertebrate abundance and percent EPT both increasing in 2020. The increases in EPT and HBI scores in the Last Chance reach were likely the result of increased flows from Island Park dam in late-April to early-May in 2019 that mobilized an estimated 555 tons of fine sediment out of the reach. This deliberate management strategy to increase flows and mobilize sediment likely improved habitat quality and macroinvertebrate compositions responded to these improvements. To learn more about the relationship of Island Park dam spring flows, sediment mobilization, and habitat quality, check out Rob’s blog discussing the topic, Why Have Insect Hatches Improved So Much in 2020? .

Trends in macroinvertebrates at Osborne were nearly opposite of what were observed at Last Chance in 2020. Percent EPT and HBI declined from 2019 but total macroinvertebrate abundance increased. These changes indicate an increase in abundances of non-EPT species while abundances of EPT species remained relatively constant. Even though there was a net transport of 555 tons of sediment out of the reach between Island Park Dam and Pinehaven, trends in Osborne in 2020 could be the result of some of the sediment mobilized upstream in 2019 depositing out in the Osborne sample index reach. Regardless, conditions at Osborne in 2021 returned to those similar to 2019 with “excellent” HBI habitat classification, decreases in total macroinvertebrate abundances, and increases in percent EPT.

The Ashton site stayed relative constant between 2019-2021 but the St. Anthony sample location had a statistically significant decline in HBI and percent EPT, in addition to a slight increase in total macroinvertebrate abundance. The decline in habitat quality at St. Anthony in 2021 reflects a return to average habitat conditions and macroinvertebrate communities for the site, down from record high HBI scores and percent EPT in 2019.

We added macroinvertebrate sampling downstream of the Ora bridge construction into our monitoring program for 2019, 2020, and 2021 to monitor macroinvertebrate communities before, during, and after bridge construction. We were able to sample in 2019 (pre-construction) and 2021 (post-construction) but weren’t able to sample in 2020 due to Covid-19. Diversity, HBI, and percent EPT did not change between 2019 and 2021 at the Ora site but total macroinvertebrate abundance declined by over 50%. If construction impacted habitat downstream (primarily with additions of fine sediment), we would anticipate changes in all three of these metrics. This suggest the decline in macroinvertebrate abundance at Ora was a result of larger environmental influences beyond construction.

Water quality data collected with sondes above and below Ora bridge construction didn’t show significant differences between sites but there were differences in seasonal flow characteristics between 2018 and 2020. Water year 2018 had significantly higher spring flows than 2020, while 2020 had higher flows during irrigation season. If habitat conditions and macroinvertebrate compositions below Ashton Dam respond similarly to the habitat and macroinvertebrates below Island Park Dam, it is plausible that the spring:summer flow ratios between 2018 and 2020 could be driving the shift in macroinvertebrate abundances from 2019 to 2021 at the Ora site. In order to dig into this further, and due to the increased popularity of angling in the reaches downstream of Ora, we intend to add the Ora site into our long-term macroinvertebrate monitoring program and sample the Ora location every two to three years.

Last, we have worked with River Continuum Concepts, Inc. (macroinvertebrate lab in Manhattan, MT, who analyzes our field samples) and they have agreed to expedite processing our macroinvertebrate samples in the spring so they can provide us raw data on Henry’s Fork macroinvertebrates by early June. This will allow us to summarize and publish macroinvertebrate data prior to peak fishing season so anglers can have a general idea of what they might experience regarding hatches throughout season. Thank you, Brett Marshall and River Continuum Concepts crew!

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