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Aquatic Insect (Macroinvertebrate) Data Results: 2015-2022

Updated: Jun 30, 2023

Beginning in 2015, the Henry’s Fork Foundation, in consultation with River Continuum Concepts, established an annual aquatic insect (macroinvertebrates) monitoring program for the Henry’s Fork. All macroinvertebrate samples are processed by River Continuum Concepts of Manhatten, Montana, under the supervision of Brett Marshall, who is one of the most experienced aquatic invertebrate scientists in the country.

Henry's Fork 101: The Ranch's Insect Balancing Act


Samples are collected from the stream bottom using a standard Hess sampler that is 0.1 square meters in area. Six samples are taken at each location. Samples are collected in mid-March, immediately prior to the first mayfly hatches of the years, so that all species in the river are large enough to be collected in the samples. If we sampled later in the spring or summer, the samples would miss species that had just hatched, as their individuals would be still in the egg or early larval stage and hence too small to be collected. We sample at five fixed locations on the Henry’s Fork every year: Flatrock, Last Chance, Osborne Bridge, Ashton (just upstream of Ashton reservoir), and St. Anthony. (Ashton and St. Anthony were not sampled in 2020 due to covid restrictions.) We also sampled at Ora Bridge in 2019 and 2021 to assess any potential effects of the Ora Bridge construction, which occurred in between those to sampling events.

Check out these blog posts from 2021 and 2022 for even more information.

Results: 2015-2022

  • Caddisfly abundance (as well as flavs and PMDs) at Last Chance peaked in this time frame on 2020 and decreased in 2021 and 2022 since then.

  • Insect abundance is highest at Ashton at around 50,000 insects per sq meter and Last Chance has about 25,000 insects/sq meter, an incredible number of insects.

  • Percent EPT (mayflies, stoneflies, caddisflies) was highest at Flat Rock (65% of insects), followed by Last Chance, Osborne, and Ashton (50%). That is off the charts for any trout stream, anywhere.

  • Hilsenhoff Biotic Index (HBI), a measure of the whole water and habitat quality, was highest at Flat Rock at Excellent (as good as it gets).

  • Last Chance, Osborne and Ashton all fall in the "Very Good" category, and St. Anthony is lowest at "Good".

What is HFF Doing to Improve Conditions?

  • Comparing 1993 (post IP sediment event) with 2015-2022, the water and habitat quality index averaged at Flat Rock, Last Chance and Osborne increased from Fair/Good to Very Good.

  • Why? 30 years of conservation with a variety of partners across the watershed has had a measurable effect.

  • % EPT (mayflies, stoneflies, caddisflies) at Last Chance and Osborne measurably increases with high springtime freshet flows.

More information on 2022 Insects and Hatches

From Science Director, Dr. Rob Van Kirk's 2022 Technical Report.

"Fish and insect abundance largely reflect conditions inherited from previous years. Given that water supply in 2022 was among the lowest in the last 90 years, the trout population will remain low at least for another two years. I expect EPT abundance to stay low for at least another year."

Timing of insect hatches was delayed in 2022 due to the cold wet spring, as anticipated in mid-April.

Prediction: “Cooler-than-average temperatures in late winter and early spring will push hatch timing later than the 2014-2021 average, at least through the spring.”

Outcome: Hatch timing and other measures of aquatic organism development lagged average until late summer and early fall not just between Island Park Dam and Pinehaven but throughout the whole watershed (Figures 45-46).

With respect to insect abundance, we are starting to uncover links between water management at Island Park Dam and quality of the aquatic invertebrate community. In this linkage, the aquatic invertebrate community lags streamflow, which in turn reflects overall water supply, by at least one year. Based on the relationship between the freshet index and measures of invertebrate community structure, I predicted lower %EPT and worse HBI in 2022, as a result of drought 30 during water year 2021. Those specific predictions did not turn out to be true based on our spring invertebrate sampling, illustrating that many factors affect aquatic invertebrates. We probably measure most of these factors but do not yet have enough years of data to quantify their effects on insect hatches.

Prediction: “It is likely that the numbers of sediment-intolerant [invertebrate] species will be lower this year even than last but not as low as they were following the 2013-2016 drought.”

Outcome: As measured by the Hilsenhoff Biotic Index (HBI), the abundance of sediment intolerant species in 2022 was higher than in years during and immediately following the 2013- 2016 drought and lower than in 2019, according to the prediction. It was about the same, if not slightly better than in 2018, 2020, and 2021, contrary to what I predicted.

On the other hand, our 2022 data showed that abundance of EPT taxa at Last Chance was the lowest in our 2015-2022 record (Figure 59). Abundance of caddisflies, Pale Morning Duns (PMDs), and Flavs were particularly low in 2022 and only a few percent of what they were in 2020, when most anglers reported the best hatches in years if not decades (Figure 60). Although our March stream-bottom (“benthic”) sampling indicates abundance only of larval insects at that point in the year and therefore is not a measure of “hatch quality,” our growing data set is showing substantial year-to-year variability that at least for some insects appears to match angler observations of hatches on the river (Figures 59-61). These correlations are strongest at Last Chance, where our data show particularly good abundances of important insects in 2017 and 2020 and very poor abundances in 2019, 2021, and 2022. Patterns at Osborne Bridge differ substantially from those at Last Chance—in some cases showing opposite trends—most likely due to inherently finer substrate and greater retention of macrophytes throughout the year at Osborne Bridge. Sampling variability at Osborne Bridge is also higher than it is at Last Chance because of lower habitat variability at Last Chance, which contributes to differences between the sites.


By most accounts, fishing experience on the Harriman Ranch reach was very poor in 2022. Our data corroborate angler experience on the Ranch in 2022: low fish population, delayed hatch timing, continuously cold weather in the spring, continuously hot weather in July and September, low macrophyte abundance, low insect abundance, high streamflow, and high turbidity combined to produce poor fishing. Of these, fish and insect abundance largely reflect conditions inherited from previous years. Given that water supply in 2022 was among the lowest in the last 90 years, the trout population will remain low at least for another two years. I expect EPT abundance to stay low for at least another year. The other factors—weather, turbidity, and streamflow during the fishing season—depend almost completely on weather and water supply in that year and so can improve dramatically in one year if weather and water supply cooperate.

In undertaking this detailed analysis of fishing conditions—made possible only by our intensive research and monitoring efforts—two factors affect fishing experience more than any others.

1. Water supply affects fishing experience both day-to-day and over the lifespan of insects and fish (1-4 years). The effects of recent drought on the aquatic ecosystem and fishing experience are lower than they would have been 5-10 years ago as a result of improved collaborative water management and conservation.

2. Sediment export from Island Park Reservoir affects water clarity on a given day of fishing and insect abundance a year or two hence. More detailed investigation of mechanisms that relate sediment mobilization in Island Park Reservoir to insect abundance will be needed to develop additional management actions to maintain the quality of hatches anglers desire.

To see these results in video form, click below.

HFF Membership Meeting 2022 -- 45:00 - 56:00

HFF is preparing a funding proposal to create an interactive website for this aquatic insect data, so that anglers can easily view this insect data over time, with more detailed information.

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