Fly Fishing Insider Podcast
Aquatic insects are used as a water quality gauge because different species tolerate different conditions. Counting mayfly, stonefly and caddisfly species is one quick measure; an observed-to-expected index compares what is actually in a stream against what should be there. Neither test names the pollutant, which is the gap current research is trying to close.
Donny Benkendorf is an ecologist with a Ph.D. from Utah State University, beginning a postdoctoral position with the Environmental Protection Agency's National Aquatic Resource Surveys team.
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How do scientists use aquatic insects to measure river health?
Bioassessment samples the macroinvertebrate community and reads water quality from which species are present and which are missing. A typical sample covers about one square meter of streambed taken with a kick net, then goes to a bug lab where trained taxonomists identify and count what is in it. From those counts researchers can estimate abundance and biomass. Much of this data, collected across all fifty states, is publicly available through the Environmental Protection Agency's website.
What is an observed-to-expected index and what does it miss?
An observed-to-expected index compares the species actually found at a site against the species a predictive model says should be there if the stream were healthy - the assemblage it might have held decades or centuries ago. A large gap between observed and expected indicates impairment. The limitation is that the index says a stream is degraded without saying why. Donny Benkendorf's research pushes toward stressor-specific biotic indices, which score each species by its tolerance for a particular stressor such as temperature, so the community itself points at the cause.
Why do mayflies, stoneflies and caddisflies get used as water quality indicators?
The common shorthand is EPT taxa - Ephemeroptera, Plecoptera and Trichoptera, the orders anglers know as mayflies, stoneflies and caddisflies. Finding many species from those three orders usually signals good water quality. Benkendorf cautions that this is a generalization: tolerance varies widely inside each order, and there are mayflies that require pristine cold mountain water alongside mayflies that live in warm, somewhat polluted water. Useful bioindicator species also occur in other orders, including the beetles, Coleoptera.
How long do salmonfly nymphs live underwater before they hatch?
Pteronarcys californica, the giant salmonfly, is semivoltine - its life cycle spans multiple years. In Utah, where Benkendorf collected them, the cycle runs about three years. That means the insect spends all but one or two months of its life as a nymph, emerging around late June and living as an adult only long enough to lay eggs and die. Because age one, age two and age three nymphs are all in the water simultaneously, several nymph sizes are available to trout at any point in the year.
Why does green drake nymph size change through the season?
Drunella grandis, the western green drake, is univoltine - one generation per year. It emerges in late spring or early summer, lays eggs, and that same cohort grows through the year and emerges the following spring or early summer. Benkendorf starts fishing green drake nymphs in late summer and early fall and increases pattern size as the season moves toward emergence. Flipping rocks tells you how close the nymphs are, and a few adults in the air is the cue to switch to dries.
Why do trout eat nymphs so heavily even when nothing is hatching?
Nymphs are not always available to trout, and two things change that. Heavy rain dislodges them from the substrate. More interesting is behavioral drift, where a nymph deliberately releases into the current and lets itself be carried downstream - probably to recolonize new water or find better shelter or food - and becomes easy prey while it does. Hatches concentrate feeding because the pupal stage, the emergence and the spent spinners all present easy targets, but drift makes nymphs worth fishing between hatches.
What should you look for when you flip a rock?
Benkendorf carries no equipment for this - no net, sometimes a small container. Aquatic insects are adapted to cling to substrate, so lifting a cobble and looking at its underside is enough to see what is holding on. He notes that where you flip matters: salmonfly nymphs are shredders that eat decomposing leaves, so he finds them under large cobble in swift but not violent water, in places where leaves settle. Heptageniidae mayflies - the flat-headed mayflies, shaped like pancakes - live in the fastest sections. Slower water holds an entirely different community.
Key takeaways
- Flip large cobble in swift-but-not-violent water where leaves collect to find salmonfly nymphs, which feed on decomposing leaves.
- Because salmonflies have a roughly three-year cycle, multiple nymph sizes are in the water year-round and a salmonfly nymph fishes well outside the hatch.
- Green drakes run one generation per year, so start small in late summer and step up nymph size as spring emergence approaches.
- Flat-headed Heptageniidae mayflies indicate the fastest water; a slower run holds a different set of insects entirely, so match your sampling spot to where you plan to fish.
- Fishing nymphs between hatches works because of behavioral drift - nymphs release into the current on purpose and become easy prey.
- EPA National Aquatic Resource Surveys macroinvertebrate data is publicly available and can be used to look up conditions on waters you fish.
