The Fly Fisherman’s Silent Spring
How the Collapse of Trout Stream Insects Dwarfs the Clean Windshield Phenomenon
Part 1: How the Collapse of Trout Stream Insects Dwarfs the Clean Windshield Phenomenon
As the river hatch ends, we’re witnessing something far more dire than clean windshields
This is Part 1 of a two-part series. Read Part 2: From Diagnosis to Restoration — How Fly Fishermen Can Help Bring the Hatches Back
Every fly fisherman over the age of fifty remembers it. That particular moment on a June evening when you’d stop your casting mid-drift, look up, and see the air above the river literally shimmering. Clouds of Pale Evening Duns drifting on invisible currents. Caddisflies dancing in their erratic, moth-like flutter above the riffles. Mayflies by the millions emerging from the river bottom in what looked like a reverse snowstorm rising toward the sky. Trout would be dancing in delight at the surface, gorging themselves, paying no attention to a clumsy fisherman who stood frozen in wonder at what Nature had just unleashed.
Those evenings are becoming memories.
We’ve all heard about the “windshield phenomenon”—the unsettling observation that our car windshields no longer accumulate the insect splatter they did decades ago. Scientific studies have confirmed this, with Danish research documenting an 80% decline in windshield insects between 1997 and 2017, and UK surveys showing 58.5% fewer insects hitting license plates between 2004 and 2021, OMG even in the Chalk Stream district. These are sobering numbers that have rightfully alarmed biologists and nature lovers worldwide.
But the windshield phenomenon, dramatic as it is, pales in comparison to what’s happening in our trout streams. While the general public muses about their cleaner windshields as a curious oddity, fly fishermen are participants in an ecological catastrophe of unprecedented proportions—and we’re the ones with the data to prove it.
The Numbers Tell a Devastating Story
Consider this: a single historical mayfly emergence event on the Upper Mississippi River or western Lake Erie decades ago showed the release of 87.9 billion mayflies into the air, transferring 3,078.6 tons of biomass from aquatic to terrestrial ecosystems in just a few hours. Radar studies—yes, these emergences were so massive they literally showed up on weather radar—have now documented that these events have declined by over 50% in recent years.
This isn’t decline. This is collapse. This is apocalypse now.
The global threat assessment tells an even more alarming story for aquatic insects. Researchers estimate that 33% of aquatic insects are threatened with extinction, compared to 28% of terrestrial species. Most strikingly, 68% of caddisfly species populations are in the worst decline—the highest rate among all insect orders. Mayflies, stoneflies, caddisflies, and dragonflies have been classified as “imperiled” in peer-reviewed scientific literature, with iconic mayfly populations showing declines of up to 84%.
For comparison, while the windshield phenomenon documents aggregate flying insect biomass decline, our trout streams are losing entire orders of specialized insects, an absence that cannot be filled.
What Every Fly Fisherman Is Seeing
Walk into any fly shop from Montana to the Catskills, from the River Spey to the River Tweed, and ask the proprietor what’s happening with the hatches. Pull up a stool at a river lodge bar and listen to the guides talk when they don’t think tourists are listening. The story is the same everywhere:
The Henry’s Fork in Idaho, once legendary for its Pale Morning Dun hatches, has seen significant declines. The Henry’s Fork Foundation’s senior scientist confirms PMD hatches are down in multiple river sections.
The Madison River in Montana documented significant declines in pale PMDs and various caddis species between 2008 and 2017.
The East Branch of the Delaware River in New York experienced a catastrophic collapse of sulphur mayfly hatches beginning in 2016. Anglers initially blamed leaf accumulation, but when the river flushed clean in subsequent years, the sulphurs never returned.
England’s finest trout waters like the River Tweed have documented up to 80% mortality in mayfly eggs.
Yellowstone’s Firehole River is seeing species replacement as traditional cold-water insects disappear and warm-water-tolerant species like white miller caddisflies proliferate.
One angler’s account captures it viscerally: driving 75% of the way from Coeur d’Alene to Superior, parallel to the South Fork of the Coeur d’Alene, the St. Regis, and the Clark Fork rivers—some of America’s most legendary trout waters—and encountering “nary a bug.” A full day of fishing produced “no more than a dozen caddis, mayflies, etc.” The fish themselves seemed confused about what to eat.
Why Trout Stream Insects Matter More Than Windshield Bugs
This is my favourite Dolly Varden fly on the Buskin River on Kodiak Island
Here’s where the trout stream collapse reveals something the windshield phenomenon cannot: which insects are dying, and what they share in common.
The windshield phenomenon documents aggregate insect decline but tells us little about which species are disappearing and why. The trout stream collapse, by contrast, provides a forensic-level diagnosis. The insects vanishing from our rivers share distinct characteristics:
-
-
- They develop in aquatic environments where dissolved gases concentrate to levels far higher than the atmosphere
- They have long larval development periods—one to three years underwater for many mayflies, up to two years for caddisflies
- They are sensitive bioindicators specifically chosen by scientists because they register water quality changes before other species
- They respire through gills or gas exchange mechanisms exquisitely tuned to specific dissolved gas concentrations
- They emerge synchronously in events that have functioned for millions of years
-
This specificity matters enormously. When scientists choose “indicator species” to monitor water quality, they choose mayflies, caddisflies, and stoneflies precisely because these insects are the first to respond to environmental chemistry changes. Their catastrophic decline isn’t just a biodiversity problem—it’s the equivalent of every canary in every coal mine dying simultaneously.
The Elephant in the Room: CO₂
The fly fishing community has been told the usual culprits for our collapsing hatches: neonicotinoid pesticides, fine sediment pollution, warming water temperatures, hydroelectric dam releases, invasive species, and habitat fragmentation. All of these play roles in specific watersheds. But none of them explains the global, synchronized, cross-watershed collapse we’re witnessing—not in pristine Yellowstone tributaries, not in protected European reserves, not in remote wilderness streams where no pesticides have been applied and no sediment has been disturbed.
As I’ve written previously about “ignoring the elephants and the insects”, there is, however, one environmental change that has affected every stream, every pond, every puddle of water on Earth simultaneously: the 44% increase in atmospheric CO₂ since the industrial revolution began.
Here’s what we know about CO₂ and water, the compound interest impact!
CO₂ dissolves into water in direct proportion to atmospheric concentrations. As atmospheric CO₂ has risen from pre-industrial 280 ppm to today’s 420+ ppm, dissolved CO₂ concentrations in all aquatic environments have risen correspondingly. This isn’t theoretical—it’s basic chemistry, the same Henry’s Law that explains how your soda gets carbonated.
Aquatic insect larvae respire through gas exchange mechanisms exquisitely tuned over hundreds of millions of years of evolution. Mayfly nymphs have gill plates along their abdomens that extract dissolved oxygen from water while managing CO₂ elimination. This system evolved under stable atmospheric conditions and operates at the razor’s edge of gas balance.
In sediments—where burrowing mayflies like Hexagenia spend their entire larval existence—CO₂ concentrations can reach levels 10 to 100 times atmospheric values due to biological decomposition. Add rising atmospheric CO₂ on top of these already-elevated sediment concentrations, and you push these organisms past their evolutionary tolerance thresholds.
Caddisflies construct cases around themselves where CO₂ can accumulate in microenvironments. As ambient water CO₂ rises, these microenvironments may become lethal chambers.
The timing matches perfectly. Major aquatic insect declines accelerated dramatically from roughly 2000-2017, precisely when atmospheric CO₂ crossed the 370-410 ppm range—a 30% increase over pre-industrial baselines that pushed dissolved concentrations in aquatic environments past critical physiological thresholds.
Why This Matters Beyond Fly Fishing
If you’re not a fly fisherman, you might be tempted to dismiss this as a problem for a niche hobby. That would be a grave mistake.
Aquatic insects are foundational to freshwater food webs. They feed not only trout but entire populations of insectivorous birds—swallows, swifts, flycatchers, phoebes—that specialize in catching emerging aquatic insects. They feed bats. They feed amphibians and reptiles. They transfer enormous amounts of biomass and nutrients from aquatic to terrestrial ecosystems, a process scientists call “aquatic subsidies.” A single mayfly emergence of 3,000 tons of biomass represents a pulse of energy that sustains thousands of species of terrestrial animals.
When this pulse weakens as it is now seen to be by 50%, the reverberations cascade through ecosystems in ways we’re only beginning to understand. Studies have already documented correlations between insect declines and collapsing insectivorous bird populations across Europe and North America.
More fundamentally, if rising atmospheric CO₂ is poisoning aquatic insect nurseries through dissolved CO₂ toxicity, the implications extend to every aquatic ecosystem on Earth. This is not a trout problem. This is a planetary aquatic life problem.
Continue to Part 2
In Part 1, we’ve diagnosed the crisis: the collapse of aquatic insects in our trout streams dwarfs the windshield phenomenon in severity and specificity, with rising atmospheric CO₂ as the one environmental variable capable of explaining the global, synchronized nature of the collapse. But diagnosis without prescription is despair.
In Part 2: From Diagnosis to Restoration — How Fly Fishermen Can Help Bring the Hatches Back, we turn to solutions. Why fly fishermen are uniquely positioned as environmental sentinels, what concrete actions we can take, and—most importantly—the ocean pasture restoration solution that offers genuine, proven hope for reversing atmospheric CO₂ on timescales that matter to our rivers. The salmon came back to Alaska. The mayflies can come back to our rivers. Here’s how.
The decline documented in this article draws from peer-reviewed research published in PNAS, Biological Conservation, and other scientific journals, as well as monitoring data from fisheries research organizations including the Salmonfly Project, Trout Unlimited, and fisheries biologists working in the American West, Upper Midwest, and Eastern trout regions. Citations available upon request.
Related Reading on This Blog: