The Aquatic Insect Decline

June 24, 2024 By: Spencer Durrant

Photo: Zleng/Flickr

Last week I reported on a study about a decline in aquatic insect populations across America. This isn’t the first I’ve heard of the decline, although any updates on the matter have been few and far between.

Or so I thought, until I did some digging.

It turns out the scientific community is aware of this problem. One study has linked insect population collapse in the Parana River, in South America, to dams. Another study has found a significant decline in the hexagenia mayfly. Yet another claims that 33% of aquatic insects are facing extinction.

Warming waters and pollution are, from my initial perusal of this information, cited as the leading causes of the declines. On the Henry’s Fork, in particular, PMD hatches have decreased significantly, corresponding with erratic water releases from Island Park Reservoir, causing warmer water temperatures, according to Tony Bonavist.

Just last week, however, I spent five days fishing a handful of my favorite rivers in Utah. On all but one we were treated to spectacular hatches – some of the best I remember seeing. Caddis, yellow sallies, and PMDs were the main attractions, along with some late-arriving Drakes.

The one “average” hatch we fished only felt that way in comparison to a trip we took last year, when that river produced more PMDs and caddis in one night than I think I’ve seen anywhere other than the Henry’s Fork.

I’m not doubting the science, or the firsthand experiences, of anglers who report seeing smaller – or altogether absent – hatches. What I’m curious about is the common thread. Are these problems more prevalent on freestone and spring creeks? Are tailwaters seeing issues? Are the declines also present in high-elevation trout fisheries? The current evidence shows the decline in aquatic insects is notable both in Eastern and Western fisheries, so I feel safe in not assuming that it’s a regional problem.

What has my interest piqued is that, especially this year, I’ve been lucky to fish some spectacular hatches. I’ve done almost all my fishing in Wyoming and Utah in 2024, two states that have been hard-hit by the ongoing mega-drought. We’ve had two great winters, so perhaps that influx of cold water spurred on great hatches, while other fisheries haven’t fared as well?

I don’t know, and I’d love to discover the connection between all the declines. I’d also love to hear from you. Have you noticed hatches that haven’t been as prolific? If so, what other factors might have influenced that? Please feel free to sound off in the comments.