Where have all the mayflies gone?

Posted 2/8/23

From what I’m reading, hearing and observing in the field, there is consensus that mayflies worldwide are in decline. 

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Where have all the mayflies gone?


From what I’m reading, hearing and observing in the field, there is consensus that mayflies worldwide are in decline. 

Many of you who read this column know that I’ve written a great deal about aquatic insects, and mayflies in particular, in the last few years. In fact, “Another Silent Spring?,” which was about the changes and decline in aquatic insect populations, appeared in the 2021 FISH magazine published by this very newspaper. 

Mayflies are considered by entomologists, taxonomists and naturalists to be the oldest order of insects on the earth, dating back some 300 million years. So the question remains: why this fairly recent decline in an insect community that has survived a myriad of environmental catastrophes, including floods, ice ages, droughts and warming and cooling climates?

There are a lot of theories about the cause of mayfly declines, and a fair amount has been written about the topic. For example, in what are some of the most pristine trout rivers in the world—the chalk streams of England—the mayfly hatches have declined dramatically. 

Researchers believe that a variety of factors are the cause, including sewage, siltation [the deposition of silt] and the application of pesticides. 

In Lake Erie, historical records associated with the Hexagenia mayfly hatch indicate this species is once again in decline after an upsurge. “Hex” mayflies are of the burrowing variety, and historically hatched in such large numbers that the dead flies had to be shoveled off bridges. 

Biologists are not sure whether the current decline is cyclic, or if the species is the victim of more sinister reasons.

So what are we seeing—or not seeing—in the famous trout rivers of the Catskills regarding the decline in the mayfly population? While I can’t comment on many of those rivers, I’m told that mayfly hatches are not as prolific as they once were on the Beaver Kill. There are some folks who believe siltation from uplands, and sand used along roadways in winter, are possible causes. 

Where I have considerable knowledge is the East Branch of the Delaware, between Downsville and Harvard, NY. I have fished this section of river for the past 40 years, and have observed significant changes in species composition and the size of mayfly hatches during that period. 

While I don’t have scientific proof, there is data in the literature to support the theory that aquatic insect populations closest to bottom releases can change dramatically, due to constantly cold water temperatures. With the implementation of the water releases legislation in 1976, when the summer release was increased from 15 cfs (cubic feet per second) to 70 cfs, hatches of sulfur mayflies in particular responded dramatically along the upper East Branch. There were huge emergences that began around June 10 and extended well into July, with the first flies appearing right around 6 p.m. 

Several years later, when the summer release was increased from 70 cfs to about 150 cfs, we saw a significant change in the sulphur hatches. Hatches that once began at 6 p.m. saw the flies suddenly appear at 1 p.m. 

The hatches were huge and lasted from mid-June well into August. There were probably several broods during that time frame. 

Then beginning around 2016, we observed a significant change in sulphur hatches. The huge, steady early afternoon hatches all but disappeared. We saw no evening hatches of these mayflies either. So what happened?

During the summer, when the release from Pepacton Reservoir is maintained at around 150 cfs, daytime water temperatures in the reaches close to the dam seldom exceed 52 degrees Fahrenheit. That temperature is constant throughout the summer. Based on my research, I believe that the normal temperature range through which sulphur mayflies evolve ran from about 33 degrees Fahrenheit in winter to highs of 80 degrees Fahrenheit during the summer. Sulphur mayflies, and other mayflies that evolved and survived through these extreme temperature fluctuations, do not appear to do well when exposed to a constant temperature of 52 degrees Fahrenheit. It appears to be too cold for several species. 

That’s why I believe that the only species of mayflies found in the upper East Branch close to the dam are what I’ll call—for lack of better terminology—early season or “cold water” mayflies. Species like quill Gordon, Hendrickson, blue quills, pale evening duns and olives all hatch when the water temperature reaches 50 degrees Fahrenheit or slightly higher. Consequently, there are no March browns, gray foxes, green drakes, brown drakes, light Cahills, Isonychias or tricos to be found in the upper East Branch. Yet these species are all well represented downriver beginning around Shinhopple, NY, where the water is a bit warmer. 

So it appears that the release of very cold water, while favorable for trout, is limiting mayfly diversity and hatching in the upper East Branch.

Assuming that cold water is the limiting factor affecting mayflies in the upper East Branch, what is causing the changes in mayfly hatches in other Catskill rivers? While environmental factors, like siltation, flooding, low flows and high water temperatures in the summer might be factors, there is no actual data to support those naturally occurring conditions as culprits. To complicate matters, there is very little historic, baseline data available to compare against current mayfly populations. 

Mayflies, like all aquatic insects, are sentinels of the environment in which they live. When their populations decline dramatically, it signals that something is very wrong.

We just don’t know what that is yet.

mayflies, environment, decline, population


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