ramblings of a catskill fly fisher

Aquatic insect decline: An update

Posted 5/29/24

In this year’s Fish magazine (River Reporter, April 4), my article regarding the decline of aquatic insect communities in the western United States was reprinted from the Big Sky Journal. In …

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ramblings of a catskill fly fisher

Aquatic insect decline: An update


In this year’s Fish magazine, my article regarding the decline of aquatic insect communities in the western United States was reprinted from the Big Sky Journal. In that piece, I outlined some of the reasons aquatic insects, such as salmon flies, have declined in some western rivers. Low summer stream flow, along with warming water temperatures, are being blamed as the causes. 

A recent call from a man who was writing about aquatic insect decline indicated that both land-born and aquatic insects are in trouble worldwide. Scientists researching this issue are blaming climate change, insecticides, habitat loss, road salt and siltation as some of the causes. They claim that small rises in ambient air temperature are part of the problem. 

While that appears to be true for some western rivers, what about here in the Catskills? Catskill freestone rivers such as the Beaver Kill have undergone very low summer stream flows and extreme water temperature fluctuations for thousands of years. For example, water temperatures rise as high as the mid-80s in summer to the low 30s in the winter. But the aquatic insects that inhabit that river have evolved over the millennia to survive environmental extremes. So it’s difficult to conclude that a small increase in ambient air temperature would impact the aquatic insect community of the Beaver Kill or other Catskill freestone rivers. 

In addition, the Beaver Kill—especially the upper river—flows through private, undisturbed forests, where none of the uplands are committed to agriculture, so there are no insecticide issues in that watershed. The same can be said for the rest of the Catskill freestones when it comes to the use of chemicals for insect control. 

Sand and salt have been used as road conditioners during winter months for many, many years. Therefore it is difficult to know whether enough of those materials find their way into Catskill river systems in large enough quantities to impact aquatic insects. Sadly, even intensive detailed water analysis regarding the impact of road salt and sand, lacking the background baseline data, would likely be inconclusive.

What we have determined through detailed research is that the release of cold, hypolimnetic (bottom) water from deep reservoirs, such as the Pepacton, have a profound, negative impact on some species of aquatic insects. What we have documented is that the number of mayflies in the East Branch of the Delaware River in the reaches immediately downstream of the Pepacton is limited to Hendricksons, pale evening duns, sulphurs and blue-winged olives. There are no March browns, grey foxes, green Drakes, Isonychias,  light Cahills or tricos. There are limited species of caddis flies, too. The water is just too cold. Yet all those species are present from Shinhopple downriver, where the water temperature warms by at least five degrees. So while the release of cold water and regulated flows are most helpful to the trout fisheries, they can impact, dramatically,  the species composition of aquatic insects in the upper East Branch below the reservoir, particularly sulphur mayflies. 

The release of cold water has also impacted the hatching times of some mayflies, sulphurs in particular. On freestone rivers, sulphurs hatch at dusk. When the summer release from the East Branch was increased from 15 cubic feet per second (CFS) to 70 CFS after the passage of the Water Releases legislation, sulphur hatches began around 6 p.m., not at dusk. Those hatches were huge with thousands of duns on the water; actually, there were too many files, which made fishing difficult. 

As the years passed and the hydrology of the East Branch/Pepacton system was studied in more detail, the releases were adjusted accordingly. As a result, the Flexible Flow Management Plan was adopted and implemented. When that program went into effect, the summer flow from the Pepacton was increased from 70 CFS to 140 CFS. Soon after, anglers began to see that sulphur mayflies were hatching in huge numbers right around 1 p.m. It’s a huge departure from their normal hatch time at dusk. 

Discussions with an aquatic entomologist at the Stroud Center in Pennsylvania revealed that sulphurs needed warmer water temperatures to hatch. Since the release water seldom exceeded 52 degrees Fahrenheit, sulphurs began to hatch much earlier in the day.

While the record indicates that insect communities, both land-born and aquatic, are in decline worldwide—with climate change at the forefront of the causes—one has to be a bit skeptical about it all. The fossil record indicates that mayflies, as an order, have been around for 300 million years, dating back to the Carboniferous Period. They are some of the oldest known insects to inhabit the earth. Therefore it is difficult to comprehend how these delicate creatures—which have survived periods of glaciation, floods, droughts and other catastrophes—are the victims of recent environmental changes.

The good news, at least for this spring, is that my angling friends have reported excellent Hendrickson, blue quill and caddis hatches along the Beaver Kill, lower Neversink and on both branches of the Delaware. Some have said that it’s “the largest number of Hendricksons we’ve seen in years.” Hopefully the hatches that follow will be just as good.

ramblings of a catskill fly fisher, aquatic, insect, decline, river, beaver kill


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  • barnhllo

    Back when I was fishing the Beaverkill regularly 65 years ago, there were five dairy farms operating in the roughly two mile stretch between Junction Pool and the 206 bridge upstream in Rockland. Turn a rock over in the river back then and you'd find many, many caddis cases. Today, you might have to turn over 5-6 rocks to find a single caddis case. Hmmm....Maybe those farm fields fertilized by raw cow manure had a fertilizing effect on the river which resulted in great insect growth. (Note: Back then, commercial fertilizers and pesticides were not commonly used.)

    The effect of these dairy farms along with the effluent from the primary sewage treatment plant adjacent to Junction Pool produced those big fish of Ferdon's and Barnharts and other downstream fishing hotspots.

    Just saying.....

    Lloyd Barnhart

    Thursday, May 30 Report this