Another silent spring?

Fly-fishers observe decline in aquatic insects

Posted 3/27/19

About three years ago, angling friends that fish Catskill rivers reported a decline in some traditional mayfly hatches. Of particular concern was the purported absence of sulphur mayflies, an eagerly …

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Another silent spring?

Fly-fishers observe decline in aquatic insects


About three years ago, angling friends that fish Catskill rivers reported a decline in some traditional mayfly hatches. Of particular concern was the purported absence of sulphur mayflies, an eagerly awaited though somewhat-frustrating little fly. Sulphurs normally hatch at dusk from freestone rivers*, but, due to the coldwater releases from some New York City Department of Environmental Protection (DEP) reservoirs, they were found hatching in huge numbers around 1 p.m., creating a unique fly-fishing opportunity. As a fly fisherman/biologist, my immediate reaction was to dismiss the claims, believing instead that the decline in sulphur hatches was a temporary phenomenon, attributable to the normal cycles that all aquatic insects go through. When talk intensified into the 2016 season, it became apparent that there was more to what anglers were reporting about the decline in sulphur hatches than typical annual population fluctuations.

An early stonefly

It was at that time a group of us met in Downsville, NY to discuss the issue. The East Branch of the Delaware River was one of the waters in question. This river is considered a tailwater, because it is fed from a bottom release of very cold water from Pepacton Reservoir. The East Branch is considered a low-gradient river and, as such, does not “flush” unless the reservoir spills and flows reach or exceed 4,000 CFS (cubic feet per second). During the meeting, we learned the Pepacton had not spilled to that extent since 2013. As result, layers of dead leaves had built up along the bottom in slower pools and a luxuriant growth of filamentous green algae invaded the riffles.

As biologist, I attributed the blooms of green algae to the discharge of cold water from the reservoir, because some species of algae are cold-water dependent. Others disagreed, believing instead that the algae was a function of increased amounts of dissolved phosphates and/or nitrates finding their way into the river. To determine whether or not those two nutrients were a problem, we collected water samples at multiple points within three miles downstream of the reservoir. Although our sampling was limited, analysis concluded that concentrations of both materials were not high enough to promote the growth of algae.

At this point, and with no other reason on which to base the sulphur’s decline, there was some consensus that the dead leaves and green algae had compromised river-bottom habitat. As a result, sulphur mayflies, and perhaps other species of aquatic insects, lost the areas they needed to thrive in the East Branch. The river needed a good flush!

In June 2107, review of USGS flow data revealed that spill from the reservoir exceeded 4,000 CFS during that spring. As consequence, much of the algae and most of the dead leaves flushed from the river, providing hope; the river bottom had cleared and sulphur populations would rebound accordingly. However, as the season progressed, hatches for the species remained well below normal. Still, we were hopeful and expected the species would rebound in the coming season. Sadly, sulphur hatches remained sparse in 2108. Now we wait to see what happens in 2109.

Later that year, we met again to continue discussions about aquatic insect populations. During that meeting, friends were asked to provide an assessment of the insect activity, or lack thereof, they witnessed while fishing. To a man, all indicated that hatches of mayflies and caddis flies in general were well below normal, not only on the East Branch, but on other Catskill rivers as well.

Then in December, the copy editor at The River Reporter indicated there were widespread rumblings of insect declines worldwide, suggesting I review an editorial printed in the New York Times, tilted: “The Insect Apocalypse Is Here.” The article dealt primarily with the decline in terrestrial-insect populations in Europe. Based on actual data, findings concluded that land-born insect populations across the continent were down by as much as 80% for the period 1989-2013. Furthermore, researchers in the United States concluded that monarch butterfly populations were down by as much as 90%. The article went on to explain the decline in terrestrial-insect populations was worldwide and could be attributed to climate change, loss of habitat and the application of agricultural pesticides.

So what about aquatic insects? I turned to the internet for guidance. I found a December article in The Guardian, a British newspaper, titled, “Insect Declines: New Alarm over Mayfly Is Tip of Iceberg, Warn Experts.” This editorial explained that modest levels of pollution in the form of fine sediments, phosphates and powerful neonicotinoid insecticides were having a devastating impact on mayflies, killing about 80% of all eggs in some of Britain’s rivers.

With this information in mind, and to see if there were problems with aquatic insect hatches in other parts of the United States than the Catskills, I contacted a number fly shops, fly-fishers and macroinvertebrate researchers across the country. I spoke with fishermen and fly shops in West Yellowstone, Manhattan and Bozeman Montana, Island Park, Idaho, Michigan’s Upper Peninsula, Texas, Pennsylvania and California.

This is what I learned from speaking with members of the fly-fishing community. Hatches of green and brown drakes were reported as all but gone from some rivers in Yellowstone Park. Their demise was blamed on the colonization of filamentous green algae, which, much like the East Branch, had compromised river bottom habitat for both species. Then a fly-fisherman I spoke with in Bozeman indicated that hatches on Big Spring Creek in Lewiston, Montana were way off. He blamed the decline on pesticide contamination from farming in the area. Next, I spoke with two men, both who fished the Henrys Fork River in Idaho for more than 30 years. Each told me that mayfly and caddis fly activity had declined dramatically over the years. One kept a record of insect hatches and indicated that in 2018 more trout were taken on of terrestrial fly patterns than on traditional mayfly/caddis imitations. The ratio changed from 80% aquatic insects/20% terrestrial insects to 65% aquatics/35% terrestrial

In summary, both blamed insecticides, erratic flows and warmer water temperatures for the serious decline in aquatic insects hatches on the Henrys Fork.

The Salmonfly nymph, from Montana, has disappeared  from some Montana rivers.

Later in my investigation, I contacted researchers in Michigan, Pennsylvania, Idaho, Montana and California to see what they were finding in regard to the status of aquatic insects communities through their studies. Despite the reports I received from the angling community, most of the researchers I spoke with were not in the process of studying aquatic insects populations on a quantitative basis. Instead, most of their research was directed at the impacts that a variety of pesticides might have on aquatic macroinvertebrates. Of particular concern was the widespread use of neonicotinoid insecticides over the last 15 years. The material is of particular concern because its application is widely used in corn, cotton and wheat farming. Seeds are treated before planting and the chemical is water-soluble, so is easily transported from farmland to river during runoff. It is also hard to detect in water and is long lived. Results in the laboratory confirmed the chemical to be extremely toxic to aquatic insects at levels in parts per trillion! What researchers could not tell me was whether the use of neonicotinoid insecticides was impacting aquatic insects in America’s rivers. What they all did tell me was that insect populations were definitely on the decline across the United States.

The last researcher I spoke with had monitored aquatic insect populations on a quantitative basis across several states. When I asked him about Montana, this is some of what he told me: Salmonfly hatches are in serious decline in some of Montana’s most famous rivers, including sections of the Madison, Big Hole, Smith and Clark Fork. The salmonfly is a species of stonefly and is a very large insect, with nymphs attaining a length of two inches or more. Because these insects take three years to mature and hatch, it is not hard to comprehend what happens to a population if even one year’s class is lost. From a fishing standpoint, salmonfly hatches are eagerly awaited by anglers because healthy populations hatch in large numbers and bring really big trout to the surface. When I asked him what was responsible for the declines in salmonfly populations, he said, “Loss of habitat through sedimentation, low flows associated with periodic droughts and higher water temperatures due to warmer air temperatures.“

So what does this all mean? Aquatic macroinvertebrates are, other than the various forms of plankton, the basis of the food chain in aquatic ecosystems. Both immature and adult forms provide food for fish, other insects, ducks, birds and bats. And because of their need for clean, clear and well-oxygenated water, they are as a group biological-indicators, and, as such, are the first organisms to make us aware that all is not well within our waterways.

So after speaking with a lot of well informed people, doing a great deal of research myself and after giving this issue a lot of thought, it is very clear: there is a major, worldwide decline in insect populations. But unlike Rachael Carson’s groundbreaking work,  “Silent Spring,” which found DDT to be responsible for the decline in predatory bird populations, worldwide insect declines are a function of several more subtle factors, and therefore much more difficult to resolve, if that is even possible.

Warming temperatures, loss of habitat and the widespread use of a variety of pesticides are just three of the known factors impacting aquatic insect communities.

Is this another “Silent Spring”?


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