Fly fishing for trout requires intense concentration, especially when there are flies on the water and fish rising freely. Under those circumstances, anglers have a tendency to be oblivious of their surroundings. That’s why it is possible, while fishing, to miss some serious changes in insect and animal behavior that has manifested itself before our very eyes, but that we did not see.

The other day, I was thinking about some of the things that are missing in the East Branch of the Delaware River. Years ago, when our RV was kept at the Peaceful Valley Campground, I spent a lot of time fishing the river from that location. I was there almost every weekend, beginning Friday evening, and leaving Sunday afternoon. As the season progressed, and after Hendrickson and March Brown/Grey Fox hatches were over, fishing tended toward evening. That began with the emergence of pale evening duns, green drakes and ultimately, our little sulphur mayfly on the second week of June. At that time, I fished well into the evening, only stopping when I could no longer hear trout rising.

As the evening progressed and the flies increased, and as dusk closed, the bats would appear in good numbers to begin vacuuming the river’s surface for the hapless hatching flies.

On more than one occasion, bats chased my fly while I was false casting. Then one evening, I felt my fly line stopped on the back cast, only to find a bat attached to my dry fly.

Rather than handle the little creature, I snipped the leader a short way above the bat and it fluttered off. I’m guessing that the little mammal did not fare well with a small dry fly stuck in its mouth.

After we left Peaceful Valley and moved upriver, there was a period of time, when the little sulphur hatch, which normally began at dusk, started at about 1 p.m. in that section of the river. That was because the water was so cold that it caused some species of mayflies to hatch much earlier than normal.

There is a good pool that we fished in that reach of river; a power line stretches high above the water there. When the hatch was about to occur, a number of barn swallows would perch on that power line, waiting for the insects to appear. It seemed that those little birds knew when flies would hatch. Once the flies began, the swallows would skim the water’s surface, picking off the little sulphurs.

Later, if there were spinners in the air, the swallows would patrol high above the river, intercepting the mating flies.

Another species that waited for fly hatches was the eastern kingbird. Those small, gray- and-black birds would lurk in the riverside trees, looking to pounce. Once the hatch began and the duns left the water, seeking refuge in the bank-side vegetation, kingbirds would intercept the duns mid-air.

These days, I no longer see bats skimming the surface at dusk, swallows massed on the power line, or kingbirds waiting in the streamside for mayflies to hatch. So what’s the issue here?

As far as bats are concerned, there have been significant declines in the population of Indiana bats, northern long-eared bats, and little brown bats, due to the white nose fungus. Some of those species have declined by as much as 50 percent, according to the New York State Department of Environmental Conservation.

Mayfly populations have also declined significantly. I’ve written about this problem several times, in the River Reporter and the Big Sky Journal.

There’s no question, based on the research I’ve conducted and with the conversations I’ve had with anglers and biologists, that insect populations both land-born and aquatic are in serious trouble worldwide.

The lack of aquatic insects hatching from Catskill rivers is an important reason we are not seeing the numbers of bats, swallows or kingbirds as we once did. There are just not enough flies to keep those species around our rivers.

Mayflies and other aquatic insects constitute the bottom of the food chain, in that immature and adult forms provide sustenance for fish, bats and birds. Aquatic insects are also biological indicators, the first forms of water-born life that make us aware of serious environment-related issues.

Currently, there are a number of theories, some of them valid and others conjecture, as to why insect populations are in dramatic decline all over this planet. Pollution, habitat loss and climate change are the most quoted as the cause, and likely the most valid.

But what about the East Branch? The water is cold and clear, oxygen concentrations are high, and flows reasonably stable most of the year. Yet, mayfly populations have declined significantly in the last six or seven years. This year on the upper river, the pale evening dun hatch was almost nonexistent. The same can be said about the sulphur hatch, which upon occasion could be good—or more likely non-existent. Historically, those two hatches provided excellent angling opportunities. Now they are at best inconsistent.

We have no answers to this very troubling phenomenon, other than to say there are significant problems associated with insect population declines, both on land and water. Frankly, I’m not sure the trend can be reversed, and time is running out.