Those of you who regularly read this column know that I have written extensively about the little sulphur mayfly. Ever since the implementation of the water releases legislation in 1976, the time and length of certain mayfly hatches have changed dramatically. From what I have observed over the last 40 years, the sulphur hatch has been impacted the most. 

On freestone rivers, the sulphur hatch begins the second week in June, lasts a week or so, and ends. Under those normal conditions,  sulphurs hatch at dusk and leave the water immediately. That is not the case with the tailwaters. When the summer release on the East Branch of the Delaware was increased to 70 cubic feet per second (CFS), the sulphur hatch began at 6 p.m. and lasted well into the night. 

Once the summer release on that river was increased to 140 CFS, the sulphur hatch began at 1 p.m. That provided anglers with some excellent daytime fishing opportunities. 

Interestingly, as the years passed, the sulphur hatch evolved so that it once again began around 7 p.m. and continued until dark, at least on the East Branch. In addition to changes in the time of day that sulphurs hatch, emergences now last well into August.

This same progression of sulphur hatches occurs on the West Branch of the Delaware River too, once the cold-water release was increased and stabilized. While sulphur hatches on the East Branch are no longer a mid-day occurrence, those on the West Branch still take place earlier—sometimes before 12 noon. That is because of the sustained release of very cold water, and early hatching happens closer to the dam. As the water warms slightly downriver a few miles, sulphurs hatch later in the day and do not last as long into the season.

A check of the USGS gage at Downsville, just below the Pepacton Reservoir, revealed water temperatures between 42 and 44 degrees Fahrenheit at this time of year. The gage for Cannonsville read 48 degrees Fahrenheit when I checked on August 10. So what we have here is a significant change in the ecology of the sulphur mayfly in relation to its environment, due to the release of very cold water on a season-long basis. 

I’ve researched this change in behavior in sulphur mayflies both online and with aquatic entomologists at the Stroud Center in Pennsylvania. What I’ve learned is that water temperature must reach at least 50 degrees for sulphurs to hatch. I believe that is why sulphur hatches on both the East and West branches of the Delaware River begin at odd times. It takes a lot of ambient heat for those tailwaters to reach 50 degrees. 

Anglers who fly fish pay very close attention to fly hatches. They pay particularly close attention to the sulphur hatch because it lasts so long and provides daytime angling opportunities, at least on the West Branch of the Delaware. And this is where it gets really, really interesting. Anglers are finding rising trout during these daytime hatches on the West Branch and are catching fish there. The same cannot be said for the East Branch. I happened to be on that river during the evening of July 27 of this year. It was a sunny, warm afternoon, with a few small olive mayflies on the water when I arrived at 6:30 p.m. Soon after, once the sun slid behind the mountain, sulphurs began to hatch. There weren’t many at first. But by 7 p.m., there was a good hatch of duns on the water. 

As the evening progressed, the hatch intensified, but there were no trout rising except for a few “dinks,” as my friends like to call very small trout. I stayed on the water until 8:30 p.m., and then found a real mess in my 7X tippet. 7X is a very fine tippet and will spin during casting, even with tiny flies. When that happens, the entire tippet turns into a bird’s nest and cannot be replaced under low light. 

Anyway, I left the river, but stayed in the meadow on the bank, watching the water. About 8:45 p.m., at least half a dozen good trout began rising downriver while I watched, rod in hand, but with no fly or tippet. I cannot tell you how many times I’ve more than implored anglers to never leave the river until dark. And if it weren’t for the issue with my tippet, I would have done just that. That being said, the question remains: why are West Branch trout feeding on hatching flies during midday, while East Branch trout wait until the last 15 minutes? 

I know that the trout in this section of the East Branch receive a lot less angling pressure than do trout in the upper West Branch, so that can’t be an issue. There are a lot of eagles around this reach of the East Branch, so I’m wondering if that is a factor. Or is it because the flow in the West Branch is three times higher, and the trout feel safer? 

Whatever the reason, it’s very interesting and frankly somewhat frustrating to see hundreds of flies hatching and no trout rising. While there is no real answer to this dichotomy in trout feeding behavior, it’s certainly an interesting issue. The kind of issue that motivates us to fish for trout instead of bluegills.