ramblings of a catskill fly fisher

Fake news for fly fishers

Some interesting facts, a few corrections about aquatic insect hatching behavior

By TONY BONAVIST
Posted 11/18/20

Because of my training as a fisheries biologist, along with some knowledge of aquatic insect life cycles, I’ve made remarks about the hatching process that mayflies and caddisflies go through …

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ramblings of a catskill fly fisher

Fake news for fly fishers

Some interesting facts, a few corrections about aquatic insect hatching behavior

Posted

Because of my training as a fisheries biologist, along with some knowledge of aquatic insect life cycles, I’ve made remarks about the hatching process that mayflies and caddisflies go through that I’ve learned were incorrect. For example, I’ve written and lectured that mayfly nymphs hatch while rising from the river bottom on their way to the surface: a revelation that turned out to be untrue. I also said that, once the pupation process was complete, caddisflies left their cases at the river bottom as winged adults. I learned that was also not true.

Over the years, I’ve been intrigued and frustrated as a fly fisherman with the difficulty in attracting trout to my flies when mayflies and caddis were hatching. In fact, not so long ago, I wrote a column about that very dilemma. Titled “Up Versus Down,” the piece explained the conflict anglers experience when presenting their nymphs and wet flies—which sink downward—as mayflies and caddisflies hatch on the bottom and rise to the surface. The naturals are moving up through the water column while our flies sink, going in the exact opposite direction. See the conflict here?

To resolve this conflict, I decided to find out the real truth about how mayflies and caddisflies actually behave while going through the hatching process. After a little research, I was fortunate enough to find a very kind man, an aquatic entomologist, who studied and actually knew how these river-bottom creatures change from immature to adult insects. He spent a considerable amount of time explaining the process that the flies go through while “hatching.”

First, we talked about the mayflies. Remember that I wrongly indicated that the transformation from nymph to dun took place in the water column? Well, this is what the entomologist told me about the different genera of mayflies and how they hatch: Those mayflies that are included in the genius Epeorus, such as our Quill Gordon, hatch at the river bottom. Then, the fully-winged duns (subimagos) float to the surface where they either drift for a while or fly off. The Ephemerella mayflies—which include Hendricksons, Red Quills, Pale Evening Duns and our little Sulphur—hatch at the surface! That means the nymphs gravitate from the river bottom to the surface, at which time they are easy prey for hungry trout. March Browns, Grey Foxes and Light Cahills (reinstated in the genera Stenonema and Stenacron) migrate to the shallows where they rise and emerge at the surface; some individuals climb out onto riverside stones. The hatching process for mayflies is fairly slow, with duns remaining on the water for a considerable length of time before flying off. Then we talked about the largest mayflies to appear on Catskill Rivers: Green and Brown Drakes. Those two species are representatives of the genus Ephemera. Like some of the other mayflies we discussed, these hatch at the surface. The nymphs are burrowers that live in soft areas of river bottom. They are fast swimmers and move through the water column quickly. Since there is no pupation stage in the mayfly life cycle, and the nymphs grow quickly from immature to adult, metamorphosis is incomplete.

Caddisfly life cycles are very different than those of mayflies. Unlike mayflies, caddis larvae go through a pupation stage before hatching, so metamorphosis is complete. And while there are some species of free-living caddisflies, they all make some type of case/cocoon before emergence. Most make their cases from sand, small sticks or river-bottom debris. Just before pupation, caddis larvae seal the case where they undergo the transformation process from larvae to winged adults. When it’s time to emerge, the pupae leave the case and either float or swim to the surface. There are small swimmer hairs on the pupae, plus a sheath of air around the insect, which may help it float to the surface. Unlike mayflies, all caddisflies hatch at the surface, with the exception that some leave the water to emerge.

So what does all this mean to us as fly-fishers? For me, it has changed my thought process with regard to what the word “emerge” means. With mayflies, it means that hatching happens at the river bottom or the surface. So, is there actually an emerger stage where the nymphs go through the transformation process in the water column? The answer to that is no. I suppose one could conclude that there is an emerger phase either at the bottom or right at the surface depending on the species.

Since most caddis pupae hatch as winged adults at the surface, and the process is so quick, is there an emerger? I don’t know. If there is, it will only last as long as takes for the adult to leave the pupal shuck. Nevertheless, the fly-fishing community talks about all kinds of “emergers,” both mayflies and caddisflies. Since many or all of them work at certain times, anglers should stick with what brings trout to their flies. For me, now that I know the truth, I still believe that it is very hard to emulate, with the artificial fly and a fly rod, the hatching process that insects go through as they ascend the water column. But based on the new information I have about how mayflies and caddisflies actually hatch, there is always the next trip to the river!

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