Mayflies can be found emerging every month of the year in the Yellowstone area. They are particularly abundant from May through October and are an important food source for trout during this time period. Mayflies are responsible for many of the great fly fishing opportunities that arise during the season, and a knowledge of their habits is invaluable for an angler.
Mayflies inhabit every trout stream and the number of species a stream can hold is often amazing. Our friend Dan Gustafson has found over sixty species on the Gallatin River alone, a sign of remarkable diversity in habitat. Of course not all rivers support so many species, and not all species are important to fly fishermen. Some mayflies are not abundant enough to warrant individual concern, while other species are so closely related in behavior that they can be considered synonymous by fishermen. Other mayflies are simply not available to trout in quantities large enough to require any attention from fishermen.
The species we discuss are responsible for virtually all of a Yellowstone angler's mayfly fishing. The list is actually quite short, and if you do not fish certain rivers or at certain times of the year, the list shrinks even more.
While each species has its own peculiarities, most demonstrate a preference for similar emergence and egglaying conditions. Knowing when the best mayfly activity will occur is as important as knowing the individual traits of a given species.
The heaviest mayfly emergences generally take place in overcast, cool weather. Misting rain or snow showers are ideal and stir the blood of all fly fishermen aware of this fact.
Though we can find no substantiation for this theory among professional entomologists, every experienced angler we know is in agreement on this point. Entomologists tell us that mayflies may actually prefer warm, dry conditions, and that they hatch just as abundantly on those kinds of days. We've been told cool weather simply concentrates the emergence over a short time period, and that during warm, dry days just as many mayflies emerge but do so by trickling off over longer periods.
We have trouble accepting that theory because it runs counter to years of observation by so many people. While inclement weather is not a strict requirement for good hatches (we have all seen great emergences on nice days), there is no doubt in our minds that many more flies come off when the weather is bad.
Of one thing we are sure: the best fishing during mayfly hatches is definitely on days when the weather is poor. If not because there are more mayflies, then certainly because they ride the water longer in cool weather, and suffer more emergence defects. Both these factors give the fish a better chance to feed on them. Too, trout in general (and browns specifically) seem to feel more comfortable feeding under overcast sides. This makes approaching and casting to them easier.
Of the mayfly spinners important to fishermen, all need moderately warm, calm conditions to lay their eggs. Wind stronger than a slight breeze, cold temperatures, or any precipitation precludes the spinners from reaching the water. Attention to the weather then, as well as learning the habits of the prevailing mayflies, can be very important to successfully fishing Yellowstone mayfly activity. Mayflies are most vulnerable and available to trout while hatching and during egg laying and subsequent spinner falls.
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