ANGLING TACTICS are determined by any number of factors, but weather is always one of the most important. When I think of the influences of weather conditions on a fishing situation, I am reminded of a customer who, a few seasons ago, walked into the Livingston, Montana fly shop where I worked. It was one of those days that would frustrate any angler or guide — a beautiful sunny June day in the 70s, but a front was pushing in, and the wind that makes Livingston famous was out in full force. This customer had tried to make the best of the situation on one of the local spring creeks, but the upstream wind had kicked the normally smooth flows into whitecaps, the hatch (what little there was) had been ripped off the water immediately, and casting was almost impossible.

Clearly beaten by the conditions, he came trudging into the shop for advice, hoping we could suggest something that would turn the day around. We just tried to console him, assuring him there was no secret fly pattern or magic technique that would improve the situation. After half an hour of pacing, he finally asked the question: "If you were in my shoes, what would you do on a day like today?" He had asked for it, so one of my co-workers gave him the brutally honest answer: "I'd go home and mow my lawn and fish when the wind isn't blowing so hard."

The point is that anglers don't get to choose the weather for their fishing expeditions. Even those of us who are lucky enough to live close to great trout fishing have jobs, and we can't abandon the rest of our lives just because fishing conditions are optimal. We schedule a fishing trip for a day off or for a vacation, often weeks or months in advance, and we try to pick an ideal combination of weather, water conditions, and hatches, but it doesn't always work out as planned. Knowing how to adapt fishing tactics to varying weather conditions is one key to consistent success. In this first part of the article, I will outline some of the generalizations that can be made about hatches and fish behavior with varying seasonal weather patterns. In parts II & III, I'll discuss dealing with the wind and weather patterns that are typical for areas like Montana's Paradise Valley.

The first thing I should note is that a number of the lessons about fishing in various weather conditions I have learned by reading the fly fishing literature and confirmed through personal experience. It is unfortunate, but I think sporting writers are particularly guilty in failing to acknowledge the contributions made by others. In the material that follows, I have tried to credit the authors who have instructed me, and though in some cases the books might be out of print, I would encourage a careful reading if possible.

Seasonal Hatch Patterns

Selective Trout by Doug Swisher and Carl Richards was my bible when I was learning to fish Silver Creek as a teenager, and one of the ideas introduced there is that hatches (mayfly hatches in particular) tend to occur at "the most pleasant time of day for the season." In spring and fall, most hatches occur in the afternoons, when the air has warmed to its highest point. In mid-summer, most hatch activity occurs in the morning and again in the evening, not during the heat of the day. Recognizing this pattern will allow the angler to anticipate hatch activity over the course of the fishing season.

According to Fred Arbona, Jr., in The Mayflies, the Angler, and the Trout, this seasonal trend is explained by a mayfly's need for moisture preservation during its adult life cycle, particularly for keeping the insect's exoskeleton flexible enough to allow it to molt from dun into spinner. Moderate temperatures and higher humidity are ideal conditions, and mayflies (as well as other aquatic insects) have adapted to hatch at times when their survival rate is highest. It seems to me that these same conditions are also critical even earlier in the life cycle, by allowing the wings of a newly hatched mayfly dun to dry properly. Cooler, humid air allows the wings to dry slowly enough that they can be fully expanded before they harden. On the other hand, for the best chance for survival, the wings should dry quickly enough that the insect can fly off to the cover of streamside vegetation before a bird or trout can pick it off the surface of the water.

Water Temperatures and the Sequence of Hatches

Swisher and Richards' "pleasant time of day" rule is not precise because it relies on an imperfect correlation between air temperatures and the environmental factors that actually trigger hatches — water temperature and light levels. Arbona points out that water temperature is the chief influence on the maturation rate of nymphs and is therefore the biggest determinant of the seasonal sequence of hatches. This is useful information for the angler because it means that aberrant seasonal weather patterns may have an important effect on the overall timing of hatches. Unusually cold weather or a late spring may delay the onset of hatches, while early warm weather and lower flows in a light snowpack year may allow the nymphs to mature more quickly than normal, allowing hatches to start earlier than expected.

For example, when we have an unseasonably warm April and May in Montana, damsels start to hatch in good numbers on many of the local lakes and ponds by the end of May. This is at least two weeks earlier than we would predict this hatch in a "normal" year. As we will discuss in part II of this article, there is a second set of conditions that trigger a hatch on a given day, but once water temperatures allow the nymphs to reach maturity, they are ready to hatch any time those conditions are met. When planning a trip to meet a certain hatch, it is a good idea to check on water levels and watch weather patterns to see whether to anticipate a shift in the timing of the hatch.

Physical Adaptations to Seasonal Weather

The need for aquatic insects to preserve body moisture has also resulted in physical adaptations to seasonal weather patterns: size and color. Just as hatch activity shifts to the cooler times of day during the warmest part of the year, the bugs themselves tend to become smaller and lighter in color as the season progresses. These adaptations result in less heat absorption in the warm summer months. On spring creeks and tailwaters, we see the shift from the big bugs of early season like Green and Brown Drakes to smaller Pale Morning Duns to even smaller Western Sulfurs and Tricos.

Although many anglers are familiar with the size and color of specific hatches, these seasonal trends can still provide useful fishing information. Mayflies that produce multiple broods during the season will be smaller with each successive brood. Callibaetis mayflies, common on stillwaters and the slower sections of spring creeks and tailwaters, may be as large as a #14 when the first brood hatches in May and June, but a #18 imitation may be just right for the late broods of September. Pale Morning Duns also show a marked shift down in size as the season progresses. (For an excellent recap of Swisher and Richards' and Arbona's writings on these seasonal patterns, pick up a copy of Neale Streeks' Small Fly Adventures in the West.)

Becoming familiar with the effect of seasonal weather patterns on hatches may provide some useful information in a given situation, but they are especially important as the background against which to interpret the effect local weather will have during a given day. Next we'll look at the effects of daily weather patterns.

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