Think of a stream or lake as a big sandwich. There is the slice of bread on top – the surface, the slice of bread on the bottom – the substrate, and the filling in between – the water column.
by Rick Hafele
Aquatic insects live in a three dimensional world. Understanding how they behave in each dimension can help you choose an effective fly and catch more fish.
Fly fishing is a three dimensional sport. First there is the water’s surface. The two dimensional surface produces that unique space where air and water meet. It’s also the one place where fish and angler get a close look at each other. Often that look only lasts a split second, but when your dry fly floats on the surface and a trout approaches with hungry intent and hangs just below your fly before sucking it in, that split second can feel like forever. Without a doubt the water’s surface provides some of the sport’s most exciting moments. But streams and lakes have depth, and trout spend more time well below the surface than near it, for safety if nothing else. For this reason much of our fishing takes place in the third dimension, that vast area that covers all the water below the surface.
Aquatic insects also live in all three dimensions. Think of a stream or lake as a big sandwich. There is the slice of bread on top – the surface, the slice of bread on the bottom – the substrate, and the filling in between – the water column. Aquatic insects move from one layer of the sandwich to another as they progress through their different life stages. Understanding how they behave in each layer can help you understand what fly to use and how to fish it.
The Bottom Slice
Aquatic insects spend most of their life on the bottom slice of bread. This slice provides both food and shelter for nearly all the hundreds of different species of aquatic insects. As a result the bottom slice of bread is a rich composite of rocks, algae, and insects – sort of a 21 grain slice – and is where most of a trout’s food lives most of the time. But the nymphs and larvae living on the stream or lake bottom aren’t static and motionless. Especially in streams, insects let go of the bottom and drift in the current on a regular daily cycle. This phenomenon is called behavioral drift, and knowing about it can increase your nymph fishing success.
Insect drift has been studied extensively in rivers and streams around the world with some very consistent results. First, it’s easy to imagine that nymphs and larvae would get washed off the stream bottom during storms and high flows. They do, and this is called catastrophic drift for obvious reasons. But day-in and day-out nymphs and larvae let go of the bottom and drift in the water for a few feet to perhaps tens of feet before getting a grip once again. Such behavioral drift activity appears to be a simple way for nymphs to find a new, and hopefully improved, home. For example, if a nymph finds itself on a rock crowded with too many other nymphs, and the food supply becomes severely reduced, a quick solution is to let go and drift downstream to a new location with fewer competitors and more food. Drifting downstream may also be a good choice if a large predator, like a golden stonefly, happens to walk into your neighborhood. What’s a little surprising is that while some nymphs and larvae drift throughout the day, there are specific periods of the day when this drift activity significantly increases. Almost always there are three peaks in drift over a 24-hour period. The first peak occurs an hour before to an hour after sunrise. The second peak lasts from roughly an hour before to an hour after sunset. And a third peak often occurs sometime during the middle of the night.
Unless you are a night owl, the night peak in drift is not something you need to think much about. But the sunrise and sunset drift peaks are definitely worth paying attention to. Since trout feed primarily on what’s drifting in the current, rather than plucking food directly off the bottom, these periods of peak drift activity provide a huge influx of food. If you’ve been fishing for awhile, you know that some of the fastest action occurs both early and late in the day. This can be due to large concentrations of insects on the surface (more on that soon), particularly in the evening, but it also reflects the increased food drifting below the surface, which occurs even when there’s no activity at the surface. So, if you are wondering when you should try your luck nymph fishing, there’s no better time than at first light in the morning and last light in the evening.
But there’s even more that we know about behavioral drift that can help you catch more fish. Namely, who’s drifting? It turns out that some nymphs and larvae are much more abundant in the daily drift than others. In fact there are a handful of species that generally make up 50 to over 75 percent of the insects drifting. Knowing what these species are can give you a big leg-up on picking a more effective nymph pattern for your morning and evening nymph fishing.
With few exceptions the two most common species in stream drift are nymphs of the mayfly genus Baetis (the blue-winged olives), and chironomid or midge larvae. Blue-winged olives are ubiquitous throughout North American streams and rivers, and consistently provide food for trout. The nymphs are streamlined little swimmers, with an emphasis on little. The largest might be matched with a size 18 nymph, but most need to be imitated with size 20’s, and many are smaller. Chironomid or midge larvae are typically even smaller. I group them into three sizes: small, tiny, and invisible. Fortunately midge larva patterns are quite easy to tie, but threading your leader through the eye of a size 22 or 24 hook in the dim light of early morning or late evening, presents a whole new challenge.
Other, less minute, insects can also be important. Net-spinning caddis larvae (Hydropsyche sp.) frequently drift in good numbers, as do the American Grannom caddis larvae (Brachycentrus americanus). These caddis can be matched with nymph patterns ranging from size 16’s to 12’s. If amphipods (scuds) are abundant they too can be active in the morning and evening drift, and can often be matched with patterns in the 16 to 12 hook-size range.
The abundance and importance of blue-winged olive nymphs, however, is almost universal, and if I had to pick one nymph pattern to tie on in the morning or evening it would be a size 18 or 20 blue-winged olive nymph. Where fishing two nymphs at once is legal, a favorite combination of mine is a size 14 olive or green caddis larva combined with a size 18 blue-winged olive nymph as the trailer. It’s important to keep in mind that the vast majority of food that’s drifting is small, so don’t hesitate to fish tiny nymph patterns.
Middle of the Sandwich
While a lot of action goes on along the bottom, the middle layer of the sandwich comes into play during critical moments in the life cycle of most aquatic insects. Put simply, aquatic insects have to find a way to successfully cross this middle layer in order to escape their watery way of life and become terrestrial flying adults. For a little insect this middle layer must be like a desolate, deadly desert that has to be crossed in order to reach adulthood and reproduce. For a trout this middle layer brings food, and when large numbers of insects cross through it to reach adulthood, food is plentiful and living is easy.
There are several methods insects use to cross this dangerous middle layer, and as an angler trying to catch trout, you want to match their behavior as closely as possible with your imitation. The most common way insects move from the bottom slice to the top slice is to simply let go of the bottom and swim as fast as they can across the desert-like water column. For the best chance of surviving this trip, members of one species generally time their swim to the surface to occur at roughly the same time as other individuals . In other words there seems to be some safety in numbers. Plus, emerging together helps make sure that if you do make the trip successfully and become a winged adult, you can find someone nearby to mate with. Thus, a well-timed mass emergence has a number of benefits. As fly fishers, we call such a large synchronized emergence a hatch, and being on the right stream during the right hatch is what many fly fishers spend all winter planning and trying to predict.
Some insects are able to race across the middle layer desert better than others. Mayfly nymphs, like the abundant little blue-winged olives, are quick swimmers, and can move quickly from the stream bottom to the stream surface. Other mayflies, like pale morning duns (PMDs), swim slower, but still move relatively well. Clinging mayfly nymphs such as March browns or pale evening duns, of the family Heptageniidae, may be the slowest swimmers and thus the most vulnerable. But when compared to the speed of a swimming trout none of these little nymphs stand much of a chance to escape once a trout decides they are on their menu.
Most mayfly nymphs swim all the way to the surface film before their exoskeleton splits open allowing the winged mayfly dun to pop out. When trout are feeding on these morsels, as they frequently do, matching the nymphs with an appropriate nymph pattern and drifting it at some intermediate depth can be very effective. For the most part you don’t have to impart much action to mayfly nymph patterns fished during a hatch. It is helpful however to let your fly swing up to the surface at the tail end of each cast. The resulting lifting or rising motion often triggers trout to rush in and take the fly before it escapes.
For some species of mayflies, the dun escapes the nymphal exoskeleton underwater, somewhere in the middle of the desert, and the dun then has to finish the trip up to the surface to escape its watery world. In this case you need to use a fly pattern that looks like a submerged dun rather than a nymph. Soft hackle fly patterns and traditional wet flies do a good job of imitating these subsurface emergers. Fish them an inch below the surface to a foot or more deep depending on how deep trout are feeding. And because duns don’t swim well and rise to surface mostly by gases trapped under their skin, drift these flies with little or no extra action other than the rising motion at the end of each cast. Pale morning duns (Ephemerella excrucians) and Western green drakes (Drunella grandis and Drunella doddsi) are some important mayflies whose duns frequently – but not always – emerge underwater.
Caddisflies and midges also cross the mid-layer desert to emerge into adults in the surface film. For these insects it is the pupal stage that makes this crossing. Caddisflies and midges progress from the larval stage to the pupal stage before becoming adults. Insects like mayflies and stoneflies have no pupal stage. The length of the pupal stage varies widely depending on the species. Caddis pupae generally take one to two months to mature. Midge pupa may complete their development in one to two weeks. In both instances, while the pupae are developing they are hidden, either sealed up in cases or stone shelters like caddisflies or covered in silk and bottom sediment like most midges. As a result during this phase pupae are not available to trout. However, once the pupae are mature they leave their hidden shelters and swim just like mayflies up across no-mans land to the surface.
Caddis pupae are excellent swimmers and cross the middle layer with a quick and erratic swimming action. During a good caddis hatch many trout feed exclusively on these swimming pupae, and that’s the perfect time to fish a caddis pupa pattern of the size and color that matches the natural. Because the naturals swim with an erratic action, it can be effective to add small twitches and short strips to your fly while they rise up to the surface. Midge pupae on the other hand do not swim quickly to the surface, and are a best imitated with a dead drift or very slow retrieve.
The last approach insects use to reach the surface is to simply crawl out of the water. Dragonflies and damselflies have adopted this method, but stoneflies may be the best known example of aquatic insects using this approach. Mature stonefly nymphs simply crawl along the bottom until the reach shore, or perhaps a boulder or log that protrudes from the water. They then keep right on crawling above the waterline, where the adult then safely escapes the nymph exoskeleton. This behavior eliminates the risky business of swimming to the surface and thus being completely exposed to feeding trout. The crawling nymphs however do get washed into the current along the stream bottom in larger numbers than at other times in their life cycle. Therefore, while this behavior eliminates the need for special emerger patterns or techniques, it does provide an excellent opportunity to fish nymphs. If you know stoneflies are emerging and you don’t see trout taking adults, put on a nymph pattern and dead drift it along the bottom. Make sure to drift your nymphs close to shore as well, since the nymphs are moving towards shore to crawl out.
The Top Slice
While the bottom slice and the middle layer of our imaginary sandwich produce a lot of feeding action by trout, it is the top slice that gets fly fishers most excited, as the surface slice of bread is where we fish dry flies to rising trout. The history of fly fishing is a long one, and the fact that fly fishing allows anglers to effectively catch fish feeding at the surface is a major contributor to its popularity. It doesn’t matter whether you’re drifting a size 22 trico spinner over picky trout or dropping a size 4 deer hair popper onto a bass-filled lake, seeing a fish take your fly never stops being a thrill. Some will even say that fishing the surface is the only true form of fly fishing.
There are two periods during an aquatic insect’s life that they find themselves on the top slice of bread, and both are brief. The first occurs when bugs change into their adult winged forms, and the second takes place after mating when they return to lay their eggs. The transition from nymph or pupa into a winged adult is the riskier of the two because it is more complicated and takes longer. Imagine a fish changing into a bird and you get some idea of the radical change that takes place. Not only do wings have to suddenly appear and become functional, but the animal itself has to make the shift from breathing and living underwater to breathing and living above it. I know of no other animal that makes such a dramatic change during its life. Amphibians come close, as they also transition from aquatic to terrestrial lives, but none that I know of sprout wings and fly.
When insects emerge, or hatch, in the surface, trout and many other fish take advantage of the situation by feeding heavily on the rather helpless food supply. And the lucky angler present during a hatch likewise takes advantage of it with surface flies that imitate the trout’s food. The trick is imitating not only what the food looks like but also how it acts when emerging.
It would seem an easy task to determine what an emerging insect looks like, but when changing from nymph or pupa to winged adult the insect’s appearance changes throughout the process, and trout often feed selectively on just one particular look of the insect during a hatch. For example, when mayflies emerge in the surface, trout may focus their feeding on nymphs floating in the surface film, on winged duns just beginning to escape their nymphal exoskeleton, on duns with wings fully formed but the nymphal exoskeleton still clinging to their tails, or on fully free duns just floating quietly before they can fly. Then there are cripples, those duns that had problems emerging and died in the surface looking like a twisted victim of a car wreck.
Fly fishers have created a variety of emerger patterns that match the subtle differences in appearance that mayflies progress through during a hatch, including the car-wreck look. The same is true for caddisflies, and midges, which also transition through several different looks during emergence. Now multiply the variety of emerger patterns with the hundreds of different species of insects that come in different shapes, sizes, and colors, and you can quickly see why picking the right fly to catch selective surface feeding trout isn’t always so simple.
Next comes the egg laying stage. After mating, either in the air or on streamside vegetation, gravid females need to lay their eggs on or near the water. Most species find it best to take the direct approach, and end up landing on the water’s surface where the eggs are extruded from their body and sink to the bottom. This produces the classic spinner fall during mayfly egg-laying events, and fishing matching spinner patterns in the surface can be extremely effective. Other insect groups, like stoneflies, caddisflies, and midges, also lay their eggs on the surface, and can be matched with appropriate looking dry flies.
But females of many species, especially caddisflies, also lay their eggs by diving through the surface and swimming to the stream or lake bottom, where they paste their eggs on the substrate. Once the eggs are laid the females let go and usually drift back up to the surface buoyed by bubbles of air on their body. Matching this subsurface egg laying behavior requires subsurface patterns and tactics.
Put simply the tactics for fishing dry flies fall into just two categories: dead drift or with action. A dead drift presentation, meaning your fly drifts completely naturally with the current without any imparted movement or drag, is preferred most of the time, which explains the frequent discussion in trout fishing books and articles about how to get a good drag-free drift. But for insects that move a lot on the surface during emergence or egg laying, imparting action to your fly can be just as important as a good drag-free float. Caddisfly adults often run across the surface after emerging or when laying eggs. Again this means you need to pay close attention to the behavior of the natural on the water. When in doubt start with a drag-free float, but if that fails try adding some twitches and subtle motion to your fly.
To improve your fly-fishing success learn to think in three dimensions. If you understand how insects and fish live and behave in each layer you will more often choose an effective fly, fish it the right way, and catch more fish.