Often as a guide I am asked by fly fishermen what fly pattern to use and how do I determine that. There are many factors that come into play such as time of year, temperature of air and water, and weather. You might get a hatch in the middle of the day, but if it's rainy, those fish might not be feeding around the water's surface. Flies that have been hatching for a few days might not be as prolific if a cold front is coming in. There is a lot for an angler to consider for sure, but having a basic knowledge of the fly fishing entomology is the foundation of fly fishing and fly selection for trout.
Entomology is the study of insects, in our case the aquatic insects of the river or stream bed. There are all kinds of aquatic critters that trout will eat, but they mainly subsist on two types of macroinvertebrates called caddis flies and mayflies. Stoneflies are also present in larger streams, but the basic life cycles are the same for all of these "flies". First of all, these bugs are always present in the stream and live under the rocks for roughly 364 days out of the year. They cling to the bottom of rocks eating very small things (think plankton) all year long. Mayfly nymphs and stonefly nymphs hold onto the rocks with their legs. Some caddis swim through the water from rock to rock, while others actually build small burrows attached to rocks. They are like little worms in a casing.
These critters hang out all year until the right conditions, water temperature, and sunlight finally tell them its their special day and they start the transformation to adults. Mayfly emergers will go from their nymph stage to an adult. They float to the top of the water column, where they get stuck in the surface film until their wings split out of their thorax and they can fly off. Trout feed readily on the stuck emerger, leaving nothing but an air bubble in their wake. They mate shortly thereafter and return to the water where they lay their eggs and then die, this is called a spinner stage and trout will also feed on the . 364 days of waiting for 1 day of partying. Caddis larva evolve to a pupa stage and they swim or float throughout the water column to the banks or dry rocks above the stream. Stoneflies do the same. They shuck behind their bodies and have wings as they emerge from their aquatic casing. As they come back to the water to lay their eggs you can see trout seek them out through the moving water.
How To Connect Fish and Bug
So what does this all mean to us? If it is the right day for these particular insects then they will be out in mass. Its not an individual survival thing, but a numbers thing. The concentration of this insect means that the trout will eat nothing else. This is what we call a hatch and usually when you see fish rising everywhere eating bugs off the surface. Your best bet is to "match the hatch". Try to skim one of these bugs off the surface, or check in some spiderwebs. It is imparitive to get your imitation the right size and color. Caddis have tent wings over the back and are kind of clumsy flyers. They almost bump into the water when laying their eggs. Mayflies are graceful and almost float through the air. Most types of mayflies typically hatch in the morning and evening hours, but understanding what life stages they are in are important, too. Look for dead spinners in a cluster on the water. Fish with this knowledge and understand that getting the right fly can mean an amazing day. However, we can prepare in advance with a little research. A local fly shop can tell you what fly patterns we use and sell you the hot fly, but what if you don't have a shop nearby? Hatch charts! Yes, hatch charts will help you take your fly fishing to the next level. Some biology student in the area has likely put together a hatch chart for the more known streams. A quick google search for "waterway you want to fish" + hatch chart and you will have a pretty good guide to what is likely hatching that time of year. For example "Shenandoah National Park Hatch Chart" and there is an example here:
This is a pretty basic chart, however it gives you some ideas of flies to carry in a particular time of year and you can already have the flies in your box.
Be patient and persistent. You might think you have the perfect fly and its not quite it. There are a wide variety of mayflies found in typical trout waters. Take the time to know the basic bugs found in typical trout streams and identify what the fish are eating. Maybe go smaller, more tippet, a slightly different color. Once you have the right idea of how it works together then it is only a matter of time and experience, and messing up plenty. When it all comes together you'll understand how to catch more fish and have all kinds of dry fly fishing stories to tell.