How does this hatch thing work?
I get this question quite often in various forms and it can be a complicated question to answer. For the new fly angler the worst thing I can do is go all aquatic entomology on you. The best way to look at what is a hatch is to begin with the definition. A hatch is when an aquatic insect that is living on the bottom of the river for up to a year must go through a metamorphosis like all aquatic insects. A metamorphosis is similar to a caterpillar spinning a cocoon and emerging as a monarch butterfly. In the case of the aquatic insect it must leave the bottom of the river and swim to the surface to emerge as a winged adult where it will mate and return to the river to lay its eggs and eventually die. This is better known as a hatch. A hatch is really why we anglers dry fly fish. It’s the excitement of fooling a trout into taking your fake fly over that of the natural.
When a hatch or emergence occurs it’s usually just one specific aquatic insect like a mayfly or caddis fly. So for the sake of answering the question it’s when one specific aquatic insect leaves the bottom of the river to swim to the surface to become a winged adult. So we are now just looking at one particular aquatic insect that will be hatching. You should know what kind of insect that will be hatching on the river you are fishing ahead of time. You can get this information via a google search for a hatch chart for the particular river you will be fishing or by your local fly shop, fishing partner or local fly fishing report. The information you should be looking for is what bugs are hatching or emerging on the river you are fishing and what time of day to expect them.
You can bell curve any emergence on any river with a bell shaped curve. So if the particular hatch you want to learn about for your local river might start in May and end towards the end of July. A bell shaped curve would show the hatch being slow or week at the beginning of May, strong through June and slow or weak by md July. You can do a 24 hour bell curve and get similar results. How strong or how week a hatch may be on a river can be observed by the bell curve method. However, as most fly angler can tell you sometimes there is just no rhyme or reason to why a particular mayfly hatch is epic one day and mild the next. Weather such a high or low barometric pressure changes or wind and rain can put down or cause a minimal hatch. So there are quite a few external factors that can complicate any kind of consistency one might wish to expect when preparing for an occurring insect hatch on a river you want to fly fish.
What’s more important than trying to figure out how a hatch works is being prepared ahead of time with the appropriate flies to fish the hatch. Only two things catch trout: the fly rod and the fly. Having a complete understanding of how a hatch works will do little to help become successful during that hatch if you do not have the flies you need to fish that hatch. In other words you must be prepared to “Work the progression” and that requires for you to have a solid base of imitative flies, that will match as best we can with fur and feathers, exactly the insect those trout are feeding on.