If you have ever fished a mayfly hatch you know that using the standard dry fly that imitates an adult mayfly may not be as successful as you would like. Sometimes it works, and works well, but other times you may struggle to hook a fish, or the fish you do hook are small. For the most part you can tell what the fish are taking by how the fish are rising. This isn't 100% accurate, but it is a good place to start. If fish are not slashing at the surface for adult bugs you need to try something different. Below are some strategies and patterns that I like to use during these tricky situations.
Emergers: There are a million emerger patterns out there, I personally have the best success with emerger patterns without wings, and the reason... I think the fish just know a bug without wings is going to be an easier meal, than one that can fly away! I usually fish these patterns in flat water to get a good idea where the fly is, or I tie them as a dropper. When tying emergers, try to imitate the nymph first and have the emerging adult be a secondary aspect to the pattern. Many emerger patterns out there are basically a fancy dun pattern with a generic shuck, try it the other way around, fancy nymph and generic adult, and see if it is more successful for you.
Cripples: An often overlooked part of a hatch, and sometimes an absolute killer pattern. During a hatch watch the water, take a sample... observe first. You will not miss any action, and if you pay attention to what is happening with the bugs, you will end up catching more fish. Sometimes adult mayflies can't make it out of their shuck, and they float helplessly down the stream... When there are high numbers of these cripples, the fish respond, and so should you. Tying a cripple is where I like the upwing emerger patterns. I tie a standard upwing emerger with a shuck, and then mash it up with my fingers. The worse it looks, the better it works.
Nymphs: Many times anglers will use a nymph under an indicator before the hatch, and once they see a fish rise, the nymph gets put away and the adult pattern gets tied on. There are a few ways I like to use nymphs during a hatch... If you see a rise where the dorsal and/or the tail are coming out of the water trail a nymph pattern from your dry fly and put a tiny split shot on to get the nymph down a few inches in the water column. During other situations you can dress the nymph with the same stuff you use on your dry to keep it in or around the film. You will catch fish on the nymph so watch the dry like an indicator and hang on!
Next time you are in the middle of a nice hatch cast less, observe more, and don't be afraid to try something new!