Isonychia Hatch: The Overlooked Large Mayfly

It is easy to get a fly fisherman excited, just start to mention large mayflies. In the northeast there are plenty of large mayflies around in the springtime, but not so many in the fall. However, there is one around in the fall and it's also around in the springtime. The Isonychia or Slate Drake hatches in both the spring and the fall, but doesn't seem to get the attention of some of the other big mayfly hatches. While some fishermen overlook this hatch the fish rarely do.

An Isonychia is a rather large mayfly with heavy hatches. In the springtime the duns range from a size 10 to 12, while in the fall they may be slightly smaller with the duns ranging from a size 12 to 14. Not only are these large mayflies that have two major emergences, but also both emergences last over two weeks. The springtime hatch generally starts around the same time as the Green Drake (mid-June), which is why it is often over looked. However, the Isonychia hatch lasts a couple of weeks longer than the Green Drake hatch. This gives the fish plenty of time to recognize an Isonychia as food. The fall emergence usually starts sometime in mid-August and can last through September. Now most mayfly emergences only last about two weeks, but when you add up both of the emergences of Isonychia's we are talking about two months. It is easy to understand why this is such an important hatch when these duns are on the water for two months during the season.

Isonychia seem to prefer faster water and there emergence strategy tends to vary. Often they will swim to rocks that emerge above the surface of the stream and will crawl onto the rock to hatch. However, this is not there only strategy, they will also hatch directly out of the water in mid-current. This is great for fishermen because you will be just as likely to see fish rising to Isonychia in midstream, as you will near the shore. Also their emergence on rocks will leave their exoskeleton behind stuck to the rock. Looking for these exoskeletons on the rocks will let you know if they are hatching or not.

My two favorite patterns to fish during this hatch are an Isonychia parachute and an Isonychia elk hair emerger. The parachute should have a dark or maroon body and a dark post, the same colors hold true for the emerger. The parachute is deadly during the heart of the hatch and can even be blind fished (not casting to rising fish) during non-hatch periods. The emerger works great on finicky fish or is great to fish near the shore like an Isonychia hatching near shore. The other pattern that is a solid stand by for an Isonychia is an Adams. The body of an Adams resembles an Isonychia, and if fished in the proper size can be just as effective as an Isonychia parachute.

On my home waters, the Catskill area rivers in New York, the Isonychia is my favorite hatch. Nothing is more exciting than casting large dry flies to large fish in fast water and that's what happens during the Isonychia hatch. Next time you are planning a fishing trip in the spring of the fall, make sure you have a few Isonychia patterns on you.