Hendrickson mayfly hatch (Ephemerella subvaria) is is a major mayfly on the freestone streams throughout Northern Wisconsin. The abundant nymphs are found in the riffles among the rock and gravel bottom. Depending on the Spring weather pattern and the stream, Ephemerella subvaria nymphs will be hatching anywhere from mid-April to mid-May. If the temperatures stay cooler the hatch can last for 2 to 3 weeks. Offering fly fishermen some great Spring trout fishing.
During this time in the mornings the trout will begin to key in on the Ephemerella subvaria nymphs. The nymphs become very active and their wing cases will turn very dark prior to emergence. A size #12 hare's ear nymph can be deadly prior to the hatch. Once the hatch first begins a CDC wing emerger or a soft hackle wet fly can take a lot of trout. When the small gray winged sailboats start to cover the water a size #12 or #14 comparadun, or the classic light and dark Hendrickson dry flies can make an exciting afternoon of fishing. The male subvaria are generally smaller and a dark reddish color. The Red Quill dry fly is an excellent imitation for the male Ephemerella subvaria dun. Tthe female duns are more of a gray-brown color and have smaller eyes. (Click here for Hendrickson mayfly patterns).
Generally in cooler Spring weather the hatch will begin around 1-2 pm in the afternoon and can last for a couple of hours. On overcast days the hatch may sporatically occur from morning to late afternoon. If the weather is unseasonably cold the duns may ride on the water for quite a long time before being able to fly off. This makes the helpless duns very easy meal for rising trout.
In the evening don't pass up the terrific Hendrickson spinner falls. They generally occur around dusk over the riffles and faster sections on the stream. Hendrickson spinner falls may also appear in the morning with warmer Spring temperatures.
Below is a short video I took of an Ephemerella subvaria nymph (Hendrickson) swimming. Notice the swimming motions of the nymph. This is why a small, dark wiggle nymph pattern works so well when the trout are feeding on the nymphs, if you have the patience to tie them. :-)