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Baetis

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Mayfly Species Ephemerella aurivillii

I have found nothing about this species in the writing of anglers, but the scientific literature shows it to be the most widespread species in the Ephemerella genus, and the only one that spans more than one continent. It may have gone unnoticed for so long because it might not produce thick hatches, and the sporadic individuals could easily be mistaken for Hendricksons.

Where & when

Time of year : Mid-June to early August

This species was first described in Europe, and has also been found in Asia and North America. I have collected them in Alaska and New York, and probably Wisconsin, although the ID on the Wisconsin specimens is less certain.

I found ready-to-hatch, definitive Ephemerella aurivillii nymphs near Fairbanks, Alaska, in mid-July. However, because this species is found across such a large geographic range, the emergence timing probably varies widely. They may go unnoticed in the East due to overlaps in timing with more abundant Ephemerella species.

Specimens of the Mayfly Species Ephemerella aurivillii

1 Male Spinner
Male Ephemerella aurivillii  Mayfly Spinner
This spinner molted from a dun after being photographed, and the dun form is listed here as a separate specimen. I've rarely found a more cooperative and photogenic mayfly.
3 Male Duns
2 Female Duns
15 Nymphs

1 Streamside Picture of Ephemerella aurivillii Mayflies:

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