Puzzling phenomenon!!! An "almost hatch"

[JasonN]

Yesterday on the Namekagon River in northern Wisconsin I noticed a really strange behavior among some Ephemerella mayfly nymphs (the genus that includes the Hendricksons and the Sulfurs). They were swimming by the hundreds up off the bottom and wiggling all the way to the surface as if they were going to hatch, but they didn't. They just wiggled around in the water column and, I assume, made their way back to the bottom.

This was happening in the middle of the day, with the water temp in the 30s. It's too cold for any Ephemerella species I know about to be hatching, although one person e-mailed me that he caught a thick hatch of size 12 BWOs on this river a few days ago. I'm baffled by that--I haven't found any Drunella nymphs in my extensive sampling of this river, and as far as I know they're the only mayflies around that size which are known as BWOs. It may be related to this weird phenomenon I saw yesterday, but I don't know.

It's also possible that this was the normal "invertebrate drift" phenomenon, but that's supposed to peak around sunrise and sunset. This behavior was only taking place during the warmest part of the day, and it seemed to intensify when the sun came out from behind the clouds every once in a while.

So, I'm baffled. I'm also not sure what species of nymphs were doing this weird "almost hatch" stuff--I collected two by reaching down and grabbing them with my fingers just under the surface. They most closely resemble Hendricksons, Ephemerella subvaria, but they're more of a ruddy brown than the olive color of E. subvaria and the abdominal tubercles aren't quite black. Below are pictures of two of the nymphs I scooped up:

Does anyone know for sure what species these are, and has anyone seen nymphs behaving like this before? I've never read about it, and it seems like the kind of activity that would really get the big trout going. I'd love to understand what was going on and why.