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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

Dorsal view of a Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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SUNY Environmental Science and Forestry, Syracuse

Posts: 71
Jjlyon01 on Nov 17, 2008November 17th, 2008, 7:29 am EST
Since the failure of my cold water tank (cooled by running 40 feet of tubing through a mini fridge; got the temps down to about 55 for 3 weeks until the old mini fridge burned out). I was going to put trout fingerlings into the tank before it burned out and already had some mayfly, stonefly nymphs, and caddis larvae along with a few scuds and a helgramite larvae. They are all doing great even though they had a sudden increase in temps and the introduction of a few fish they have never seen before (black ghost knife and german rams). I figured they might live a few days like this and either die in the heat or be eaten by the fish. They, however, seem to be thriving and the caddis even show signs of maybe hatching soon. This changes my mind about some summer streams. I have the water well oxygenated. I think many streams dont support huge hatches in the summer primarily due to the lack of oxygen. Does this conclusion that I have made make any sense?
"I now walk into the wild"
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Site Editor
Plano, TX

Posts: 1311
Taxon on Nov 17, 2008November 17th, 2008, 9:30 am EST

I think many streams dont support huge hatches in the summer primarily due to the lack of oxygen. Does this conclusion that I have made make any sense?

That could certainly be one factor, but there are others. Among them would be clean water, appropriate habitat, and adequate food supply.
Best regards,
Roger Rohrbeck
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Bellevue, WA

Posts: 2758
Troutnut on Nov 17, 2008November 17th, 2008, 11:05 am EST
Many invertebrates found in trout streams have a wide range of temperature tolerances and will thrive at room temperature during the summer. Some will not.

It's very interesting to consider why there are fewer hatches during the heat of summer.

I doubt that dissolved oxygen is a primary reason. There certainly aren't enough insects in most streams to significantly deplete what oxygen there is. The overall level will be lower when it's warm, but there are still plenty of nymphs in the stream that are waiting to emerge during the fall, or even at night.

It's more likely that insects are just trying to avoid the hot, dry midsummer midday conditions, because they dry out the adults and the heat probably increases their metabolism and makes them burn through their limited energy more quickly. This idea fits well with the observation that hatches move toward the very early morning and very late evening when it's hot outside.

Another possible reason there are fewer emergences during midsummer may be that floods and droughts are more common and unpredictable at that time. A poorly timed flood at emergence time could really screw up a population.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist

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