Why do BWOs (and Mayflies in general) hatch better on cloudy days?

[SperryWater]

I think we've all noticed this or at least think we have noticed this. Any theories as to why?

[AaronJasper]

I would not make that generalization. Some of the best hatches that I have ever seen have been on sunny days.

I think that we as anglers tend to associate the best hatches with fish feeding on them. For example when it's sunny and there is a major insect emergence fish tend to take the nymphs and ignore the duns, for the most part. Of course there are exceptions.

When it's cloudy fish tend to be more relaxed and the sun is not stressing their eyes. Remember, trout don't have eyelids so that have to deal with bright sun. So when it's cloudy, they are inclined to feed more heavily on the insects due to the lack of intense light.

So I think that it might be more of a fish feeding issue, not one that has to deal with insect emergence?

[Troutastic]

Some people argue that on heavily fished waters the fish will ignore duns during hatches, often BWOs, because of the history of getting fooled by dryflies. I know that in some areas around here people only fish with emergers for the big caddis hatch (coming soon!) for that reason. The fish only key on the nymphs and emergers.

Your thoughts?

[steve z]

Pure speculation: perhaps it is an evolutionary trait. It is harder to see into the water on a cloudy day so perhaps the fish are less fearful of aerial predation since it would be harder for a bird to pick off a fish on a cloudy day then on a sunny day.

[rbnielsen]

My guess is cloudy days somehow offer a greater chance for the BWOs to hatch and get flight, and not get eaten.

[AuSableTrophyhunter]

One theory is that duns need to be protected from dessication from the sun. That is why as summer approaches the mayflies get lighter in color, even white to reflect solar radiation. The BWOs could also avoid being dried out if they emerge into humid air or overcast skies or at dusk.
In early Spring, the bugs are dark colored to absorb heat because the water and air is colder. I believe this theory makes sense.

[AaronJasper]

Good point there as I have seen many mayflies with darker wing have them burnt off by the intense sunlight during the summer months. Ever think of the reasons why tricos do their thing so early in the AM? I thinink that it's mother natures way of taking care of her own.

I am sure the the angle of sunlight and the season that is is has a lot to do with the effect of sunlight on the wing of a dry.

[big south]

Mayflies have no mouths. The primary cause of death is not starvation, but dehydration. Cloudy days are preferable to hatch in order to maintain hydration. True, mayflies do also hatch on sunny days and occasionally on arid days, but sunny/humid days are a good second choice to cloudy/humid days.

[AaronJasper]

Good points and I agree with both of them!

Btw welcome to TPO