|Pteronarcys californica, the Salmonfly|
Salmonflies are a mythical insect for many western fishermen. I've spent more than a few seasons doing something I call chasing the hatch; dropping everything important and driving to the river and turning over rocks daily, obsessing over flows and water temps. Sometimes I feel like no matter what I do, I always seem to miss them. If there's one thing you could say about the Big Orange, it's that they lack consistency, from year to year, from river to river, and from reach to reach. Here are some of the things I've learned about salmonflies over the years, from reading, studying, and fishing.
There is only one species of salmonfly in the western US, Pteronarcys californica. There aren't any small salmonflies. They are all big, and the nymphs usually take at least three years to mature. If you sample aquatic insects in the spring, you often find three different sizes of nymphs, one for each age class. For this reason there are always salmonfly nymphs present in the river and available for trout, regardless of time of year. A #6 to #12 girdle bug is always good to have on hand.
Salmonfly nymphs require high oxygen content and are an excellent indicator species for high water quality. The nymphs are usually found in rapid, boulder strewn areas and these types of rivers and stretches have the best hatches. It's worth mentioning that giant golden stones tend to hatch along with or just after salmonflies, and the adults are often confused with each other. There really isn't any sure way to tell the difference without keying out the bugs, but salmonflies tend to be orange, while goldens are usually yellowish-brown.
|Salmonfly exuvia, on a burnt tree above classic stonefly habitat.|
When salmonfly nymphs are ready to hatch, they migrate to the edges of the stream and stage under rocks, before crawling out of the water. Usually when you find them right next to the bank, it's only a matter of hours or days before the hatch is on (unless the river blows out. That's bad). The nymphs congregate in the flooded willows or horsetails near the bank, on bridge abutments, or under overhanging rocks. Trout follow them. Anglers often find nymph "shucks" or exuviae around these areas; that's the technical term for the nymphal exoskeletons left behind by the adults. Yes. I am a super nerd.
In my experience, a salmonfly hatch lasts for a VERY short time. These big bugs are sitting ducks for fish, birds, mammals and predatory insects. Also, stonefly adults, like mayflies, have no working mouth parts, so they can't eat or drink. This all means that in order for salmonflies to survive, they all hatch, breed and die pretty quickly. It's a coordinated burst. A few might be early or late, but they have a lot harder time mating and not getting eaten. There are usually only adults on a given stretch for a few days at a time.
I've said this before a million times, but if you find a lot of shucks and no adults in a given area, the hatch is pretty much over. You might still get fish to eat the adult imitations, but that's it. Hasta. As we speak, on a few stretches of Missoula rivers, the hatch is already done. On others it has yet to begin. If you want to be there when it happens, I know of only one way to do it. Drive to the river and look; turn over some rocks, check the flows, and hope you get lucky!