Look at that! Those trout are feeding on duns.”
My buddy Nolan pointed over the starboard bow of our drift boat: “Do you see those fish rising near the bank about forty yards away?”
We were floating the Yellowstone River in Paradise Valley, south of Livingston Montana. It took me a few moments to spot the three trout whose noses kept poking up from the surface. But I didn’t see any tiny mayflies in their dun stage. I thought Nolan was arrogant to make such a claim.
How could he have the 30x vision of a spotting scope?
After I scoffed at him, Nolan explained that he couldn’t see those insects any more than I could. Rather, he made the call by watching how the trout were feeding. While it is not an exact science, you can generally figure out what trout are feeding on by watching their behavior.
1. Noses mean duns
If you see noses poking through the surface, the trout are feeding on mayflies in their dun stage. Sometimes, these trout appear to be standing on their fins, up to their eyeballs in water.
The dun stage is the first of two adult stages of mayflies. A Parachute Adams may work fine. But in some cases — slow, clear water or a specific hatch — it might pay to use a Comparadun or Sparkle Dun pattern. Some kind of cripple pattern may work, too, given that most aquatic insects do not make the transition from nymph to adult stage and remain stuck in the surface film.
2. Fins mean nymphs
If you see only a dorsal fin or tail (and not the trout’s nose), then the trout is feeding on something just below the surface.
This is a good time to use unweighted nymph, which floats just beneath the surface. Or, you can use an emerger pattern which sits low and protrudes into the film beneath it. A pattern which rides high, like a Parachute Adams, will not work well unless it gets water-logged and disappears from your sight.
3. Dimples mean midges or spinners
If you see a small dimple in the water, chances are are the trout are feeding on midges or spent mayfly spinners. You may or may not see the trout’s nose. Sometimes you will even see the trout gently roll through the surface with the grace of a dolphin.
Aside from specific midge patterns, a size #20 Parachute Adams works well for midges. Mayfly spinner patterns have light bodies and wings which lay out to the side (like airplane wings) rather than shooting up from the body at a forty-five degree angle.
4. Splashes mean caddis
If you see rising trout making splashes, they are likely feeding on caddis flies. The reason for the splash is that these flies are fluttering on the surface, and the trout go into attack mode. Some kind of elk hair caddis pattern will do the trick.
Of course, watching surface behavior is only one part of your knowledge base. Knowing which hatches happen in the river you’re fishing at particular times of the year and even specific times of the day is critical to making the correct visual assessment. As always, talk to the experts at your local fly shop or read their reports online. Then keep your eyes open to watch what is happening on the river’s surface.