In the last section I talked about in-the-middle-of-a-hatch dry flies floating on the surface film, viewed from below while looking up into the infamous Snell's Window. So far I've done my best to steer clear of the Selective Trout issue.
From Selective Trout:
The Au Sable River in Michigan and other even-flowing rivers like it all over the country have many different species of aquatic insects hatching every day of the season. These rich rivers have high lime content that results in tremendous hatches. In fact two or three different species will often be on the water at the same time. On these prolific waters, the fisherman must not only match the hatch but also discover which hatch the fish are taking.
To the extent "selective trout" really is an issue I think it's important to focus and limit it's scope to heavily fished and smoothly flowing, flat-surfaced waters found on spring creeks and tail water fisheries below man made dams. Free flowing freestone rivers like the Yellowstone, Madison, Jefferson and Gallatin are often difficult to fish, but not because of selective trout behavior.
I remember one day fishing with my old friend Don Labbe, on the Yellowstone. Don was on his way to meet a group of riends in Eastern Montana, but he came an extra day early so the two of us could drift the Yellowsone together. I was no longer a licensed guide but I did row the whole day while Don Fished. The Yellowstone can be hot as a pistol but it can also be among the most frustrating places to fish I know. We had a tough time of it that late summer day. We tried everything, and I mean everything. The only fly that produced at all was a heavy bead head Prince Nymph fished deep, several feet off the bank and most ofall at the heads of riffle drop offs. We didn't catch any big fish that day but once we got onto the Price Nymph rhythm we did catch quite a few--small rainbows and white fish anyway. On a day when there are feeding fish and actively hatcing aquatic insects it still isn't that hard. You see what insects are hatching and change flies until something starts to work.
In Montana midges and Blue Winged Olives sometimes hatch together, in the early season. The midge fishing then is often excellent, but if the BWOs start to hatch there is no mystery. Any dimpling fish will be taking maylies at that point. They won't be eating midges again until the mayflies stop hatching. During Salmon Fly time the fishing often gets slow in the late afternoons and then picks up again for an hour or so before dark, when tan/brown summer caddis first start swarming over the water. You can still catch the occasional fish on a big salmon fly then, but you are more likely to do well with caddis, right before dark. The good Salmon Fly fishing starts again the following morning. We sometimes see Green Drakes and Pale Morning Duns hatching on the same day too. In those cases I think the Green Drake would likely be everyone's easy choice.
In the Green Drake case it's easy to imagine an individual fish demonstrating a sudden vulnerability to a Rene Harrop Emerger, after repeatedly refusing a Sparkle Dun. But I also wouldn't expect that Harrop Emerger vulnerability to survive more than one or two bad casts. At that point it might be time to try a spent wing pattern or even a soft hackle wet fly. Alas three different mayflies hatching simultaneously is a delemma I've ever experienced in Montana.
In late Summers on the Paradise Valley Montana (Yellowstone River Valley) spring creeks there used to be an intermittant evening hatch of pale cream-colored mayflies we guides refered to as "Sulfurs." There were not at all the same as the Sulfur hatches of Pennsyvania. I'm not sure what they were but those mayflies spread their wings and flew the instant they hit the surface. On a good hatch night there would be flying mayflies all around you, but none of them floating on the surface of the water. And no fish would take a dry fly. You could fish 'til you were blue in the face with dry flies and still come up skunked.
Almost anything below the surface film worked like a charm however. We often used a tiny cream-colored thing that was just a hook with slightly darker ribbing over sparse dubbing. Or we fished soft hackle wet flies. You go with what works. That hatch is alas now gone for the most part. We used to see it for a week or so in late July. Now not at all.
On the lower reaches of the Gallatin, Madison and Jefferson Rivers in September there is an occassional hatch of large white mayflies that is quite a spectacle to see. There are not that many flies but they are large and fun to look at. And they too can be seen flyig overhead as you pull on the oars of a drift boat, down near the headwaters of the Missouri near Three Forks. They too spread their wings and fly the instant they hit the surface. And during that hatch too can only catch fish with wet flies.
That's an interesting but rare case of selectivity at work, for both imitations and naturals. You might even say the fish were keying in on drifting emergers, but there were no floating adults to reject. Like the spring creek sulfurs those mayflies spread their wings and flew the instant they hit the surface.
One of my fondest memories involves a river camping expedition on a central Montana river in September, a time when that river is usually to low to navigate. We had an odd summer of nearly constant rain about that year. There were four of us in two boats: Hal and Mooky and me and Randy. The ranger where we got our permit said we would be the only ones in the canyon. The Spruce Budworms were hatching and the fish were eating #12 and #14 Elk Hair Caddis like kids with cotton candy at a county fair. We caught so many fish Mooky and Hal clipped their hooks off so they didn't have to release any more fish. Their fun from that point on was all about who got the most rises. The really odd part about that amazing trip was the wet fly fishing. It didn't work. Not well anyway. Those central Montana fish were keyed in on floating Spruce Bud Worms. No argument there. But there were no wet flies to reject. Spruce Bud Worms are not aquatic insects that rise up from below and then fly off. There were zillions of them flying overhead and perching on overhead branches. Those that lost their way and landed on the water got eaten long before they even had a chance to sink.
During the more important and more reliable Baetis and Pale Morning Dun hatches that form the bulk of Montana's most important hatching events, I have not seen anything like fish "keying in on emergers." Imitations do get refused, often at the last possible moment, moreover which imitations get refused constantly changes. Real insects tend to get eaten, regardless profile. In the last section I did talk about fish occassionally rejecting real insects. That's real but rare, and in that context it had more to do with visual confusion, rather than decisive preference for, for instance, emergers over fully-formed duns.
Actively feeding fish take possession of a feeding lair and defend it against intruding competitors. They adopt a semi-regular feeding rhythm and rise to the surface film to suck in drifting insects like clockwork, like an extra-slow metronome swinging left and right on a piano. Once feeding vigorously they tend to take everything real that comes their way, in on or just below the surface film. They can be very hard to catch on arificial imitations and some flies do work better than others. Which fly works best right now often changes every few rises. But that's because they are at that point influenced by predatory fishermen, rather than anything having to do with the natural flow of events in their watery habitats.
Chaning flies a lot is the best thing about spooky fish. Changing flies a lot means you really do need a rich selection of patterns in your box. Changing flies a lot means you really do need to stay busy tying flies all Winter long.
Among the flies that do work it doesn't much matter if it's wet or dry. Or a spent wing or a nymph. Either it works or it does't. And if it doesn't work right now, try something else.