Many fly fishers live to fish the big major hatches. They dream of hitting that one perfect day when that dream hatch blankets the water and every trout in the stream has its nose poked through the surface film sipping them in like a hog at a trough. Been there and done that, and if they had a shirt for it I would have one, however, as wonderful as it is to witness such a natural phenomenon the fishing under such conditions are often less than spectacular.
Many years ago I witnessed a blanket hatch of mayflies on the Rail Road Ranch water on the Henry's Fork in Idaho. This was long before the Ranch became Harriman State Park and I parked at the mail boxes [the old timers will remember this] and walked into the river below Bonefish Flats. It was a beautiful summer day, and the Tetons just over the border in Wyoming stood out clearly. When I arrived at the flats hatching mayflies covered the water and looking upstream I only saw one other angler well upstream and on the opposite of the river. Wow, I thought that I had died and gone to heaven. I collected a few hatching mayflies and sat down on the bank to select a suitable fly. After tying on a fly I started looking for a target. I scanned the water, nothing, I concentrated on the water along the bank, nothing. Finally I saw a rise but I sat watching the spot where the fish rose for several minutes and it never came again. The same scenario repeated itself three or more times over the next hour. A fish would show once but never twice. I stalked the banks looking for a subtle riser hugging the banks but nothing showed. When I looked up to see how the angler on the other side of the river was doing he was gone. Later, when I checked in with my friends at Will Godfrey's Fly Shop they told me that the hatch had been going on for days and the fish were literally stuffed to the gills.
I have also confronted the opposite problem, a blanket hatch where it appears that every fish in the stream is on the feed. The first problem for the angler is to find one fish and concentrate on that one fish rather than casting to one fish and then casting to another. I call this the scatter gun approach and it usually results in few if any strikes. When confronted with this situation experienced anglers will scrutinize the various rising fish and try to find the best fish and concentrate their efforts on that fish. Patience is essential since a blanket hatch of insects' offers many targets and the fish will allow many insects to pass before it feeds again. Usually a fish will establish a pattern, an interval between each rise that can be quite precise. Larger fish are more prone to establishing a pattern while smaller fish often rise erratically. Sometimes it is also advantageous to show the fish a pattern that is slightly larger than the naturals. I don't know why this works but I have witnessed the success of this method many times when a rising fish turned down what appeared to be the proper sized imitation.
One of my favorite approaches when I'm confronted with a blanket hatch is to look for a single fish that is out of the main flow of hatching insects. The Salmon fly hatch [Pteronarcys californica], that is prevalent on many large western rivers, is one hatch that brings the largest trout to the surface, but it can be one of the most frustrating hatches to fish. Rivers can be high, cold and dirty, the fish can feed up on the emerging nymphs as they crawl toward the banks to emerge and thus nearly completely ignore the adults. When the flying adults are hitting the water they often do so in great numbers and it's during that period that I like to look for fish holding in a small side channel or in a current line that is not carrying lots of floating insects. Often this is a good fish and since they are not seeing as many insects as those fish that are holding in the main current flow they are quick to smash a proper imitation.
While I enjoy the thrill of a blanket hatch my preference is to fish behind the hatch, or, in the case of such insects as Salmon flies, ahead of the hatch. By fishing ahead of the hatch the angler can often use suitable nymph imitations that can result in some excellent angling opportunities. However, since I prefer to fish dry flies my favorite method is to fish behind the hatch. For a few days after a major hatch has ceased at a given location the fish will usually continue to eat suitable imitations even though the insects are no longer available. Some hatches blow through and then they continue to trickle for a few days after the main hatch has moved on. This is the ideal situation since the angler is no longer competing with a great number of naturals and the fish are still eager to eat something that resembles the natural.
In addition to blanket hatches anglers often encounter blanket spinner falls. I love fishing spinners especially those spinner falls that occur in those last few hours of daylight at the end of a long hot summer day. Like blanket hatches these spinner falls often carpet the water with insects and the angler can experience some excellent angling opportunities. I have many fond memories of fishing evening spinner falls but one experience stands out in my memory. I had spent most of the day fishing this particular stream with moderate success. It was a mid-summer day and the stream was low and clear and the fish were somewhat flighty. As the sun began to slip toward the western horizon I began to think that I would call it a day. It was a long drive home and I had been fishing for several hours. As I was contemplating calling it a day I noticed a cloud of insects hovering over the water and realized that a large cloud of mating mayflies was forming over the water. I was standing in the tail-out of a long flat that flowed along an area where a meadow came right down to the water's edge. The river pushed hard against the far bank which was the deepest part of the flat and was undercut and overhung with grass. As I stood watching a few fish began to rise along the current line that flowed against the far bank and by the time I made my way across the stream trout had taken up feeding positions along a stretch of bank about 50 yards long. Starting at the bottom I began working my way upstream, and until it was too dark to see I was consistently hooking fish. As I reeled in as darkness enveloped the stream I could still hear the fish rising.