Have you ever looked at the bottom of a shallow riffle and seen hundreds of mayfly larvae moving about and wondered why fish aren't stuffed to their gills with these critters? Have you ever noticed that nymphing in the Sierras is excellent in June and July, declines in August and September, gets better in October, and declines again in November? Have you ever noticed that nymphing is generally slow in midday, but better in the mornings and evenings?
Years ago, while I was in school, an important paper came out with the title "Why Is The Earth Green?". The argument went as follows. The rules of life are that populations are controlled by their food sources and the food source is in turn controlled by these same populations. That's true in predator-prey relations. It's true in parasite-host populations. Why then do rainforests in equatorial climates have luxuriant foliage? Why aren't the herbivores in the rain forest taking full advantage of their food supplies? The answer is that only the soft, young shoots of plants are preferred by grazers. Once a plant reaches a certain size it is no longer a preferred food item. This study has always stayed with me because it shows the difference between food presence and food availability.
When I crouch over a riffle at Waddell Creek, initially I see absolutely no life. If I am still for about twenty minutes, the area will look like an ant farmmayfly nymphs crawling everywhere. If I make any sudden moves during that time, all life will vanish immediately. When I had an aquarium with mayfly larvae in my bedroom people wondered what it contained. It looked like bare rocks in the daytime. When the light was turned on during the night, larvae scurried for cover in all directions.
Nymphing "experts" often like to point out that nymphing is a superior form of fly fishing because hatches are fairly rare occurrences. Nymphs, meanwhile, are always present. The nymphing devotees seine the water column and show the presence of the greatest amount of food in the bottom strata (see bar graph). They argue that since hatches occur during 10% of the time, trout are feeding on nymphs 90% of the time. The implicit assumption is that trout are feeding constantly.
Gary LaFontaine wondered about those same questions. He researched the scientific literature and found the following numbers from a study in England:
|October through April|| trout fed mainly on the bottom.|
|May|| 88% surface food|
|June|| 78% surface food|
|July|| 59% surface food|
|August|| 59% surface food|
|September|| 95% surface food|
Gary concludes as follows about why fish prefer to feed on the surface.
"The reason is vulnerability always a more important determinant of feeding than relative abundance. So what if the weed and bottom gravel are crammed with crayfish, scuds, aquatic worms, leeches, and insect larvae and nymphs. Those organisms are relatively safe in their natural sanctuaries from fish. Any food item perched in or on the surface film, outlined against he sky, is totally exposed."
Good feeding studies evaluate food consumption, not food presence. The largest biomass in the stream comes from insect larvaethe nymphers are right about that. However, the greatest food source comes from hatching bugs. This explains why fish caught during periods of insect hatches are plump and full of vigor. Fish caught during other times are relatively emaciated and poor fighters. Yet the larvae are always there, regardless of the hatches.
Now that it is established that the best nymphing surrounds the period of hatching, we are no longer burdened with the concept that the best strike zone is the bottom six inches of the river. The bottom zone may be the optimum area just prior to a hatch, but the zone to fish the nymph in will depend on what's hatching and the stage of the hatch. Each bug has it's own way of doing it. Eventually, some of the best nymphing is just under the surfaceas most soft hackle addicts knowbecause the transition through the surface tension at the airwater interface is most difficult for a hatching insect.
Please note that nothing mentioned so far suggests that dry fly fishing is more effective than nymphing. Nymphing properly during hatches with imitations which represent larvae, pupae, and emerging adults is superior to fishing dun imitations. However, the notion that high-stick dead-drifting attractor patterns just off the bottom is a superior technique is inaccurate.