Story by Harvey Thommasen with Photographs by Mike Wigle

Stir up a gravel streambed on British Columbia's Pacific Coast in the spring and you'll uncover all kinds of creatures: fish eggs that look like fat pink pearls; developing fish, called alevins; small, wriggling, black lampreys and a vast array of aquatic insect nymphs and larvae.

Research has shown that streams in the Pacific Northwest can produce as much as 100-200 pounds of nymphs and larvae per acre of gravel bottom. The kinds of aquatic insects present - but often hidden from view - is amazing. Mayflies, caddisflies, stoneflies, and a host of dipteran, the two winged fly species that includes chironomids, No-See-Ums, blackflies, mosquitoes, and craneflies.

In most streams there are many different species of the same fly. In the Atnarko River, for example, there are at least 27 different kinds of mayflies (Ephemeroptera), 40 stonefly species (Plecoptera), 40 caddisfly species (Trichoptera) and hundreds of kinds of dipterans.

Deciding what fly to use can often be a challenge. Time spend examining river stones and the streambed gravel is never wasted. But the surface of the water also provides a clue. If you are fishing fast, choppy, whitewater, you will encounter different insects than if you are on a slower run, with a steady current, because aquatic insects are finely adapted to their habitat.

Some insects are built to cling to rocks and woody debris in strong currents. They tend to be streamlined and equipped with specially designed hooks, claws, suction discs, silk lines or heavy cases to help them hang on. They are known as the clingers.

The stoneflies as a group (Claasenia sabulosa, Hesperoperla pacifica, Pteronarcys californica for example) are well adapted for life in swift, fast-flowing riffles and more turbulent whitewater.

Blackfly larvae (eg Simulium sp), Net-winged midge larvae (Blepharoceridae) are sometimes so numerous in this kind of fast water habitat that they form a brownish black carpet over the stream bottom, which some people mistake for algae.

There are also fast current caddisfly species (eg Neophylax rickeri, Rhyacophila sp, Dolophilodes pallides) though the caddisfly group as a whole is better adapted to slower flowing waters. Fast current caddisfly species include the Autumn Mottled Sedge (Neophylax rickeri), the Green Sedge (Rhyacophila spp), and Net-spinner Sedge (Dolophilodes pallidipes)

Mayflies found clinging to rocks and woody debris in the fastest flowing waters include the Small Western Ginger Quill (Cinygmula uniformis), Western Quill Gordon (Epeorus grandis), Medium Blue Quill (Epeorus longimanus), Little Yellow Mayfly (Epeorus nitidus) and the Western March Brown (Rithrogena hageni/robusta).

Mayfly nymphs you'd expect to find in slower currents, where the water isn't churned up in frothy, white bubbles, include the Dark Grey Quill (Ameletus validus), Little Blue-wing Olive (Baetis sp), the small and large Western Green Drakes (Drunella sp), the Palemorning Mayfly (Ephemerella sp), and Slate-winged Mahogany Mayfly (Paraleptophlebia sp). You might find trout rising to adult mayflies in fast water, but it's likely the nymphs emerged and transformed in slower water upstream.

Stonefly nymphs found in this kind of slower water include the Little Black Stoneflies (Capnia sp), the Rolled-winged Stoneflies (eg Leuctra sp, Despaxia augusta, Paraleuctra sp), the Medium Brown and Yellow Stoneflies (Isoperla Sp, Megarcys sp, Rickera sorpta, Skwala parallels), and the Little Green and Yellow Stoneflies (eg Alloperla sp, Sweltsa spp).

Caddisfly larvae include the Green Sedge (eg Rhyacophila sp) the Great Grey Spotted Sedge (eg Arctopsyche grandis) the Little Sister Sedge (eg Cheumatopsyche oxa) and tube-case Caddisfly larvae like the Giant Caddisfly (eg Dicosmoecus atripes), Great Late Summer Sedge (eg Oncosmoecus unicolor), and the Snow Sedge (Psychoglypha sp).

In the slowest flowing backwater habitat, where the mud, silt, sand and fine gravel are deposited, you'll find the chironomids, No-See-Ums (Ceratopogonidae) and the cranefly larvae. That's why, even though you may see clouds of chironomids dancing over a marsh along the river, you'll find the trout in a nearby riffle are keyed to something else entirely. There the fish will be keyed on the insects they see emerging from the rich, river bottom.