What Fish Eat
Aquatic insects make up the majority of a fish's diet. The four key insect orders (listed first and not including Streamers or Terrestrials) that emerge from streams and lakes are:
The Gospel on Caddisflies
If you fly fish for trout in the West, could Gary LaFontaine's Caddisflies be the most important bug book to read?
Green Sedge Caddisflies
Green sedge caddisflies are members of the Family Rhyacophilidae or commonly called the Freeliving Caddisflies. These Freeliving types are the only caddisflies in which the larvae don't live in a case or retreat. Larve roam freely under rocks and within spaces in the rocky substrate. As they crawl, they spin a fine silk thread and anchor it to the substrate which helps them to navigate in fast current.
Cool, clear, and fast flowing water is their preferred stream habitat. Most freeliving caddisflies are found in the fastest sections of riffles under loose boulder and cobble. Sometimes they are in slower sections of streams, but always in current. Which helps to explain the following...
Most fly fishermen move along a stream at a steady pace, either up or down, hitting every good-looking spot. They pop casts in the backwaters, along the edges, and over the broken pockets in succession. The problem with such a straight-line method of reading water is that these anglers are probably using an imitation suited to one type--not all types--of water.
Will an imitation of a specific insect work best in only certain types of water? Bet on it. A good imitation of a Rhyacophila larva is going to catch a lot of trout in swift, bouncing stretches of stream. The same fly is going to do poorly in slow areas of the same stream.
In one of my experiments on the Wind River in Wyoming, the two best nymph fishermen in my group, Graham Marsh and Wayne Huft, moved upstream side by side. They cast to every prime piece of holding water. Graham used a fast-water fly (a size 12 Green Caddis Larva) and Wayne used a slow-water fly (a size 12 Dark Cased Caddis Larva) They covered a quarter-mile of river.
Their results were very different:
Fast-water areas: Wayne caught 11 fish Graham caught 22 fish
Caddisflies by Gary LaFontaine