Stoneflies are a small, but diverse family of insects that are adapted to well oxygenated streams and rivers. Stonefly nymphs have fixed gills that can only extract oxygen in moving water. If trapped in still water they die quickly. Most stoneflies crawl out of the water to hatch. The normal procedure is for nymphs to migrate to shore to hatch. This migration activity is attractive to trout, grayling and whitefish. Some smaller species do hatch mid-river at the water's surface much like mayflies do. Nymphs of several giant stone flies live in our rivers for up to three years before they hatch into winged adults. They are available to trout in several sizes, nearly year round. As the nymphs grow, they must take ever larger territory.
The redistribution of territory usually happens in mass with many insects changing territory at the same time. This is called a behavioral drift. The largest behavioral drift cycles of these stone fly nymphs occur mid-September through May. These mass movements of insects occur during the twilight of early morning and late evening. During these periods nymphs move down stream. Many are exposed to the currents and swept along out of control. Trout key on these vulnerable morsels. You should carry patterns in several different weights to cover different water depths and speeds. These flies will cover all of the subsurface "giant stone fly" nymph activity. There are patterns such as the Little Golden Stone which cover many smaller species as well. Several weeks before the hatch, nymphs of most species of stone flies, including the giant ones such as salmon flies and golden stones will start to migrate to staging areas along the banks. No other occurrence in our rivers produces a feeding frenzy like a migration of big stone fly nymphs. Even the largest trout find it hard to refuse a big stone fly nymph fished deep along the bottom. The peak of this activity is the last two weeks before the hatch.