The stonefly is available to trout in two stages: the nymph and the egg laying adult. Nymphs are normally found in cool, well oxygenated, fast water and rapids that have stony or rocky bottoms. However a few species have adapted to living in slower water with silt and weed, mainly near stony lake or reservoir shores. The Stonefly nymph is an important source of food for the trout and other fish. It cannot swim. This should be kept in mind when fishing. It spends it's life crawling over the gravel and stony river and lake beds. They eat small invertebrate animals, decaying plant material, organic matter, algae and bacteria. They get oxygen from the water by diffusion or by means of external, feathery gill tufts. These may be presented on the head, neck, or thorax. They prefer cold water as it holds more oxygen than warm water. As the nymphs grow they develop long tails at the end of their abdomen and segmented wing pads on their back. They can molt their skins up to 30 times and some species take up to 4 years to develop from egg to adult. Pale colored soft skinned nymphs that have just molted their skins are a particular attractive target for the local fish. As their shell hardens it darkens in colour.
The nymphs crawl from the water to hatch on land where they are safe from hungry fish. The trout follow them in their migration. They also crawl up overhanging vegetation, rocks and tree branches if they are available. Once out of the water they pop out of their shuck and emerge as a winged adult.
The adults look exactly like the nymphs except they have folded flat wings over their back when at rest. Some newly hatched adults try to fly too early and flutter down to the water surface a number of times before reaching land. This causes excitement on the water from the lurking predatory fish.
The montana and stoneflies can be fished at virtually any depth with any line, floating, sinking or intermediate with a long leader!