Turn a stone in a cold, fast alpine creek and the flat, two-tailed nymph that scuttles off the underside is a stonefly. The trout know it is there. Most days they eat other things first.
What you see
Flat, two-tailed nymphs under the stones of fast, cold creeks; the odd large dark adult crawling on a rock at dusk
When
Cold, clean mountain streams; dusk in summer is when the adults matter
Tie on
A Stimulator in 10 to 14 at dusk; a stout dark nymph on the bottom by day
Stoneflies are the insect of clean, cold, fast water, and in Australia that means the small mountain creeks. One Snowy Mountains destination guide describes a creek that supports an abundance of mayflies, caddis and stoneflies together, and notes that on dusk the larger Stimulators and stonefly patterns come into their own as the fish take their last surface meal of the day. The Victorian Fisheries Authority lists stoneflies among the standard bottom fauna a trout water is managed to grow. They are here, they are eaten, and on the right creek at the right hour they are worth a fly. What they are not is the event they are on the big freestone rivers of the northern hemisphere. Do not go looking for the hatch; carry the fly.
The nymph is the useful stage. It lives on the underside of stones in the fastest, best-oxygenated water for a year or more, and it is flat, dark and stout with two distinct tails, which is the mark that separates it from a mayfly nymph. It crawls out onto a rock to hatch rather than hatching in the water, so the fish eat it on the bottom and in the drift, and the adult is taken when it falls or flutters back onto the surface, mostly at dusk.
The fishing is not a specialty. A stout nymph on the bottom of a fast run by day, and a big, buoyant, hackled dry in the last light, are what a creek with stoneflies in it asks for, and both are flies you would be carrying anyway.
What they are
Stoneflies are an ancient order of insects with an aquatic nymph and a poor-flying, crawling adult. The nymph has two tails and two claws on each foot, holds itself flat against the stone, and is usually brown to black. The adult folds its wings flat along its back and runs rather than flies. Australian species are mostly small to medium, and the big dark adults that give the insect its name overseas are not the common sight here. Where you meet them is the cold headwater creek with a stony bed and good flow, and their presence is a mark of clean water.
When and where
Cold, fast, stony mountain creeks. The Snowy Mountains alpine creeks, the headwaters of the Victorian high-country rivers and the highland streams of Tasmania are the kinds of water where a turned stone shows a stonefly nymph beside the mayfly nymphs and the caddis cases. Summer dusk is when the adults are on the water and the fish notice them. By day the nymph is on the bottom and so is the fish that is eating it.
There is no stonefly season worth planning around in this country. Treat the insect as a reason to fish a bigger dry in the evening and a heavier nymph in the fast water, and as a good sign about the health of the creek.