Fly Fishers' Republic


August 19 2006

by Brian Chan


The presence of stoneflies in a river or stream has always been a good indicator of a healthy aquatic environment. These members of the insect order Plecoptera are found in cool, well oxygenated flowing waters and occasionally along the wave-swept shoals of northern latitude lakes. Stoneflies have a worldwide distribution and thus are an important fish food source in both the larval (nymphal) and adult stages. Their life cycle which can include as many as 3 years in the larval stage is a significant reason why they are of such interest to fish. The larvae are benthic dwellers, crawling amongst the rock and rubble of the faster moving parts of the stream known as the riffle and run zones. Depending on the species, stonefly larvae can be herbivorous, omnivorous or carnivorous. Smaller aquatic invertebrates such as mayfly nymphs, dipteran larvae, algae and detritus are all common food sources of stonefly larvae.

Life Cycle

large stonefly nymph

Stoneflies undergo an incomplete metamorphosis which includes egg, larval and adult stages. Larval development of many species takes from a few months to one year with a few species developing over 2 and 3 year periods. Depending on the species, larval stonefly bodies can range between 5 and 50 mm in length. Distinquishing larval features include 3 pairs of well developed legs each ending with 2 claws and two distinct tails at the end of the abdomen. Larvae typically go through 12 to 25 instars or molts before becoming fully developed and ready to transform into the adult stage. Actual adult emergence can occur at all times of the year with the most intense emergences happening from late spring to mid-summer in most north temperate waters.

stonefly skin

Fully developed larvae typically crawl along the stream or lake bottom to shore where they climb out of the water to complete the transformation to the adult. During a heavy emergence the rocks and shoreline vegetation will be covered with larvae and emerging adults. Most larval migrations begin in the late afternoon period and continue well into the evening hours. Mating generally occurs in the riparian vegetation along the stream or lake shore, although, sometimes the adults travel considerable distances away from the water to reproduce.

Females, laden with fertilized eggs, return to the water during the afternoon to early evening period. Most common egg deposition is by flying over the surface of the water and dipping the tip of their abdomen in the surface film and releasing eggs. Small clusters of eggs are released during the brief contact with the water and so the motion is repeated several times before the females either fall onto the water or flies back to land. Some species crawl back into the water and deposit eggs on the underside of rocks or woody debris. Adult stoneflies are poor fliers and on windy days many land clumsily on the water which makes them an easy meal for cruising trout.

Fishing the Hatch

Anglers and trout anxiously await the annual stonefly larval emergence migrations and adult egg laying flights. Both events can bring on some extremely aggressive trout feeding activity. Fishing with stonefly larval imitations during the weeks and days prior to the actual emergence can be extremely effective. Prime water to fish is the fast riffle and run areas that are between one and two metres in depth. In most situations the angler is fishing with a floating line and weighted larval imitation. The key is to present a naturally drifting larval pattern as close to the stream bottom as possible. This often means an upstream presentation while maintaining as much control over the fly line and larval pattern as it drifts back downstream. A strike indicator is often used to better control the depth of the fly and to also help detect even the slightest take of the pattern.

The arrival of egg laying adults means dry fly fishing action. The trout are looking for adults that have landed on the water and are drifting naturally downstream. Deer or elk hair winged patterns and foam bodied patterns are standard choices for imitating the adults. The key is to present the fly drag-free so appropriate mending of the floating fly line helps ensure a proper presentation.