There is no better telltale sign that stoneflies have emerged than the large shucks of these prehistoric creatures clinging to shoreline rocks, trees and bridge piers. Stoneflies are one of the three major insect orders in streams, and are some of the most important food trout feed upon. Knowing when and how to match a stonefly hatch will improve your fishing success in streams.
Life of a stonefly
Stonefly nymphs need cold clean well-oxygenated water and therefore spend most of their time crawling along the bottom in the fast flowing water of riffles and runs. The turbulent movement of the faster water mixes in air, allowing oxygen to dissolve easier. Clean looking smooth-faced rounded rocks are signs of cold clean well-oxygenated water. Lifting any of these rocks along the shore often reveals a stonefly or two, especially in the spring.
Stoneflies have an incomplete metamorphosis as their life cycle has no pupa stage and consists of an egg stage, aquatic nymph stage and flying adult stage. Hatching from the eggs, nymphs molt up to 24 times before emerging from the stream. Depending on the species, the nymphs spend anywhere from a year to up to three years in the water before emerging and hatching into flying adults. Stoneflies do not actually hatch on the water surface – they emerge from streams by crawling along the bottom where they stage in the shallow water before crawling onto land. The migration to shore usually starts in the late afternoon, emerging from the water the stonefly nymph will crawl onto virtually anything it can get a hold of. Once settled, the exoskeleton of the nymph breaks open along the dorsal side, allowing the adult to crawl out and fly away. The adults live anywhere from one to three weeks.
The timing of the emergence of stoneflies varies among species and may start as early as February, continue into the spring, but some species may emerge as late as early August. The emergence of the different species is dependent on a combination of factors that include an increase in water temperature, the overall accumulated heat in the days and weeks leading up to emergence, and the amount of daylight.
The variation in the timing of hatches from year to year is often caused by the variation in accumulated heat. In years with long winters and/or cold springs you can expect the stoneflies to emerge later. Geography also influences the timing of emergence, because as you move up in elevation and upstream, hatches will be later – anywhere from a few days up to a few weeks.
Stoneflies as trout food
Most of the time stoneflies cling to the bottom of rocks where fish cannot access them easily, but there are instances when stonefly nymphs are exposed to trout. Nymphs clinging to the rocks in fast currents are often sporadically washed downstream when they lose their grip. And since they are poor swimmers the nymphs are carried a long distance downstream making them easy prey. The nymphs are also exposed to increased predation when they start their migration to emerge or during dispersal drifts. Stoneflies can re-colonize a stream through dispersal drifts. At certain times during the season the nymphs let go of the bottom to disperse to new areas, drifting a few metres at a time. These drifts usually peak the hour before sun-up and the hour after sundown, and less so during a full moon.
Stoneflies are clumsy flyers and are easily blown onto the water by wind. Some males do not develop adequate wings and are unable to fly at all. Instead these males crawl along the shore and scurry across the water surface. The females are especially exposed to predation when they return to deposit their eggs. They fly either low over the water dropping their egg sacks or they land on the water and dip their abdomens into the water to release their eggs. Some species may even scurry across the surface to release their eggs.
How many stoneflies do you need to imitate?
There are currently 131 stonefly species known in British Columbia, but not all are important to fishing. Based on their general physical description to matching the hatch, stoneflies can be simplified to six different stoneflies (in order of importance):
1. Golden Stoneflies
1. Golden Stonefly
Golden Stoneflies are the most important hatch as they are large, and the nymphs occupy most streams in good numbers. The Golden Stoneflies in British Columbia include five different species in the Perlidae family. These species are similar in size and appearance, but they do emerge at different times and the adults behave differently.
Golden Stonefly nymphs are tan coloured with dark brown markings on the dorsal side and grow to up to 40 mm in length. At the end of summer you will often find bright yellow to cream coloured nymphs in riffles. These are Golden Stonefly nymphs that molted in the early morning, and over the next few hours they will harden and change colour into the characteristic mottled tan and brown nymphs.
Golden Stoneflies emerge after spending two years in water. Depending on the species, Golden Stoneflies emerge as early as May and continue to emergence through June, July, and even into August. This protracted emergence results in adults flying around for most of the fishing season. The adults are active from midday and into the evening.
The nymphs and adults are imitated with flies tied onto 3X long size six and eight hooks. There are many versions of golden stonefly nymph imitations available and any of these can fool a fish or two. The adults are imitated with flies such as yellow Stimulators, Sofa Pillows, or any large foam-body dry fly.
This stonefly is called Salmonfly as the adults have a pinkish/salmon-flesh colour on the ventral side of the abdomen and thorax. In contrast, the nymphs are dark gray to black and grow up to five centimetres in length. Due to their size Salmonflies are arguably the most impressive aquatic insect. In British Columbia there are four species collectively known as the Salmonfly. The nymphs and adults of these species superficially look identical to one another and are imitated with the same fly patterns. The species also behave the same for the most part.
Salmonfly nymphs live for three years in the fast moving water among the rocks and boulders of a stream. In year four, peaking in the month of May, the nymphs emerge from the water to hatch as adults. In some colder or northern streams eggs may take up to two years to hatch resulting in a life cycle that lasts up to five years instead of four years.
Nymph and adult imitations of this stonefly are tied on 3X long size six and eight hooks. The nymphs are typically tied as all black nymphs and the adults are imitated by classics such as the orange Stimulator or Sofa Pillow. As a multi-year species the nymphs can be fished year round.
3. Stripetails (Yellow Sallies)
Stripetails belong to the Genus Isoperla, with 12 species in British Columbia. This stonefly has a number of names. The nymphs are known as Stripetails as they have three dark brown stripes on the dorsal side along the length of the abdomen. They are also known as Little Yellowstones as the nymphs are bright yellow on the ventral side. The adults are known as Yellow Sallies as they are mostly yellow with some species having a bright red rear-end. Many adults are also to olive to light brown in colour.
As the largest of the small stoneflies, trout key in on Stripetail nymphs during their migrations to shore. With a one-year life cycle the emergence peaks in June, but may start as early as April in the southern parts and lower elevations of the province. The emergence continues throughout the spring and summer until August.
The mature nymphs and adults usually vary in length from 10 mm (males) to 15 mm (females) and are best imitated with flies tied onto 2x long size 14 hooks. Flies such as the Stripetail nymph and Yellow Sally dry flies imitate the nymphs and adults, respectively.
4. Lime Sallies
Lime Sallies are the most well-known group within the Chloroperlidae family and include 27 species in British Columbia. Sally nymphs are about 8 mm long with a slim appearance and drab in colour with shades of brown and olive. These small nymphs are often in the drift and fish will feed on them during this time. As they are in the same size range as many mayfly nymphs, imitations of mayfly nymphs can easily double as Sally nymph imitations and vice versa.
The adults are also small and slender in appearance but are either lime green, olive or yellow in colour. Much of the attention has been on Lime Sallies and less so on the olive and yellow coloured adults, most likely as Lime Sallies have a distinctive bright green colour that is hard to miss. The Olive Sallies and Little Yellows (different than Yellow Sallies) are probably lost among the many mayflies hatching at the same time.
In British Columbia Sallies emergence peaks at the beginning of July, but you may see some as early as April at lower elevations, and as late as August at higher elevations and further north.
You find Sallies along sections of streams where there is plant growth close to the shoreline. Wide-open exposed runs with big gravel bars are not going to shelter a lot of Sallies. Especially the Olive Sallies, who are abundant and a quick perusal of the riparian vegetation almost always reveals a few Olives Sallies, scurrying away. The best success with Sally dry flies is during the egg-laying flights of females in the afternoon and early evening. Adult Sallies are imitated with flies that are slender in appearance and tied onto size 16 and 18 hooks.
5. Skwala stoneflies
Skwala stoneflies are named for its Genus Skwala which belongs the Perlodidae family. Skwalas fame originates from the large hatches on some Montana streams. Anglers flock to these streams to fish the pre-freshet hatches of which the Skwala hatch is the most prominent. In British Columbia Skwala hatches are not nearly as intense but under some circumstances anglers can imitate these stoneflies with good success in streams that are fishable in April and May.
Mature Skwala nymphs are 20-mm (males) to 25-mm (females) long and are best imitated with flies tied onto 3x long size eight to ten hooks. The nymphs are dark brown to gray-olive with yellow banding between the abdominal segments. The ventral side, especially the thorax area, of the nymphs is bright yellow to orangey-gold colour. The adults are similar in size and colour as the nymphs. A stimulator in Skwala colours of yellow and olive can be very effective to imitate the adults.
7. Snowflies (Winterstones)
Members of the Capniidae family, these tiny slender black stoneflies are often ignored as they mostly emerge during the winter months from December to March. And as they are often seen on snow banks many call them Snowflies.
Many anglers in the Lower Mainland and Vancouver Island are very familiar with Snowflies when pursuing coastal cutthroat trout as these stoneflies are a common site. In the interior, anglers will see Snowflies on the warm days of March and April. This is not a hatch that you will fish often, but having a few imitations in your fly box might just avoid getting skunked. Both the nymphs and adults are tied onto size 16 and 18 hooks.
I always believe that if you find the food, you will find the fish. Getting to know stoneflies a little
better adds to the overall experience while fishing streams and rivers, and it should help you catch
a few more fish. After all, nothing beats stimulating a fish into taking a big bushy stimulator off the
Stoneflies, Stimulators and Happy Anglers
by Danie Erasmus
First published in May/June 2018