Change is coming, spring is just around the corner, the early bird gets the worm, and you should have been here yesterday. The last throes of winter are a harbinger of things coming soon to a river near you — both on and under the water. Trout fishermen who are in the know keep their eyes open for new items coming on the menu starting in early spring. The term Benthic Macroinvertibrates — while a mouthful to say, represent a crucial concept for trout fishermen to understand. The world of subterranean insects that live in the several feet of saturated rocks and gravel under your boots when you're wading are what drive stream ecology and trout behavior.
It's taken me quite a while to wrap my head around the seasonal changes that streams undergo in terms of biology and the strategies to address them — and I'm still learning. Flowing water in winter typically is in the immediate proximity of ice and snow, and not much different than the temperature of that. Even water in lakes under ice is typically warmer than natural winter freestone stream flows, owing to ambient earth temperatures at depth and the insulative factor of a solid overhead surface to protect from frigid overnight temperatures. To a lesser extent, this applies to tail waters below dams as well, though the draw point in the water column from the dam spillway or takeoff pipes can change things dramatically and provide consistent winter activity. Year round feeding is one reason that fish in productive tail waters tend to be larger than fish from freestone biologies.
Water temps in the 30's, while capable of providing maximum saturation for dissolved oxygen, slows the fish's cellular metabolism to a crawl and reduces the need to feed. Fish are only minimally motivated to eat, holding in very gentle, shallow lies that require minimal effort to maintain position in, and aren't inclined to make large expenditures of energy chasing a meal. Insect forms in winter streams are typically limited to hatches of tiny midges, and smaller annelids and crustaceans living in the gravel and rocks under the river that accidentally make their way into the current, which is generally at gentle annual low flows. But in late winter, the rising angle ot the sun into shallow water and the increasing frequency of milder spells of weather drive biological changes in streams.
Once water temperatures reach 40 degrees, however, things start to happen. The warmer environment starts to jump start the mitochondria of the fish, requiring them to be more active in searching out food to supply the increased metabolic stress. At the same time, water temperatures are warming the saturated ecology of the river bottom, signaling dormant insect species living in the gravel that are adapted to slightly warmer conditions to begin making their way to the surface of that environment — he bottom of the river — and to begin seasonal life cycles. One of the first insect species in a trout stream to make their appearance are the large family of small mayflies known as Baetids.
The Blue-Winged Olive mayfly is a bread and butter species for the fly fisherman (certainly in the Western US), and there exists considerable scientific debate about the taxonomy of these insects.
Smallish in size (#18 - #22), with an olive green body and slate gray wings, BWO's (as they are commonly called, or simply "Olives") are usually considered the first real surface hatch of the year. Indicators of pristine water quality, BWO's make their first emergences in mid-March, continuing through May when runoff conditions provide the second seasonal transition on most rivers, Olives seem to hatch most prolifically during early afternoons with overcast conditions. It has been argued to me that the reason for this is because they have adapted to take advantage of conditions where, as darker insects, they will be less visible to trout against a camouflaged background of grey skies. As BWO's tend to be slow flying insects with a very arduous molting process in the surface film of the river, this would make sense€š — though how insects that live under rocks in the bottom of the river would be able to determine weather conditions has not been explained to my satisfaction. I do know, however, that there is no denying that millions of insects can almost simultaneously emerge over miles of river given the right conditions, with fish going ballistic over the sudden overwhelming presence of food.
Once Olives have started making themselves known in the spring time, fish will begin to enthusiastically seek them out in whatever forms they might be available in, whether nymphs, emergers, or adults. Many excellent patterns to imitate these insects in all their life stages have been developed over decades. As the surface hatch of these insects is typically right after lunchtime, on guided trips, I usually have my clients rigged for nymphing in the mornings, changing to dry flies for the afternoons.
Probably the single most famous Baetis pattern out there is the venerable Pheasant Tail nymph, which exhibits all the traits of BWOs that make them visually distinct from the midge patterns of mid winter, including representation of tail structures and legs made from the fibers of the tail feather of Pheasant, hence the name. I think I've taken more heavy trout on PT's than any other pattern in my box, and they are usually fished in tandem or triplicate with flies of other designs. After a winter of picking at midges, and fished with a good drift, big fish devour them on sight.
The next indispensable Olive pattern would be the more technical RS-2 emerger, fished as a nymph. Designed in the early 1980's, it sports a dark grey wing case of Cul de Canard feather, and a split tail of microfibbet nylon, though this is now sometimes substituted with a couple strands of crystal flash. The combination of the wing case and accurate tail design make this pattern deadly, especially right before the main event of the surface hatch.
Finally, dry fly patterns abound to imitate this insect, but some are more effective than others. A fully hackled #18 BWO pattern with grey wing fibers can get excellent results, though it can be difficult to see, especially in low light conditions. I usually will girth hitch a small section of poly yarn dressed with floatant about 2.5' above the fly to help with discerning the takes. While traditionally hackled flies are productive, my preference is usually toward patterns tied "parachute" style, which ride lower in the film and imitates a crippled insect, triggering an attack response from fish. A Parachute Adams is tough to beat for this, but if the fish are being picky and you're getting refusals, having a micro-tipped black Sharpie pen in your pocket will allow you to make a few strokes through the upright poly wing post to darken it to more accurately match the naturals. Some days, it can make all the difference. An RS-2 can be fished in tandem with dressed dry flies as well, with the emerger being allowed to be suspended in the surface film after attaching it to the bend of the dry fly with a small section of flourocarbon tippet.
With the coming of spring, it's time to shake off the cobwebs, put on some layers and get out there. Keeping your eyes open for the first mayflies of the season and responding accordingly can pay big dividends.