The Blue Wing Olives have been coming off regularly on the Truckee River and you need to check out Matt Coles blog and he'll give you some good insight as to fishing the BWO hatch. You can check out his weblog at www.gilligansguideservice.blogspot.com
With that said, here's Baetis Complex, Part II, Behavior and Habitat
The Baetis Complex or Blue Wing Olive mayflies are said to be the most important to the fly fisherman. Knowing the behavior and habitat of these mayflies can be most beneficial when you encounter them on your favorite stream. We often see BWO's in the eddies and soft water as we float the Lower Yuba and the Lower Sacramento River.
Baetis, BWO Nymphs
Nymphs for the Baetis complex are found in most flowing water habitats. In freestone streams they live in quite pools, slow eddies and even white water rapids, but are most common in shallow, sun-struck riffles where algal growth is most abundant.
Though the BWO nymphs are found in fishable numbers in freestone waters, these nymphs often reach their greatest abundance in gentle tailwater and spring creek currents. Our local Lower Yuba River has these elements and a good population of baetis nymphs, although their population is subject to seasonal scouring which can affect their habitat.
Behavior of the nymphs of the Baetis complex is predicted by their nature as being a member of the Swimmers group. Quick pulsing movements of the abdomen combined with flips of the tail propel them in short bursts of three to five inches. When they stop swimming they immediately cling to the substrate facing upstream. Their active behavior causes them to be a common component of stream drift. This makes them available to deep nymphing methods almost 365 days a year.
The Baetis complex nymphs feed on "diatoms" or small "detrial" material they scrape from the surface of submerged vegetation or rocks. The nymphs go through as many as 27 instars. The mature nymphs, those ready for emergence, are easy to recognize by their dark well developed wingpads. By turning rocks over and inspecting the nymphs you fins, you can predict with fair accuracy the likehood of a coming hatch. If you see the baetis nymphs with the darker colored wingpads, a hatch is probably coming up.
Baetis, BWO Emergers and Duns
To emerge a baetis nymph releases it hold on the bottom and floats or swims to the surface, drifting downstream with the currents as it rises to the surface. After breaking the surface film, the dun quickly bursts out of the nymphal cuticle. The dun normally leaves the surface immediately but on cold days they may float as long as fifty feet before getting airborne.
The surface film can be a substantial barrier for these tiny bugs, especially on smooth currents. Wind and broken water breaks up the barrier and makes their emergence much easier. When BWOs are hatching in calm waters many fail to make it through the surface film. The result is a high percentage of cripples, stuck in the surface film. When this happens at least some of the trout focus on feeding on them and will ignore the duns on the surface. The fish may refuse your best dun imitations and your best presentations. This is time when you need to fish emerger imitations.
As with most match the hatch fishing, observation is the most important tool in your bad of tricks. If you ever carry a pair of binoculars, this is where they really help. Watch the BWO duns as they float on down the current and when a trout rises and breaks the surface, check to see if the duns are still floating on downstream. If they are, theres a very good chance they are keying on the emergers and cripples.
Baetis, BWO Spinners
Seven to eight hours after emergence the duns molt to spinners. Mating flights usually occur mid-day, from late morning to early afternoon. After mating, females of many Baetis complex land on protruding rock and sticks and crawl underwater and lay their eggs. After laying their eggs they either crawl back up to air or they let go and are swept downstream and are often eating by awaiting trout. This submerged egg-depositing behavior is unique, among mayflies, to members of some of the Baetis complex. Again some and not all.
There are other species in the complex that lay their eggs on the open water's surface. The females soon die after all the eggs are laid.The trout will sometimes feed on the tiny transparent, empty hulls. This is a time when a spinner pattern may solve this problem. When fish are taking spinners it is often easy to overlook and not an easy portion of the hatch to solve.
You can pick up Dave Hughes book, "Western Mayfly Hatches" at Amazon books by going to the following link. It is where most of this information came from.