Baetis or the Blue Winged Olive
For our purposes the little mayfly from the family Baetidae qualifies as a Blue Winged Olive. This little mayfly is an important part of the fishermen's armory when the larger flies are not hatching. Although they are small (#16 – 22) they can hatch in large numbers. In the higher altitudes we have to wait until March or April to expect the big hatches of Baetis although if the weather suits they will hatch a little earlier. The better hatches are on cloudier days with a little snow or rain. The cloudy day allows the mature adults wings to dry more slowly and consistently so nature has built in that timing device to encourage them to emerge under those conditions. This gives rise to prolific hatches at this time. Furthermore, the longer they spend on the water drying their wings, the more time there is for the trout to feed on them.
There are over 100 types of Baetis or BWO's but because of the similarities in their appearance, we do not consider it useful for the trout fisherman to get to involved in trying to specifically identify each fly. Suffice to make sure one has a range of sizes colors and patterns to enable one to be able to mix and match if success is elusive. The nymph is dark brown. Generally the dun has smoky blue wings. The body colors range from gray olive to brown.
They live between rocks and other spaces on the bottom of the rivers and feed on diatoms and algae. The nymph of the baetis is a swimmer. This means that they will move from time to time in the water becoming prey to a trout. Consequently, fishing baetis should not be confined to hatches. A nymph on the bottom at any time will work if there is a feeding fish about. In addition when the nymph is beginning to mature and gets active prior to hatching, it will leave the bottom and drift and kick swim in the current prior to attempting to rise to the surface to hatch. Sometimes they may attempt to rise to the surface several times making them easy prey.
Consequently it is productive to fish nymph patterns both prior to and during any hatch. As the nymph is a swimmer, allowing the fly to drift in the current with some weight emulates the nymph on the move. They live in all parts of the water but tend to congregate in the slower water so when picking that section of the water to fish, try and identify the region where the water slows a little. As it is a small fly, trout are naturally going to go for densest concentration.
When the nymph reaches the surface film it will first hang as a floating nymph as its casing splits allowing the dun to emerge. During a hatch when the fish are taking them from under or on the surface, it is logical to watch for the slower eddies where debris will gather on the surface. There may be a little froth. The emerging and crippled baetis will be in the same place. The wing mass renders the emerger more visible than the suspended nymph making it easier for the trout to see. At this point it is helpless and easy prey for the trout. Where the dun fails to emerges and languishes it is described as a cripple. They slowly die suspended in the film.
The final stage of the Dun is the spinner. Baetis lay their eggs either on the surface of the water or, in some cases underwater when the dun crawls down a rock or stick to lay the eggs lower in the water. In this stage, if the dun is dislodged and drifts underwater it is prey for the trout. When it lays its eggs it will then drift in the current as well. Therefore in some cases, it is useful to fish a wet dun pattern. Usually a soft hackle will emulate this stage. The other stage is the spinner fall when the spent dun is laying on or under the surface.