The Baetis of spring are a welcome sight to anglers around the west, especially here in SW Montana. The emergence of these little olive mayflies signifies that winter is fading away and reliable dry fly fishing is soon approaching. The Blue Winged Olives - a common name for the various members of the Baetis genus – can be found in just about every waterway in SW Montana on cloudy days throughout the spring. So, if you’re out on the water the next few weeks and see scads of little bugs with the tell tale upright wings – chances are you’re in the middle of a BWO hatch and there will be fish coming up to the surface.
BWO’s typically hatch throughout the spring, but wet and overcast days are notorious for being ideal conditions to fish the hatch. These conditions provide some degree of safety to the insects from awaiting predators both in the air and in the water, plus the ambient humidity, and characteristic low atmospheric pressure of these conditions makes for a more efficient transition through the water column and surface. There are dozens of hypothesis out there as to why they seem to be more prolific in these conditions but the bottom line is that the fact remains that you’ll find more BWO’s on the water on cloudy days!
Baetis are a member of the mayfly family and the members that we generally refer to as Blue Winged Olives. They have what is known as an incomplete metamorphosis which basically means that their lifecycle only consists of the egg stag, nymph stage, and the dun stage. Caddis, on the other hand, go through a complete metamorphosis which includes a larvae and pupa stage as well. BWO’s nymphs are quite active and are classified as swimmers which are evident by their relatively streamlined body.
The complexity and diversity of our aquatic entomology is really amazing when you stop to think about it and there are dozens of places to learn more – one of our favorites is a pocket-sized book called “Hatch Guide for Western Streams” by Jim Schollmeyer. There are DVD’s, books, and websites dedicated to all things entomological so check them out to learn more – in the end it will make you a better fly angler.
The nymphs go through several molts a year as they grow and are generally tan to black in color. Their wing pads get very dark just prior to emergence so if you see them on some rocks or on your waders and their backs are really dark – expect to see some adults on the water very soon. The pheasant tail is probably the most widely recognized generic mayfly nymph and is always a great pattern to start with when you know that BWO’s are out and active. Sizes can range from as large as 14’s down to 20’s or smaller but the most commonly fished sizes are in 16’s and 18’s. There are hundreds of patterns out there to imitatate BWO nymphs and trout do seem to prefer nymphs patterns of varying colors depending on the fishery and conditions so it never hurts to have a selection of BWO nymph patterns in both weighted and unweighted options.
Once the nymphs begin to transform into the adults, things get very interesting for the angler. Typically - but this dies vary depending on the species – the insects will swim up through the water column as they begin to separate from their nymphal shuck, finally crawling free as they break the water’s surface. Known commonly as “emergers” this stage can be very important for fly fisherman as the insects are particularly helpless during this stage of their life and easy prey for feeding trout. Emerger patterns are usually fished in or just below the surface film. Again, there are hundreds of emerger patterns out there and their effectiveness will depend on the fishery and conditions. Patterns along the theme of a CDC Biot Emerger are a standard for BWO emergers and are usually fished in either a gray or olive color. Fishing a slightly weighted version of a pheasant tail suspended under a large dry can be very effective as an emerger pattern as well.
Fishing emerger patterns can be the most effective way to catch fish throughout a Mayfly hatch in general and certainly during a BWO hatch. Trout feeding under the surface are less spooky than those feeding on top and usually far outnumber the fish that you actually see rising. Plus, a fish that is feeding on the surface will often still take a properly presented emerger fished just below the surface. During a heavy hatch, the water’s surface will be covered in adults and trout often will feed freely with less specificity than you’ll find when the hatch is sparse. Heavy hatches are often best fished with an attractor pattern such as a Royal Wulff, H&L Variant, or Parachute Adams in a 16 with a small emerger dropped about 12-18 inches below the dry. The dry serves as an indicator for the emerger, but trout will often take the dry as well when the hatch is at its thickest.
The rise is what we’re all looking for during a BWO hatch and is the essence of what trout fishing with a fly is all about. Few hatches provide for better dry fly fishing than those of BWO’s in the spring on the waters of SW Montana. Once the nymphs escape their nymphal shuck and make it to through the water’s surface tension, their wings quickly unfold and extend as they dry. Trout can usually be found rising steadily to a constant stream of insects floating down along the various current seams. This is the time to get out the dun patterns and enjoy classic “match the hatch” fishing at it’s best.
Presentation here is probably more important than anything, but it always helps to have a pattern that is both imitative and easy to identify as yours on the water. Classic feather-hacked patterns like a Thorax or Parachute are hard to beat, but sometimes patterns with deer hair wings like the Comparadun will float better and can be easier to see as well. Regardless of the pattern, we always like to go with a dry that is dark olive in sizes 16 or 18.
If you haven’t used powder desiccants before like Loon’s Top Ride – this is the place to use it. It’s very important to keep your dry floating high on the surface – more so that you can see it than anything else – and desiccant quickly dries a fly while also adding a hydrophobic coating. We usually start with a gel floatant like Gink or Aquel and then rely on Top Ride to quickly dry a saturated fly.
There is always variability in nature so be adaptable to what you see in terms of conditions, insects, and fish behavior. When fishing the dun stages of a BWO hatch, we find a longer leader in the 9-11’ range with either 5x or 6x to be the best place to start in terms of the actual rig, If the water is “bumpy” don’t hesitate to fish a little larger fly, shorter leader, and heavier tippet. Trout feeding on the duns are often found in the quieter waters of tailouts, side seams, and eddies – thus the usual longer leaders and finer tippets.
So, when heading out to your favorite waters around SW Montana this year, expect to see some little Mayflies from later morning through mid-afternoon on cloudy days. The Baetis hatch is a welcome relief from months of winter nymphing and a great primer for the Mother’s Day Caddis hatch which is only a month or so away. Remember that there are several stages to a hatch and that there are almost always trout feeding on the various life stages of the BWO’s throughout a hatch. If you can’t get them to eat a dun pattern, try fishing emergers either below a dun or by themselves. If this doesn’t work, try dropping a small beaded pheasant tail a foot or two below a small pinch on foam indicator. If the trout are feeding, they can be caught!