Kosciusko Dun           

This Mayfly is present in several local rivers as well numerous waters along the Great Dividing Range as far north as the Snowy system. Particularly good populations can be found in the Mitta Mitta and Swampy Plains Rivers. It affectionately known by those who fish with the fly as a Kossie Dun and news of their hatching usually conjures visions of extreme surface feeding and large fish. Best hatches occur from Mid-November continuing but tapering during the warmer months. Another good emergence occurs in early to mid autumn.

This particular mayfly is found in the fastest stretches of river and due to its size is easily identified and collected. When mature it averages between 20 mm and 30mm in length including tail. The best place to find them is under stones in the fast water as it drops over a ledge. They require a very high oxygen concentration which explains their preferred habitat. Their underside is light coloured and any imitations should endeavour to represent this.
The sub-imago or dun of this particular Mayfly is the most important to the fly fisher. The emerger is no way near as useful when chasing fish on kossies as they emerge quickly with very little time being wasted crawling from their nymphal shuck. Our studies indicate the average size to be roughly 25mm from the tip of the head to tip of the tail with some being up to 25% larger or smaller. Emergence takes place in fast water and therefore for the most part ultra realistic imitations are not necessary.
The spinner or imago is slightly smaller than the dun and can be found in the days following emergence. Generally the next night but sometimes (depending on climatic conditions) one or two nights later. It has a clear wing and more brilliant colouration with a darker upper abdomen with light belly. The females (which this one is) will often have a bright pale yellow egg sac attached at the rear of the abdomen. It comes out during the last hour at night and is most prolific in the tails of pools.

General Info

Fishing of the nymph and emerger is not particularly successful in regards to this family of Mayfly. The nymphs do not often get washed away from the rocks and are therefore usually only available to the fish on their swim to the surface. This does not take them long as they are a large and strong Mayfly and reach the surface in a very short distance unlike other types that make require several attempts and rests. Having said this a large nymph pattern with plenty of inherent movement will take fish where these Mayfly are present. Mike Spry had a particularly good pattern for imitating the Coloburiscoides and it can be found in Australia's Best Trout Flies. It incorporates the right size, shape, colour and movement. This makes it hard to beat when fishing rivers like the Swampy and Mitta Mitta.

Once the nymph gets to the surface he emerges very quickly. In fact, so quickly that fishing a dedicated emerger pattern is largely a waste of effort. Especially as the hatch is often right on dark and changing flies at this time is often frustrating and especially time consuming. The dun itself doesn't spend much time on the water and if there is any sort of breeze their wings dry almost instantly sending them fluttering off to the nearest bit of streamside vegetation. However for the most part good numbers will remain on the water and many fish, sometimes very large specimens will rise with much vigour. Sometimes hatches will be just too large and getting a fish to take yours amongst the naturals is just about impossible. Either walk downstream into the pool as recommended by Phillip Weigall in his article in FlyLife Issue 11. This gives the majority of duns a chance to fly off leaving a lesser number on the water but still plenty to encourage the fish to rise. Sometimes much better fish too live in this deeper stretch of river. Or you can put on a greatly oversized pattern say a number 6-8 and repeatedly cast over working fish. This hatch is usually short in duration and extremely heavy in numbers of insects emerging. What this means is that often very large fish will move from their hides to take up the prime feeding spots at the top of the pools where they get first choice of the duns. Patterns should be light coloured underneath with a very prominent wing. A thorax tie is best for the fast water and a paradun for those fishing further back into the pools.

The Spinner of the Kossie is impossible to miss. It comes out on evening especially on those nights when the wind backs right off leaving that last hour or two becalmed. Mating spinners interlock at a point above the riffles and the females carry their fertilised eggs forward into the pool tail out where they deposit them by dipping their abdomens into the meniscus. Fish feed voraciously when this occurs and you will often find numerous smaller fish slashing them from the top. After successfully mating or depositing their eggs they fall to the water spent where they again provide protein for the fish. Large concentrations of them can accumulate in backwaters and the best will often have a very good fish mopping them up with very gentle sipping rises. A great tactic that many fly fishers use is to  a backwater that can be viewed looking west which gives the angler a great view utilising the fading light.