After massive spring floods in 1993, I made a December visit to a favourite mountain stream. Hoping for the best but fearing the worst, I arrived (as I often did that summer) to a rearranged streambed, including vast amounts of ‘new’ gravel and rubble. I went on to catch some of the best trout I’ve landed from that stream, but equally as memorable during that session were the Coloburiscoides mayfly hatches, aka hatches of the fabled Kossie dun.
When Kossie duns thrive, the trout quickly slap on size and condition.
Previously, I’d only ever seen solitary examples of this giant mayfly on this particular water, yet that late afternoon and evening, they poured off and the rise was heart-stopping. I’ve never seen a Kossie dun hatch like it on the stream since. But with the long-lasting and widespread floods this season, maybe they’ll be back there.
What all the fuss is about: a newly-emerged Kossie dun.
Elsewhere, I’ve already seen a few Kossies coming off in the high water, but it’s reports from friends of the duns hatching in their thousands – one every few feet – that have me really excited. One mate actually described a recent hatch as the best he’d seen… and he’s seen a few over the years.
When flows are consistently good, Kossies thrive in habitat like this on the upper Indi River.
That floods should equal huge Kossie dun hatches may seem counterintuitive. However, we know the nymphs are big and tough and thrive in the spaces between medium to large streambed stones in the faster currents.
A newly-vacated shuck – that’s a big nymph!
Long dry periods can result in a lack of ‘flushing’ flows, with settled silt and sand filling the spaces Kossie nymphs like, and in some cases, virtually cementing the stones in place. But apply the strong flows of the last few seasons, top them off with 2022’s winter and spring floods, and prime Kossie real estate is likely to be abundant.
From now on, if you head to the fastwaters, make sure you’re carrying nice big Kossie dun patterns like this, plus some out-sized dark brown nymphs.
Nature will have the final say of course, as she always does. Nevertheless, I won’t be going near a fastwater this season without some size 10 duns and nymphs in my vest.
