I want to discuss the Iron Blue Dun (it is too much hassle to write Iron Blue Dun all the time so I will refer to it as ‘IBD’ in this post). For such a tiny insect it has generated a huge amount of words in angling literature, and rightly so. From the earliest history of fly fishing the IBD has been recognised as a hugely important species. I won’t bore you with the minute differences between the three separate species as they so closely resemble each other it requires a magnifying glass to differentiate. I am more concerned with the practicalities of catching trout when the IBD hatches out.
There is a common misconception that the IBD only hatches out on cold, windy days when no other fly life is active. While I have certainly seen them hatch in just such conditions I have also seen them coming off the water on mild and calm days too so I am inclined to treat the whole ‘bad weather fly’ theory as highly dubious. What is not in doubt is the high regard the fish have for the IBD. Back in the days when I fished the Aberdeenshire Don it was not unusual to see heavy hatches of Large Dark Olives, March Browns and IBDs on the same day. The Olives would come down the rivers in a pretty steady trickle and the March Browns would just appear in explosive bursts, none on the surface one minute then a host of them like miniature speckled sail boats the next. Within the space of a few moments the March Browns would be gone again only for the process to repeat itself 20 or so minutes later. The IBD hatch can vary widely. Some days they would gradually build from one or two flies to a heavy hatch over the period of up to an hour while on other occasions they never seemed to appear in more than a trickle.
The problem for the fly fisher is to initially spot the IBD when the hatch starts. They are tiny and in moving water can be hard to spot, especially when other species are present at the same time. I have been guilty of missing the start of an IBD hatch more than once because I simply didn’t see them. Surely an experienced fisher would not not caught out like this? Well, let me tell you that when olives and March Browns are drifting down the stream and trout are rising all over it is very easy to fix your concentration on what seems to be the obvious and automatically cast your olive or March Brown copies at the risers without stopping to study the water for the possible appearance of something else. I have been fooled many times into thinking I knew what was happening because I had seen fish actually take a few olives. Trout will accept the olives all right but they much prefer the IBD when it is available in my experience.
That brings me to the question of tactics and fly patterns for fishing the IBD hatch. There are a number of traditional spider patterns which imitate the IBD such as the Snipe & Purple and the Dark Watchett. Fished up or downstream as the situation dictates these are very effective catchers of trout. Avoid the temptation to use flies bigger than a size 16. The naturals are very small and size seems to be one of the triggers for the fish. Some IBD patterns sport a crimson tag and while I have netted numerous fish on flies with this adornment I remain skeptical they add significantly to the success rate.
Personally I prefer to tackle trout rising to IBD’s with a dry fly. I find that they take a floating dun imitation well and will come to it even if the hatch is light. Tiny klinkhammers and parachutes are ideal for this job. Brightly coloured wing posts are a real help to those of us with dodgy eyesight.
You can expect to find the IBD in numbers from now until mid-May so be prepared to scan the water closely for them. It is so easy to miss them but you can be sure the trout won’t. Those minute flies which resemble spots of ink in the water can provide you with some unforgettable sport if you keep your eyes peeled.