Yellow pearls

Don’t ignore the Yellow May Dun, says OLIVER EDWARDS who explains why is vital the successful angler has these lethal patterns to hand

On my initial quick flick through the December 2008 copy of FF&FT, I was checked at page 63 by the fine photograph of the Yellow May nymph. After reading the accompanying piece I turned back a page to find that the author was Peter Lapsley.

Peter admits to being intrigued by the insect, and, as an angler of southern streams, rather than being totally dismissive, he at least has done some homework. Nevertheless, there is still a good deal of misunderstanding about this Mayfly and I bet I’m quite safe in saying that chalkstream fly fishermen have only scant regard for this beautiful dun. Even beyond the southern chalkstreams the myth still persists in some fly fishers’ minds that the Yellow May – Heptagenia sulphurea, is of little fly fishing value.
I believe it long overdue for this myth to be quashed, or at least qualified.
For starters, I would like to describe my own observations of the Yellow May on southern chalkstreams. I have only fished a few, so my experience is limited. However, most hours have been spent on the river Test, where for several years I was a co-instructor on fishing courses. During the late May and June courses, I would occasionally see the odd Yellow May. Most times the insect was airborne. Any I did see floating along never got eaten, as far as my eyesight could follow. And I always did. However, I’m talking about very small numbers, a handful. Over several years the observations were always the same – the odd ones on the water, odd ones flying – none eaten. Yet, when I did kick-sampling for the entomology part of the course, I nearly always found their nymphs, sometimes quite a few.

The conclusion I came to then, and remains so, is that there are simply not enough Yellow Mays on chalkstreams to create the necessary feeding trigger. Remember, this mayfly is a ‘trickle’ emerger, and while it is possible to turn up what may seem a good number of nymphs, their trickle emergence behaviour never results in concentrations of duns. So trout never see enough.

As obvious and eye-catching this large yellow fly may be to us on the bank, far, far too few to stir the fish.

So, I, too, never actually saw a Test trout rise and eat a Yellow May, and, if what I saw is typical, then I can quite see how this mayfly has been ignored on the chalkstreams.

More of the same
On streams like the Test, where, outside the Greendrake (‘Mayfly’) hatch, the most abundant, and by far the most available surface borne food items are the olives and other similar pale upwings, the various Baetis, Centroptilum and Ephemerella species. These are seen by the resident fish very regularly and over a long period. As the emergence of one species fades, another starts, and within a whisker this new species will look and behave more or less the same as the previous one. In this situation the trout get accustomed to the regular sight of the same dun and they will feed in a similar way – same, regular and confident. As on any river, trout key-on and will ‘selectively feed’ on the most abundant and available food item in the river. Those two words – abundant and available – are very important, when both have a high score; trout – and other fish – are virtually guaranteed to be feeding on it/them. Trout – and grayling, too – soon ‘learn’ when the day’s emergence will start. They simply take up a good feeding station on what can be a well stocked conveyor of ‘same things’, and sip away at their leisure.

Now into this large mix of ‘same things’, widely scatter a tiny number of ‘different things’ – Yellow Mays for instance – and see what happens. Even a deep-pocketed, short-armed Yorkshireman would put money on their not being eaten.

It is this dependable and ordered regime which made our southern chalkstreams world famous – they were insect factories (I’m using the past tense because we are told things aren’t what they used to be) where fat trout were virtually guaranteed to be rising very regularly to small duns for much of the day. Even when the situation changes and trout are rising to extremely huge duns, as for instance during the annual ‘Mayfly’ carnival, the same two boxes – abundance and availability – score ten out of ten.
Enough of chalkstreams … come north, then on into Scotland, or journey into Wales, or the West country and you will find the Yellow May story quite different – on some rivers even opposite to that of the chalkstreams.
The two rivers I fish mostly in Yorkshire, the Ure and Wharfe, have in their middle reaches, huge and sustaining populations of Yellow Mays, and it has always been so, at least during these past 55 years of my fly fishing. I, for one, have never understood all the concern about the Yellow May, to me it has always been a great hatch, and my fly boxes have always had rows of yellow patterns, right from my early days.

Flat nymphs
Today on the middle reaches of both rivers it is quite common to find every stone you turn over will reveal many of their flat nymphs, and in most instances they greatly outnumber the olive nymphs. The insect appears to have a definite preference for calcareous water which is why our middle reaches suit it, thank to the various ‘craven faults’ through which both rivers pass, washing both in beneficial limestone and pushing the pH well up the scale. Readings on our Wharfe stretch are typically in the 8 to 8.5 range with a recent high of 9.2 (Environment Agency figures). Like all members of the Heptageniidae, they prefer a rough, stony substrate, with quite swift water, which these two rivers have in abundance.

The Yellow May emergence is something I eagerly look forward to, as this usually signals the start of at least six weeks of good fishing. Also, this upwing is one of those which seem to ‘pull’ the better fish – sometimes a clonker!

Our trout and grayling take the dun with a confident head-and-tail rise, occasionally the take is an aggressive swirl reminiscent of a caddis take. It is also not uncommon to find some fish which actually go ‘selective’ on this big yellow fly.

However, the dun is just one part of this insect’s appeal. The nymph is also eagerly taken, but it is the mature nymph which is targeted, particularly in the days/hours leading up to emergence. Now the nymph becomes very active and restless, it will also be displaying plenty of the adult yellow colouration, particularly on the underside. (This ‘ripe’ colouration is in quite marked contrast to its earlier instar colour of overall black or very dark grey/brown with white spots and flecks).

The nymph in this pre-emergent state spends a considerable time on the exposed upper surface of the rocks and stones it was previously hiding under, and, although the nymphs of the Heptageniidae are not noted as efficient swimmers, these can, with a bit of dog- paddling, and tail lashing, progress through the water column. It is also suggested they may make short exploratory swimming trips to adjacent rocks immediately prior to emergence. Now the nymph is both abundant and available, and, of course, very vulnerable. Fish quickly key on to this nymphal activity and many are eaten.

At this time a weighted pattern, one having all the correct triggers, fished quartering upstream, classic style, and dead-drifted through likely spots will usually result in very positive takes. Sometimes a touch of speed-up or cross-current working will also give you a thumping, vigorous take.
The emerger is also eaten in quantity, but feeding on these goes unseen as it happens within the water column, not at the surface. So many anglers miss out on another excellent opportunity for sport.

Emergence of the Yellow May is interesting, even controversial. Quite a few species of nymphs don’t just rely on one method of emergence. They need to get to fresh air quickly, they are opportunists, and will take the most practicable quickest way possible. In the case of the Yellow May, perceived wisdom suggests that two methods are used to transpose from nymph to dun. It seems the deciding factor is all to do with the depth of water the pre-emergent nymph finds itself in. Where the nymph is in fairly deep water – say above knee deep – the nymph grips the upper surface of the rock very firmly with all six feet, the nymphal cuticle splits on the dorsal median line and out comes the dun – underwater – with its large forewings embryonic and un-pumped. It then makes the perilous journey to the surface. As it rises through the water column, the wings are being expanded to full size, so that when it reaches the meniscus and pushes through, the wings are virtually fully erect, and it floats away, or, as is often the case, immediately flies off. I have observed this mini miracle countless times, sometimes right by my waders.

In more shallow water, do-able by swimming you could say, the nymph behaves in a more conventional way; it swims to the surface, pushes its wing-buds/thorax through the meniscus membrane, the nymphal cuticle splits and out pops the dun. Capturing the underwater emergers can only be done via one method – finding them in a fish’s stomach. (Being eaten instantly arrests any further development of the wings) This is how I first discovered them. Most late May and June fish I autopsied showed heavy feeding on these black-eyed, pale yellow jobs. It wasn’t at all rare to find a trout with its stomach packed with them. (This was years before catch-and-release days, when a brace of half pounders were breakfast fish!) This was quite a discovery, and naturally I copied them, no science; I just made mine look like them. Then enjoyed myself and kept stumm – for a while anyway.
The dun first starts showing usually around the second week of May on the Wharfe stretch I fish, but it can vary. These very early ones go un-noticed and un-taken they float away or fly off, but you’ll only see maybe half a dozen all day. After only a few days the rate of emergence builds quickly and many more are seen, by the third week of May there is usually a good heavy trickle. They can be ‘on’ when I arrive, say ten-ish, and go on all day if the weather is kind – warm and sunny. By good heavy trickle I mean at almost any time while I am fishing I will see many scattered within my field of vision. Now fish will be switching on to them, and among these early feeders may lurk a big fish.

By the end of May, start of June, as well as singles, groups of three’s and four’s will be floating along, many others will be airborne. Now you can confidently fish a dry pattern, and even if it the weather is less than ideal, you can still ‘pull’ fish by simply prospecting through the likely spots.
From my own observations this is a ‘fine weather fly,’ and settled, warm, sunny still days, definitely boosts numbers, unfortunately in such conditions they’ll be off the water almost straight away. If a strong breeze sets in, all you’ll see are yellow splodges scooting past you! By now the entire river and its bankside communities know all about the yellow Mays – ducklings, wagtails, chaffinches, swallows, martins, everything is downing them. On the Ure at Masham, at the peak of the emergence, the black-headed gulls from a nearby flooded quarry actually hawk them in the sky.

Read more of Oliver's article and some of his patterns in the March 2009 issue of Fly Fishing & Fly Tying magazine.