Fly Fishers' Republic

Undercover Olives

May 9, 2006

by Andrew Petherick

Large Olive (Beatis Rhodani)

First I’d like to explain that olives are not a single species, or even a single family of insects. They are in fact a collection of species from a number of genera of insects from the order Ephemeroptera or the upwinged flies. Each species is called something like a Medium Olive, Pond Olive, Blue-winged Olive, Lake Olive etc.. You get the idea. The common thread being ‘olive,’ and given for the obvious reason that the dun adult has a body more or less olive in colour. These insects are also very similar in general appearance. They emerge at similar times and are thus easily confused. A lot of the time distinguishing one from another isn’t actually going to catch you any more fish, so most of the time we just call them olives. It’s easier that way. Right then, now I’ve got that off my chest, I’ll continue.

Here in the UK, we wait all year for the Mayfly hatch, Duffers Fortnight. A time where big, fat, free-rising trout, take big flies; a time when, hopefully, the fish are easy to catch. Don’t get me wrong, this is excellent sport, but due to its prolonged season, and a one or two other handy characteristics, there is another upwinged hatch that the angler should keep in mind. I am of course, talking about, Olives. Happily, these hatches occur in a large number of waters both still, and running.

So, What on earth is he warbling on about? How are the two hatches linked? Let me explain. There are a couple of separate elements here. The hatch timings, in that they cross over (Mayfly hatch May-June, Olives hatch March-November); and the more important topic of this feature, the olives themselves and their importance to the fly angler. Of several species of olives important to the fly fisher, in an episode I’ll relate to you in a moment, one of bankside frustration, the European Blue-winged Olive (Ephemerella ignitia) lay at the heart of a conundrum.

Hatch timings – The steak & nut theory

In the past I’ve spent months preparing for this time of year. I have, like most anglers, been preoccupied with the main hatch of May and early June – the Mayfly (Ephemera danica). This, due to the hype, is an easy trap to fall into. I remember being stood on the banks of a well-noted southern chalk stream, watching rafts of mayfly struggling on the surface, trying to become airborne.

I saw fish rising freely. I could see them, they were good fish, not recently stocked fish, but fin perfect, resident fish that every angler dreams of catching. Watching them, I thought that they were feeding on the large mayfly, the obvious thing for them to be chowing down on at that time of year. I tried everything to tempt them; every mayfly pattern, nymph, emerger and adult. I had to get that strike! I tried all sorts of tactics, long-range casting, ultra long (16ft+) leaders, even, dare I say it, downstream dry fly! I don’t think the editor will allow me to type exactly the level of frustration that I was feeling at the lack of interest I was getting from the fish. Lets just say I was not a happy bunny. I just could not fathom it out. I left the bank that day, fishless.

At the time, I worked as the fishing manager at Orvis in Stockbridge. It was rare that I caught nothing, and I was hugely frustrated. The following day I was in the shop serving the ‘Mayfly army’ that descended on that part of the world, at that time of year. I got talking to a gent, I wish I had asked his name, and told him of my frustrations the previous day, a knowing smile broadened across his face.

This experienced angler told me he’d been in a similar position a few years earlier. As I listened he told me about the steak and nut theory. He was convinced that more experienced fish (the ones we want to catch!) simply got fed up with mayflies after gorging on them for any length of time. He likened it to a human gorging on steaks for a week; despite its initial appeal, it would soon wear off. After this level of feeding, any sane being would turn to a smaller food source, he used nuts for his example. Essentially, the Mayflies were the steaks, and Olives were the nuts. Now this theory may not be open to debate but either way it certainly gave me a new outlook.

With new perspective, I prepared for my next trip on the river. Armed to the teeth with olive imitations I hit the river the following Saturday. Sure enough, the mayfly were out in force, and the same situation I had experienced the previous week, kicked into gear once again. This time I was ready.

Initially, as before, I saw nothing. Mayfly are big insects, and their shucks litter the river making it hard to see anything else in the drift. The fish were rising freely, but with the aid of a small pair of binoculars I spotted some adult olives sat on the surface. In fact they were blue-winged olives (BWOs). Sure enough, I tracked a few down the river, and they were gobbled up in preference to the mayfly.

Needless to say, I tackled up with a dry BWO imitation, and scored well that day, six fish to be exact all on CJ’s Ducks Dun, an excellent BWO imitation. All were better resident fish from the slack water at the tail of a large pool. I learnt four things that day:

1. You never know it all.
2. Be observant; look closely, even if the solution looks obvious.
3. If you are failing with the obvious solution (Mayfly) there is a reason!
4. I owe that gent a beer!

Since experiencing the masked hatch on the chalk streams, I have seen similar scenarios play out on some of the UKs stillwaters; notably, Avon Springs in Wiltshire and Lechlade (also Bushy leaze) in Gloucestershire.

So when you’re faced with a tricky situation maybe this could help you with a solution. Of course it will help to know a little more about the olives themselves so as to take best advantage when the light bulb goes on and we spot olives on the menu.

The Olive Family

Olives are found as larvae in rivers and streams year round. They emerge as adult flies from February through to November, but hatch in their largest numbers in July and August where they can be seen flying above rivers and streams and near bankside vegetation. The earliest species to emerge in the UK is the Large Dark OliveBaetis rhodani followed soon after by the Blue-winged Olive Ephemerella ignita that continues to hatch through to November. In the US, besides the Light HendricksonEphemerella rotunda, the Blue-winged Olive Baetis sp. is the earliest of the mayflies to emerge. Adult emergence timings are heavily dependant on temperature, weather, and locality.

Life Cycle

The general olive life cycle starts with the males forming a swarm above the water and the females flying into the swarm to mate. The male grasps a passing female with its elongated front legs and the pair mate in flight. After mating, the male releases the female, which then produces a ball of around 1200 eggs. This egg ball is held under the female’s tails and dropped into the water as she flies over it. After releasing the eggs, the female usually falls, spent, on the water surface.

olive nymph

The eggs fall to the bottom of the water where they stick to plants and stones. The eggs develop quickly in the autumn until they are just about to hatch. They then stop growing and spend the winter at this stage. When the water warms in the spring the eggs hatch and the tiny nymphs start feeding. They grow rapidly through a series of moults (or instars) and the first adults are usually seen from late June, but ealier in one or two species. The nymphs usually swim in short bursts, interspersed with periods of clinging to submerged plants and stones. Known as ‘darters’ and ‘moss creepers’ olive nymphs feed by gathering fine particulate organic detritus from the sediment and the flow of water.

Emergence of the adults takes place on the surface of the water during daylight and at dusk. Upwinged flies are unique as insects in that they have two adult forms the sexually immature dun (sub-imago) and the sexually mature spinner (imago). The first adult stage emerges from the nymphal skin as a dull-coloured adult that seeks shelter in bank side vegetation and trees. After a period of a couple of hours or more, the dun once again sheds its skin to transform into a more deeply coloured adult spinner. For futher detail you can read the details in the entomology section of Fly Fishers’ Republic.


So the main thing is to remember the key stages of the olive’s life cycle. Nymph, emerging nymph, hatching dun, dun, spinner. With those in mind let’s consider some details:

Rise Forms

Over the years since my experience on the river that time, I have been fortunate enough to fish and work with some of the best names in the business; notably, Charles Jardine. Charles opened my eyes to studying the rise forms of fish, before tying a fly on.

Once you have identified the species the fish are feeding on, stop and watch for a few seconds. Fish bulging/boiling beneath the surface will be intercepting the nymphs. If they are high in the water, just breaking, or just below the surface, consider using unweighted nymphs, or having a CDC dry to hold a nymph in the upper layer of the water column, in the feeding zone. The CDC will also act as a strike indicator, unless it is the one taken! If the fish are confidently rolling over the adults, then dry fly reigns supreme. Don’t get too caught up in this though, be aware that they will also take nymphs while feeding at the surface.

In my experience they are less likely to look at dries when on nymphs, but more than likely to look at nymphs when feeding on dries.

Good flies to use:

Olive Thorax Dun

Thorax Dun

Hook: 14 – 24 dry fly
Thread: Primrose Uni 8/0
Body: Fine olive dubbing
Rib: Nylon Mono
Wings: Hen hackle points
Hackle: Dun cock hackle clipped or singed.

Waterhen Bloa

Traditional hackled wet fly - Waterhen Bloa

Hook: 14 – 18 down eyed
Thread: Primrose Thread
Body: very sparsely dubbed mole
Hackle: Morehen primary covert

Pheasant Tail Nymph

Pheasant tail nymph

Hook: 14 – 20 down eye
Thread: Black Uni 8/0
Tail: Cock pheasant tail
Body: Cock pheasant tail
Rib: Fine copper wire
Thorax: Hares mask
Wingcase: Dark cock pheasant tail
Beard Hackle: Brown partridge

Be flexible

OK, so you have sussed out that the fish on the water you are fishing are feeding on olives. Don’t expect the hatch to start at the same time every day, it wont. Olives are very susceptible to climatic variation. Extremes in pressure and temperatures can mean that they will hatch very early in the morning, or late evening. Keep your eyes open, and be prepared to change as soon as you start to see obvious indications.

Whether on river, or stillwater, whatever species of olive you are looking at, the setup happily, is broadly the same. I would recommend nothing heavier than a 5-wt, and a leader tapering to a 3-4lb tippet (5-6X). These are small flies and need delicate presentation to convince the fish. Due to their ecology, there will be a lot of naturals around when a hatch starts, don’t make your fly stand out for the wrong reasons.

We have already looked at the nymph and adult stage, don’t forget the spinner. This is the only time I would jump up and down and say that it is pointless to fish anything else other than a single fly. Accurate casts are needed to feeding trout. Also, try and make the leader as long as you can handle, while maintaining turnover. Enjoy!


Size Matters: Make sure that the size of your imitation matches the flies that are showing. The fish just might not recognise it as food otherwise!

More Dries & Emergers: Olive CDC, 'F'Fly, Pent Spinner, Dear Hair Emerger, Loop Wing Emerger, CJs Ducks Dun

More Nymphs & Wets: Hares Ear Nymph, Greenwell's Spider.