The Large Dark Olive – How to catch spring trout with 9 great fly patterns

The first sight of a Large Dark Olive (Baetis Rhodani) drifting downriver, drying its wings, is heart-warming as it signals spring has arrived or is near.

This early season up-winged fly starts hatching on the Welsh Dee during March. However, after a mild winter, they are seen on mild-February days whereas, after a severe, late winter, it can be April before significant numbers of LDO hatch.

Late or early, their appearance it’s a welcome sight because it provides an opportunity to start dry fly fishing for trout or grayling, especially after spending the winter euro nymphing for grayling.

Habitat & lifecycle of the Large Dark Olive (LDO)

The LDO (also known as the winter olive) is found throughout the UK but only in running water. It flourishes in all types of running water from fast-flowing mountain streams to slow rivers, including weedy, chalk, and limestone rivers.

LDO’s begin to emerge in late February on the Welsh Dee, with the main hatch occurring in spring (March & April). After sporadic hatches throughout the summer, a secondary hatch occurs in autumn (September and October).

Usually, the best hatches of this hardy up-winged olive occur on dull, cool days, but I’ve witnessed some good hatches when there is snow on the ground. 

On warm and windy days, newly hatched flies quickly leave the water because the dry conditions almost instantaneously dry their wings. In this situation, often, trout and grayling focus on taking the ascending nymphs.

The lifecycle of the large dark olive is similar to that of other up-winged flies:

• The nymph crawls out from its underwater habitat and swims to the surface (usually between 11 am and 4 pm).
• At the surface, nymphs emerge from their shuck and transform into duns. 
• Duns sit on the surface while their wings dry and then fly off to find cover and mature into a spinner.
• In the evenings, adult male & female spinners mate. The females then land on semi-submerged rocks and bankside vegetation to crawl down into the river to lay their eggs.

Often, you can…

set your watch by the start of the LDO hatch

Usually, the hatch will begin slowly around 11 – 12 pm, gradually increase to a peak between 1 – 2 pm, and then peter out by about 3 pm.  In warmer weather, the hatch will start earlier and finish later.

Large Dark Olive Nymph

The LDO nymph is torpedo-shaped (ca. 10 – 13mm long) with tails and belongs to the order of agile darters. It has a dark olive body with pale yellow ribs and darker wing buds, which become almost black when it’s ready to hatch. 

Trout and grayling feed on the nymphs all year as they dart between rocks and weeds on the riverbed. However, spring hatches can send…

hungry, overwintered brown trout into a feeding frenzy.

Fishing Large Dark Olive nymph patterns:

While waiting for a hatch to start, I often fish a pheasant tail nymph pattern in the riffles & pool heads.

A slimly dressed pheasant tail nymph (size 14) is probably the best imitation of the LDO nymph because it has stood the test of time. (NB. Buy flies that have a thin body profile ca. 3 mm max).

So, what nymph patterns do I use?

Frank Sawyer’s Pheasant Tail Nymph is first up because…   

• It catches fish
• Only two materials are required: copper wire and a pheasant tail feather
• Easy to tie – especially if you are starting fly tying. 

Frank Sawyer’s Pheasant Tail Nymph

• Hook – HENDS BL354 #12 to16
• Thread – Fine copper wire
• Tail – 4 to 5 pheasant tail fibres
• Body – 3 pheasant tail fibres
• Thorax – Fine copper wire
• Wing case – 5 pheasant tail fibres

Tying Instructions

This nymph is a good choice for streamy water.

However, for slower flowing water, where trout and grayling are selectively feeding, I use a more life-like large dark olive pattern that has legs.`It is more complex to tie but worth the additional effort. I always carry a few in my fly box for the occasions when Sawyer’s PTN does not work.

PL Pheasant Tail Nymph

• Hook – HENDS BL354 (#12 to 16)
• Thread – Tan 8/0 Uni
• Tail – pheasant tail fibre tips
• Body – pheasant tail fibres
• Rib – fine copper wire
• Thorax – Hare’s Ear dubbing
• Legs – Partridge neck feather 
• Wing case – WASPI Mottled Oak Thin skin

In fast streamy water, use a heavier pattern to reach fish nymphing close to the riverbed. In that situation, I fish a pheasant tail jig nymph sporting a heavy tungsten bead.

Pheasant Tail Jig Nymph

• Hook – HENDS BL154 (#12 & 16)
• Bead – 2.5mm tungsten (silver & gold)
• Thread – Brown UNI-Thread 8/0
• Tail – 3 pheasant tail fibre
• Tag – Orange floss (optional)
• Body – pheasant tail fibres
• Rib – Fine copper wire
• Thorax – Olive dubbing  

It’s a versatile pattern, which I tie with, and without the orange tag and using a range of coloured tungsten beads (i.e. neutral, silver, gold, and copper).

Large Dark Olive Dun

Identification is easy because, apart from the March Brown, the LDO is often the only fly on the river at the start of spring. Also, the two species differ in both size and wing colouration. 

The LDO has a dark olive-brown body (ca. 10mm long) with yellow-olive bands, which is roughly 3/4 the size of the March Brown. However, wing colouration is a more noticeable difference between the two species. The LDO has pale grey wings with brown veins, while March Brown has mottled brown wings.

Fishing for trout feeding on LDO

LDO nymphs start to swim to the surface to hatch into a dun between 10 and 11 am. During the hatching process, it unfolds its wings and floats on the surface while drying its wings, which can be slow during cold, damp conditions. During which time they are…

“Sitting Ducks”

Fish take the nymph as it emerges or while it’s waiting for its wings to dry. If your dry fly should start to sink, wait a few seconds before recasting because fish often take the drowning fly.

Often the best hatches of LDO occur at the heads of pools, fed by oxygenated water from gravel and rocky riffle sections. On the Welsh Dee, examples include:

• Duncan’s Pool
• Hollybush Pool
• Pipe Pool
• The Long Meadow

Studying the general structures of the pools above gives the novice angler a clear understanding of what to look for on other rivers.

Wet fly/spider fly fishing

In spring, I aim to be fishing by 10 am to ensure I don’t miss the start of the large dark olive hatch. Also, I like to make the most of these short spring days because they often involve a 2-hour round-trip drive to the river for me.

I’m always eager to wet a line and often start fishing with a team of wet flies or North Country spiders. Usually, this approach catches a few trout before LDO start fluttering off the river, and I switch to dry fly fishing.

My standard wet fly cast consists of a team of 3 flies spaced 3ft apart on a 3 lbs copolymer leader (point fly – #14 pheasant tail nymph; middle dropper – #14 pearly-butt Bloa; to dropper – #14 pearl-ribbed March Brown spider). 

This team fished upriver is often deadly during the early stages of a hatch of large dark olives, especially when trout are taking the emerging insects and seem to be ignoring the flies drying their wings on the surface.

When there’s an intense hatch of LDO leaving the water almost immediately after emerging, fishing this team of flies can sometimes be better than the dry fly.

If you are not comfortable fishing a team of wet flies, start by fishing a single PRMB spider. If no takes come after 10 to 15 minutes, change the fly to a pearly-butt Bloa.

The pearly-butt Bloa and sparsely tied PRMB spider are highly effective imitations of olives – as such, there’s always some in my fly box.

Pearl-ribbed March Brown Spider

• Hook:    Kamasan B170 #14
• Body:    Tan UNI-Thread 8/0
• Rib:       Pearl Mylar #14 (1/32)
• Hackle:  Brown partridge neck

Tying Instructions

Pearly butt Watehen Bloa

• Hook:    Kamasan B170 #14 & #16
• Thread: waxed primrose silk
• Body:    lightly dubbed mole fur
• Hackle:  Waterhen wing slip

Tying Instructions

Dry fly fishing

Spring is a civilized time of the year for the dry fly fishing pursuit because the LDO is a late riser, with hatches rarely starting before 11 am and over by 4 pm.

No early start or late finish on the river is required.

Overwintered, hungry trout waste little time intercepting large dark olives as they appear on the water, triggered by a subtle rise in air temperature.

Hatches range widely, from a stop-start affair to a constant stream with an intensity ranging from a small flurry to a blizzard of duns coming off the water. Also, the hatch can be very localized, with the pool alive with hatching duns while the next pool is dead.

Therefore, it is best to cover as many pools as possible in search of a hatch and rising trout. Once found, fish the pool thoroughly until it’s exhausted and then move on to a new location.   

It’s always wise to avoid spooking rising trout, but presenting a dry fly to hungry brown trout is often straightforward. Even a…

Poorly presented fly is engulfed as it passes overhead.

A simple dry fly leader set-up is often sufficient at this time of year. I use a 9ft 4X tapered copolymer leader with 2-3ft of 3lb fluorocarbon tippet tied onto the end.

Now, let’s take a look at fly choice…

Large Dark Olive dry fly patterns

Fortunately, a multitude of dry fly patterns is not required to successfully imitate the LDO. I’ve caught plenty of trout and grayling using three fly patterns that are relatively easy to tie.

In early spring, the Jingler fly is my preferred dry fly pattern, which is successful in slow to fast water. It will also take trout when allowed to swing around in the current.

Jingler

• Hook: Kamasan B170 #14 & 12
• Thread: Olive 8/0 Uni
• Tails: Badger cock fibres
• Body: Hends body quill BQ-32
• Hackles: grey partridge & ginger cock

Jingler tying instructions

The front partridge feather should be tied facing backwards. So it concaves forwards over the eye and produces a bushy Jingler that trout find more attractive.

Next is the CDC Olive emerger, a deadly fly when olives are hatching on the Welsh Dee. It’s relatively easy to tie, and in my fly box, I have several variations (size & colour).

CDC Olive emerger

• Hook:  Kamasan B100 #12 to 18
• Thread: Olive UNI-Thread 8/0 
• Thorax: Hare’s ear dubbing
• Wing: 3-4 CDC feathers

Olive emerger tying instructions

To imitate a spring hatch of LDO, I use either a size 14 or 12. However, a size 16 or 18 is better for an autumn hatch, especially when fishing for grayling.

If the above fails, my backup is a… 

Olive Quill Emerger

• Hook:      HENDS BL345 #16 & 18
• Thread: Tan UNI-Thread 8/0 
• Body: Peacock quill
• Thorax: Peacock Herl
• Wing: 3 CDC feathers

If you’re unable to tie or get hold of the above patterns, then a traditional shop-bought Greenwell Glory dry fly (size 14) is a good general representation of the LDO and will catch early-season brown trout. 

Large Dark Olive Spinner

The female LDO spinner has a reddish-brown body with olive rings and transparent brown-veined wings. After mating, she will lay her eggs in the river by crawling down partly submerged rocks and bankside vegetation. 

As such, spinners are often of little significance to the fly fisher.

However, when trout are spotted taking spinners, they can be tempted with a size 14 pheasant tail spinner.

Pheasant tail spinner

• Hook:      HENDS BL345 #14 & 16
• Thread: Olive UNI-Thread 8/0 
• Tail: Ginger cock
• Tag: Pearl Mylar
• Body: Pheasant tail fibres
• Rib: Fine gold wire
• Wing: Cream Antron

Finally, I hope you have found this article on the large dark olive interesting, and if you want to practice this learning, there are some excellent fly fishing day ticket waters on the Welsh Dee.