Fly Size decisions

Big or small, bushy or sparse? Kiel explores a confusing topic.

It’s the age-old debate: does size matter? The wind is down, and the lake’s alive in and out of the ripple line. Rising trout clipping and sipping. The odd rise, subtle but you know that’s the larger fish. The day’s been long, sun’s dipping and the light turns from bright to golden as dusk arrives. You know the majority of trout are feeding on micro midge but you’re waiting for the right time, when the midge ball up and you have your best chance to realistically match the hatch.

A size 18-16 Griffiths Gnat?

A Klinkhammer in your favourite colour, size 18-16?

Teased dubbing or kept thin?

Green or brown, maybe grey?

But for some reason, you reach into your fly box and tie on a size 8 black Woolly Bugger, and cover a rise. On the third strip you connect. It’s a battle and you land a midge feeder.

Why size 8? Why did the Bugger work? Was it mistaken for a mudeye? Was it a good cast? Was it luck? Did you really need to buy those half dozen size 22 midge flies at The Flyfisher? Did the fish really care? Does any of this matter?

I know it’s not just me. At 10:45pm, I’m walking around the house wondering who’s been outsmarted and who’s been fooled. A trout with a brain the size of a peanut, or the supreme hunting machine: me. 200,000 years of hunt and survive evolution. And I’m fairly sure, most of the time it’s me being fooled. So lately I’ve tried to change the way I choose the flies to tie on, the decision-making, and the way I look at flyfishing as a whole.

The bigger the fly, the bigger the fish?

This one took me a long time to get my head around. As a kid, I remember flyfishing Hepburn Lagoon. I’d always choose a slightly larger fly, mostly because Dad emptied his ‘second box’ into my hands. Mrs Simpsons, bead-head Woolly Buggers and some sort of scruffy brown nymph in a size 8. These all have their place in lake fishing for trout, but they’re not the sort of flies I’d now fish all day long.

If I was fishing estuaries as a kid, I’d opt for a large Clouser Minnow, thinking that would filter the smaller bream and only hook the big fish. But of course, this proved not to be the case.

Today we drift the banks and flats in a boat, still chasing those larger bream, but I’ve had more time to think about what they feed on, at what times and, most of all, size, weight and colours.

Size 8 bead chain Hammerhead.

A lot of the time, those bream aren’t feeding on baitfish three inches long – aka a Clouser Minnow – but they’re cruising the holes in shallow sandflats looking for shrimp and worms, or rolling on rocks along the banks, eating crabs and other crustaceans. Fishing light tippet, I’m looking for correct bead weight/ fly size depending on wind, tidal current and depth. Turns out, a BMS Hammerhead tied on a size 8 shrimp hook covers a lot of that bream food. In years gone by, we used size 6 and even size 4 hooks, but we’ve found a lot of better fish coming to the boat with this smaller fly.

Things to think about here are:

  • Smaller fly equals less chance of spooking fish in shallow water, or even in the deeper spots where the fish are sitting mid-water column, such as near snags or jetty pylons.
  • We’re somewhat matching the hatch. Rolling over rocks and scouring through weed beds, we rarely see shrimp larger than 40mm and most are smaller.
  • If fishing deeper water, no need to go bigger or heavier – sink-tip leaders or lines, or intermediate or sinking lines, still allow the smaller/lighter fly to get down in the bream zone.

In this case, size does matter, and opting for a more realistic fly size is more viable due to the way in which we are fishing the flies. I believe slow sink rates can help strike rate, with a lot of fish taking on the drop. If your bream fly is big and heavy, and dropping too quickly, you’ll miss out on fish.

Stand out in the crowd

It’s overcast, 40% chance of rain and muggy. It’s mid-spring and I’ve scored a day off from guiding, so why not go see if the mayflies are popping? There have been reports of good numbers of trout being caught at Moorabool Reservoir, and I arrive at the carpark around 2.30pm. I notice it’s warm, and the air feels thick and damp. Perfect.

There’s a slight breeze over my shoulder as I get to the lakeshore, and straight away I notice a rise. Good numbers of mayfly duns are emerging, sitting from the calm water and out to ripple line, a full cast offshore. They’re not having much luck trying to dry their wings before the swallows and trout eat them up. Taking my time, I watch for trout and I pick up a real dun; size 14, brown; a tiny sailboat in the light breeze before I ruined her drift.

Right, I opt for a size 14 Possum Emerger. Waiting, waiting…There he is, a good brown cruises in and starts clipping duns and emerging nymphs stuck in the meniscus. Easy to tell what direction the trout is feeding in, cover it with a cast…

Waiting, waiting… Nothing. He swam past. No refusal. No spook. Just swam past and kept feeding.

Okay, fly change. Size 14 brown paradun, matching the hatch. Again ignored. A little frustrated at the nice brown which just outsmarted me, I think about throwing a rock, but instead keep my cool and walk to the next bay.

Same thing, duns trying to dry their wings with the odd trout sipping. I choose my target, cast out and sit my paradun out there amongst the real mayfly. The trout sips, clips, rolls and eats four real mayfly, leaving three and my fly. Ahhh, come on!!

Last fly change before I throw that rock! Opting still for a paradun, but in a size 12 – bulkier and a larger hackle. Waiting again, waiting again… Here comes the fish, good cast landing my fly out there with the real mayfly. Nose comes out slowly sucks the size 12 down. Hook, set and to the net.

Releasing him back into the water, I’m left wondering why that fly worked. This was four years ago, and I’ve been fishing many lake dun hatches the same way since. My size 12 paradun stood out in a crowd of size 14 mayfly.

I fish this technique when there’s a lot of real mayflies of the surface and when the trout are feeding intensely. I don’t change my pattern, just up the size. Getting noticed amongst the chaos. Still somewhat matching the hatch, just larger. An attention seeker.

You will find times when this won’t work – on rivers when there’s not as many duns floating down, or when the hatch isn’t thick, and the trout have time to suss the fly out. But amid feeding frenzies and chaos, sticking with matching the hatch but upping the size, can definitely work.

Things to think about here are:

  • How many trout are up and feeding & how many insects on the water? Lots of both, maybe go up a size?
  • The bigger the fly, the easier it is for us anglers to see amongst a crowded water’s surface. Easier fly to see = quicker on the strike.
  • Confidence. Get out there and try a new trick.
  • This idea of standing out will work in most heavy concentrations of insects and other fish food. Examples include gum beetles, caddis and baitfish balls in the salt.

Territorial aggression/ ‘fly swat’ strikes

This is about choosing larger flies with the intention of irritating or aggravating a response out of the fish. It especially applies to territorial species, such as Murray cod, Australian bass and even a trout working its beat.

Scrambling through the temperate rainforest-lined rivers of New South Wales’ South Coast is a favourite way of mine to fish, for a favourite fish: the Australian bass. In freshwater, bass are often feeding on terrestrials; waiting for cicadas, grasshoppers and the like to fall into the water before inhaling them. This is another situation where I’ll match the hatch, but up the size, aiming to fish surface flies larger than the bugs the bass are feeding on. I can move some water with these big flies. I’m thinking, make some noise; coax them to the eat.

It’s not very often you watch a live cicada land on the water, then make a huge bloop. But that’s what we can do with our larger flies, to bring those bass out from their homes and out to investigate. More often than not, they’ll then eat.

Large Big Mac thanks – and an angry topwater bass.

Trout in shallow water, cruising slowly looking for frogs, sticking their noses into the mud and rocks tailing. Eating nymphs and snails. Subtle swirls reveal them.

Reeds parting as they swim with their dorsal fins and backs out of the water hunting.

It gets my blood pumping just writing this!

Choosing larger flies in this situation works well due to the trout’s limited vision – the shallow water hinders their sight so upping fly sizes helps get their attention. A chunky fur fly splatted down like a frog next to a trout nymphing in the margins, will often get a reactive take.

Things to think about here are:

  • As discussed above, this technique works better on some species than others – although it’s worth a try on most.
  • Keep in mind that territorial aggression in many species is likely to be heightened close to breeding season, e.g. brown trout in late autumn, rainbow trout in winter/ spring, cod in early summer (the spring cod closure is based on the vulnerability to angling of aggressive breeding cod in spring). 

One from the reeds.


This is what most of us flyfishers hope for – well, some of the time. And yes, this challenge definitely has its place.

Most of us have been confronted by the ‘dark side’ of match-the-hatch, when we’ve tried every fly in our box, every tippet size and length, yet still no takes. This usually occurs when the fish home in on one food source to the exclusion of all else: it’s size, shape and colour, sometimes even scent and taste.

I’m reminded of a backcountry New Zealand trip in late February. Cicadas singing all around, mountains climbing into the bright blue sky, the river tumbling down a valley in between, and the odd mayfly peeling off the water. Scott X and I were slowly making our way upriver sighting big brown trout, which sometimes gave themselves away with a rise. Upon finding the first fish that was possible to cast to, I snuck down and crouched in the long grass lining the bank. I covered it twice with our go-to fly, a size 8 cicada pattern. Both times the fly drifted straight past the trout. Scott had a slightly smaller cicada pattern, a touch lighter in colour. Same thing, except this time, the brown did a little dance sideways to move out of the cicada’s way. Wary of spooking it, Scott and I gave the trout a rest and discussed why it wouldn’t eat our fat, plump, juicy cicada patterns.

As we were discussing (read: arguing) about this, we saw the trout nose up and sip a mayfly from the surface. This led us to conclude that for whatever reason, the trout was fixated on the apparently small number of mayflies drifting down. Strange it would let a big meal like a cicada drift by, but sometimes you get those fish. And matching the hatch, or at least one of the hatches the trout appeared locked on, should work. Change of fly from the size 8 cicada to a size 14 grey Parachute Adams.

Again, I was Ninja-like. Crouching Kiel, hidden trout. A fairly decent cast, landing the para Adams just to the left and two feet above the brown. Fly lands, drifts a foot, trout moves left and, without hesitation, scoffs the fly. Epic battle downstream and across the river he finally came to the net. That’s all it took. Patience to figure out what this fish was feeding on – and he was already feeding. No need to entice the fish into an eat, he was already dining. Just swap a size 14 mayfly natural for a size 14 mayfly pattern.

The mayfly muncher.

Things to think about here are:

  • When you cover a fish with a good presentation or two and it refuses the fly, change. If you have the option, rest the fish whilst you think, watch and pick the right fly. Tie it on and recast.
  • Fish don’t think like you and me. Sometimes they just want a chip, not the whole Big Mac meal. If you keep throwing that cicada over it, chances are the fish will spook.
  • Look for a food source. If a trout’s swaying off its station left right/ left right, but won’t eat your nymph, roll over some rocks. This will give you a good idea of what’s drifting down.
  • Get to know likely food sources at that time of year, for the water you’re fishing: basic sizes, colours, behaviour, etc. Keep in mind that if there’s enough food for the trout to lock in, there will often be enough of it for you to see and identify it as well.

Deliberately go small 

There are times when deliberately going small works. European nymphing is a good example. Thin, sparse ties allow nymphs to cut through the water, getting to depth quicker, and allowing a longer drift in the ‘fish zone’ on the bottom.

Spending a lot of time last year on central Victorian lakes, when the water was high, I found trout in reedy pockets and gaps in the rushes. Trout slowly made their way in and out of these gaps searching for food. For this fishing, I’d choose a small unweighted black flashback seals fur nymph or a hare’s ear, size 14 to 12. Unlike Euro nymphs, the coarse, long, wiry strands of fur aided a slow sink rate. When a trout was about to arrive in a gap, casting the small, unweighted nymph allowed a very slow and natural sink rate. With a little, unobtrusive nymph in the face of the trout, it wouldn’t have any option but to have a taste.

A brown caught on a sparse pink jig tag.

Then there’s blind searching a lake on a dull day, when there aren’t a lot of visible bugs about. Yes, fishing a big wet like a Woolly Bugger, Magoo or Green Machine is the natural choice, but tying a small nymph or similar three feet off the back or three feet up on a dropper, can work well. The larger fly gets the attention, but the smaller fly gets eaten.

Things to think about here are:

  • Fly size and dimensions can affect sink rates – weight for weight and hook size for hook size, bulky flies sink slower than sparse, skinny flies.
  • While bigger flies can be easily noticed, smaller flies are non-threatening and harder for the fish to pick fault in.
  • Small flies help delicate, non-alarming presentations. Perfect for river backwaters and lake shallows.


So fly size can, and often does matter, but in the context of circumstances which are continually changing. Don’t fish a heavy crab pattern to tailing trigger fish in a foot of water. The splash will spook it. Use a smaller, lighter fly and allow the tide to drift the crab to the trigger.

In any fishing situation, think about water clarity, light intensity, current speed (if any), depth, fish species and likely food – all these factors and more will affect what size fly is best. It’s flyfishing, and there are few tidy rules. Get out there, explore and experiment.