Size M T You wouldn’t guess at the shape or

Size M
You wouldn’t guess at the shape or
a hatch, so why are you guessing at
rout survive by becoming efficient feeders. They constantly adapt
to the available food sources and key in on the ones that are most
prevalent at any given time. Each bug they eat is positive reinforcement that encourages them to look for more of the same. As a
result, fish become selective and ignore other food items in the drift. Fly
anglers who can identify the food most available to trout and match a fly
pattern to it greatly increase their chances of success.
Most experts agree that size is the most critical aspect of matching the
hatch. But surprisingly, experienced fly fishermen misjudge fly size more
often than not. Get the size close, and you will catch a few fish. Use the right
size fly, and you have a better chance at catching a lot of fish.
by Troy Pearse
color of the natural insect during
the size? Here is how to get it right.
Size Survey
Last summer, I gave a presentation on local hatches at
the 2010 Western Idaho Fly Fishing Expo, during which
people were asked to estimate the size of nine bug samples
and three flies. All participants in the study were experienced fly fishermen and fly tiers. The bug samples were
in individual glass vials of isopropyl alcohol, and the flies
were mounted on foam.
Results of the survey showed that 72 percent of size
estimates were wrong, and 19 percent were off by two or
more sizes. The best any one person did was accurately size
half the samples, and that person was a professional fly tier.
One common pattern noted in people’s guesses was having
a preconceived notion of the expected size of a bug. They
expected midges to be tiny, so most guesses were too small
for a size 16 midge sample. Similarly, they did not expect
the net-spinning caddis to be so large, so they guessed the
typical size of 14 when the actual insect was a size 10. Part
of the confusion is that there isn’t a standard for hook sizes
(see sidebar on page 42).
Being off by one hook size may not sound significant, but it can often make the difference between a trout
accepting or rejecting your fly—especially on pressured
waters. There is a 25 percent difference between the shank
length of a size 18 and size 16 dry fly hook, a difference
that trout will notice.
Measuring Bug Size
Most fly fishermen wouldn’t dream of using the wrong
type and color of fly during a hatch, such as a Tan Caddis
imitation during a BWO hatch. Yet they will routinely and
inaccurately guess at the size for their fly. The key to selecting the correct fly size is measuring the bugs that fish are
eating and then selecting a fly that matches. It’s that simple,
MAY/JUNE 2011 I 41
but you do need two things: a reference tool and a bug sample.
Here is where most anglers fail. One can often tell the type of a
bug as it floats by 20 feet away and get a good idea of the color, but
until you have an insect in hand, you are only guessing at its size,
and as demonstrated by the survey, most likely guessing incorrectly.
If bugs are hatching, try to collect one from the surface first.
Look in back eddies for drowned naturals, or wade out quietly
below rising fish and grab one. Using a piece of mesh—such as
a mesh bag that fits over a landing net—makes it much easier to
scoop bugs from the surface. If nothing is currently hatching, then
look around for bugs onshore in bushes, underneath leaves, or
caught in spiderwebs. Turn over rocks near shore and see what
nymphs are present. Better yet, use an insect collection seine and
gather a sample from a riffle that is one to two feet deep. Put the
seine on the stream bottom and kick over rocks directly upstream.
By collecting a bug sample for fly selection, you have taken a
huge step forward and are now ahead of most other fly fishermen out on the river. Hold the bug up to different flies in your
fly box until you find one that matches the size, shape, and color.
Make sure the body length of the bug matches the fly (excluding tails, wings, antenna). For dry flies, pay particular attention
to the color of the underside of the bug because this is the color
that fish will see most.
It is very helpful to know the hook size that matches the bug,
so that you can communicate that size information to your fishing partners, make entries in your fishing journal, and later tie or
purchase flies that match the hatch. To make measuring bug size
easier, I created a bug/fly size chart called the Bugometer. (See
page 43.) Simply hold your
bug or fly up to the chart, and
you have the correct size. Sizes
in the chart are based on the
Size Differences
hook-shank length of Tiemco
standard, 2XL, and 3XL hooks.
I have been using the
Bugometer for a couple of
years now and have found it
Having a variety of sizes in each pattern is a cornerstone of matching the
hatch. In this case, the larger size 12 (2XS) fly better matches the netspinning caddis than the smaller size 14 (2XS) does.
indispensable for choosing the right size fly. There have been
many times on the water when I was getting refusals to the fly I
was using. But when I took a few minutes to capture a natural,
measure the size, and changed my fly to match, I immediately
started catching more fish. The scale reproduced on page 43
is the exact size of the Bugometer, which you can photocopy,
laminate, and keep in your vest to measure bug size. You can also
purchase a transparent plastic version online through DryFly (Editor’s note: The author originated the
Bugometer but allows another party to manufacture and market it.
He does not receive compensation for sales).
Bug Size Variations
The average angler is able to recognize a hatch situation and can tell
that fish are taking mayflies, caddis, or stoneflies, but too often that
is the extent of their analysis. They open their fly box and choose a
fly without knowing the actual size of the hatching bugs. This works
sometimes because there are typical sizes for the hatch, such as size
16 for a pale morning dun (PMD), size 20 for a blue-winged olive
(BWO), or size 4 for a salmonfly. But the problem is that each bug
Hook Size Varations
Hook Style
Size Differences
consistent across hook manufacturers and styles. A hook shank can be standard, short or long, and
there are general conventions about what style of hook is used to tie different patterns. For instance,
scuds and caddis pupa are often tied on a curved 2X-short hook; mayflies on a standard-length hook;
grasshoppers and other large terrestrials on a 2X-long hook; and stoneflies on a 3X-long hook. This
means a size 10 Royal Wulff is one-third shorter than a size 10 Stimulator because the Wulff is tied
on a standard shank hook, while the Stimulator is tied on a 3X-long hook. Similarly a size 16 Z-Wing
Caddis Pupa tied on a curved 2X-short hook is 25 percent shorter than a 16 Elk-Hair Caddis tied on
a standard-shank hook.
Although hook manufacturers use a common size numbering scheme, there is no standard for
hook length. For example, the shank length of a size 16 Dai-Riki and Mustad hooks are 25 percent
smaller than the same size hook from Tiemco and Daiichi. These complications make it essential that
you collect a sample and match it directly to the fly you intend to use.
Common Sizing Mistakes
The Bugometer
hatch is different, even within the same species. The
insects aren’t always the same size, which means that
fly-size guesses based solely on typical insect sizes or
hatch guides are often wrong.
For example, many mayflies are larger earlier in
the hatch season and grow progressively smaller over
time. A PMD may be a size 14 when the hatch starts,
a size 16 a few weeks later, and a size 18 by the end
of the hatch. Similarly, BWOs are typically larger in
the spring, smaller in the fall, and show a similar size
reduction through the hatch. This variation in size
isn’t limited to mayflies; the size of midges and stoneflies also varies through the season, year-to-year, and
among different rivers. The size of salmonflies hatching may be a huge size 2 (3XL) one year, but be a size
or two smaller when they start hatching upriver or in
the following year’s hatch.
People also tend to think of a nymph as always
being the same size as the adult, but the size of
nymphs continually changes through their life cycle.
A Skwala stonefly nymph might be a size 8 (3XL)
when it hatches in April, but in January and February, the nymph is a size or two smaller.
It is important to remember that the size of bugs
changes over time and location. You may have had
the right size fly last week, but that doesn’t mean it is
the right size fly today. Check the water to see what
fish are feeding on before choosing a fly.
LEFT: Preconceived notions of insect sizes are often wrong, as this size 16 midge shows.
Many anglers just assume midges will be size 18 or smaller. And these bugs are often
bigger in spring than in fall. RIGHT: Skwala stoneflies are typically a size 8 (3XL), but the
nymph can be much smaller prior to maturation. TOP: The Bugometer scale.
Here are five common mistakes anglers make when
sizing flies, along with suggested remedies.
1. Guessing at Size
The biggest mistake people make is guessing the fly size instead
of measuring it. The simple solution for this is to catch a bug
and measure its length, and then select a fly that matches.
moving a couple miles upstream or downstream can result in a
different size hatch. Try a smaller dry fly or a larger nymph—or
better yet, collect a new sample and verify its length. While catching your new sample, look for other food sources that fish are
eating, such as a spinner that is flush in the surface film.
5. Hook Size Differences
2. Including the Wing
Some people mistakenly include the wing or tail in the length
and choose a fly that is too large. Wings on caddis are usually
much longer than the body. Make sure to turn the bug upside
down and check the body size.
3. Secondhand Information
Too often people choose a fly based on secondhand information,
such as a fly-shop recommendation, a hatch chart, or a fishing
report. These are all good references, but they aren’t always going to match what you find on the river. Be ready to verify and
switch pattern size if the fly you first use doesn’t produce.
You bought or tied a size 16 fly expecting a certain size. But the
length of a size 16 fly can vary significantly, depending on the
style of hook and who made the hook. Before you tie on that fly,
make sure to double check the actual length—you might be surprised to find out it is a size larger or smaller than you expected.
See the sidebar to learn more about hook size differences.
4. Bug Size Changed
n their book Selective Trout,t Swisher and Richards state, “The
fly fisherman who knows what is hatching and has realistic imitations will consistently be more effective than the angler relying on
trial and error methods.” Fly selection doesn’t have to be a guessing
game. Take a few minutes to capture a bug, measure its size, and
then choose the right size fly, and you will find yourself saying more
often, “There he is!”
Last week you were killing them on a size 16 Yellow Comparadun
fly pattern during the PMD hatch, but this week you’re getting
refusals. Remember that bug size changes over time and that just
Troy Pearse is the originator of the Bugometer scale, and an avid fly
fisherman who works in Boise, Idaho, as a software engineer.
MAY/JUNE 2011 I 43