Stealth: The importance of (in)visibility to the trout fisher

Flyfishers understand that they should take care to make sure they are not seen by trout.

This page explores the question "What is a safe distance?".

If we apply a bit of maths - and some common sense - it turns out that a useful "rule of thumb" is:

Multiply by 6 your height above the water, and stand at least that distance away from trout you are targeting.

You can either stop reading now and bear this rule of thumb in mind the next time you go fishing. Or, read on if you are interested in the physics behind the rule. And learn about exploiting a trout's 'blind spot'.

Because of the laws of refraction, fish cannot see any part of an object which lies below an angle of 10 degreesto the water surface at the edge of their "window".

The red triangles in this diagram show this "blind spot" extending outwards all around the edge of the trout's window.

Diagram adapted from Fig. 6 in Professor L T Threadgold's book "Dry Flies: An Improved Method of Tying"

The head and upper body of the standingangler would be visible to a trout lying beside the clump of midstream vegetation on the right hand side of the photograph.

But if the angler crouches, only his head would be visible to a trout lying beside the clump of midstream vegetation on the right hand side of the photograph.

We can begin to answer our question: What is a safe distance?
Angler is:	Height of angler (in feet)	Distance angler invisible to trout (in feet).
Standing	6 feet	=>34 feet
Crouching	4 feet	=>23 feet

Things to bear in mind:

This doesn't mean you need to have 23 or 34 feet of fly lineoutside your rod tip, because:

1. the leader is probably 9 feet or longer
2. your rod is probably 7 feet or longer

Assuming your leader straightens out, and you hold the rod straight out in front of you with the tip at water level:

• if you are standing, you might get away with casting just 18 feet (34-(9+7)) of fly line
• if you are crouching, you might get away with casting just 7 feet (23-(9+7)) of fly line

But of course these assumptions are very unlikely (for example, your may want your leader to contain slack to avoid drag etc.) so you should cast a few extra feet of fly line to be safe.

In addition, the depth at which the trout is lying also has an important effect.

So far I have assumed that you are casting to a trout lying close to the surface: A trout lying 6 inches below the surface has a small 5 inch window (Shown on the far right of the diagram below).

But if the trout is 6 feet beneath the surface (far left in the diagram below) its window has a radius over 5 feet. So the radius needs to be added to the "rule of thumb" introduced at the top of this page, because the law of refraction operates from the edge of the trout's "window".

Or to put it another way, a deep-lying trout is closer to you (in optical terms) than you think

Exploiting a trout's blind spot

Sea trout are notoriously 'spooky' fish. But sea trout and brown trout have a 'blind spot' behind them that can - with great care - be exploited to get very close. This effect is shown in the video clip below

This Facebook post from Tony Andrews (CEO 2008 to 2016 at Atlantic Salmon Trust) captures what it is like to sit by a river containing sea trout at dusk. Tony makes an observation that I have never seen described before. He calls it “sub aqua heaving”. or ‘heaving of the water’. I see this as it gets dark, but never in daylight, and call it 'the water rocking'. I think it's caused when larger sea trout leave their daytime lies and move into shallower water. It may be particularly noticeable on smooth pools on relatively narrow rivers. He also describes how the "community of living things slip back into their normality after the rude intrusion of our arrival. This does happen in daylight and is an example of the benefits of stealth. It can help fish and angler to simply sit still for a few moments and "let nature come back to you". It will, and you'll see little and larger fish within a few feet and maybe a fish rising a short distance away.

Bibliography

Gierach, John (1989). Fly Fishing Small Streams. National Book Network.

Pritt, T.E. (1885). Yorkshire Trout Flies. Goodall and Suddick.Available online

About the author

In this video Bob Wyatt ties his Snowy Shoe Hare Emerger

Bob uses this material in place of CDC because he has found that CDC tends to be "a one fish fly" which is an absolute no-no for guides on local rivers.

These articles would not have been possible without the help and encouragement of Bob Wyatt. Bob is an artist, author, Certified Fly Casting Instructor and long-time angler. Born in Canada, he fished the freestone streams of southwestern Alberta in the late 1950s. He now lives on New Zealand's South Island. His articles have appeared in Fly Fishing & Fly Tying(UK), Gray's Sporting Journal, Fly Rod & Reel, and Flylife Magazine (AU). He has published two books: Trout Hunting: The Pursuit of Happiness (2004) and What Trout Want: The Educated Trout and Other Myths (2013). In this interview by April Vokey he discusses his “prey image” theory, trout fishing and the early days of steelhead fly fishing.