Fly Fishing Devon: Instruction & Guiding on Dartmoor Rivers

Quote from John Gierach "Fly Fishing Small Streams"

"I think stealth , and its various manifestations rates it own chapter. It's an underrated skill in fly fishing that's often listed after casting, entomology, wading, and even fly tying in order of importance, but I could introduce you to several fly fishers around here who can't cast worth a damn, don't know a mayfly from a barn owl, and wade like buffaloes, but who still catch lots of trout because they know how to sneak up on them."(John Gierach "Fly Fishing Small Streams")

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


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