If you want to catch trout with a fly rod, you’ll have to be sneaky. Trout possess incredibly acute senses that keep them safe from predators such as herons, kingfishers and minks. These senses also prevent them from being caught by fly fishers like you.

If a trout detects you, it will usually bolt for cover and stop feeding, making it uncatchable. In fly fishing, your mission is to drop flies in front of fish that are blissfully unaware of your presence. This article shows you the range of “trout radar” and points out some ways to sneak past it.

Vision

Vision is the trout’s primary sense for hunting and avoiding predators. The trout sees in a much different way than we do, partly because it has evolved special adaptations to its environment and partly because water has unique impacts on optics.

Eye Position

A prey species needs a large range of vision to avoid becoming dinner for a predator. A predator species needs binocular vision (depth perception) in order to hunt successfully. Since a trout is both a predator and a prey it has a combination of both the aforementioned visual adaptations.

The trout has eyes on the sides of its head, which give it a large field of monocular vision (approximately 160 degrees) on either side of its body. In addition, the trout has a field of overlapping binocular vision of about 30 degrees in front of its nose. This binocular “zone” is critical for locating and gobbling the small insects that make up the bulk of a trout’s diet.

With the combination of binocular and monocular vision has come a compromise. Trout have a blind spot of 30 degrees directly behind them. This gap in the trout’s vision is critical for fly fishers. You can sneak up surprisingly close to a trout if you approach them in their blind spot.

In case you were wondering how it’s possible to sneak up on a trout, keep this in mind: Trout are lazy and swim around very little. They pick a good spot in the river and sit there relatively motionless all day, waiting for tasty insects to drift by over their heads. They almost always face directly upstream. So if you approach a fish from downstream, you are likely sneaking up in its blind spot.

You can also approach a trout from the front or side. You will, however, have to stay a good deal further away to avoid detection. This is explained below.

The Retina

The trout’s retina has rods and cones, just like the retina of humans. In fact, trout have even more rods than humans do. This means that their eyes are extremely sensitive to movement. Trout can detect motion even when light levels are very low.

Just how sensitive is a trout’s eye? Biologists speculate that the moon and stars can provide enough light for a trout to feed successfully at night.

Trout also have cones in their retinas. Cones enable them to distinguish color. For the angler, this means that color is an important consideration when choosing flies. If the trout are feeding on little yellow stoneflies, you’ll probably have better luck with a yellow fly than you will with a brown fly, for example.

The Lens

The lens of a trout’s eye gives it a 180 degree field of vision. The lens has a short focal length that provides a huge depth of field. In fact, objects from 2 feet to infinity can be in focus at the same time.

However, when a trout is focusing on objects closer than 2 feet away, like when feeding on particularly miniscule flies, objects beyond 2 feet will become blurry. For the fly fisher, this means that you can sometimes approach a trout feeding near the surface more closely than normal.

Vision Underwater

Water and particles suspended in water are quite effective at diffusing light. At best, a trout can probably see about 20 feet underwater. Your legs in the water are not your main concern when stalking trout. It’s the part of you that sticks up out of the water that the trout will most likely notice first.

Vision Above Water

Understanding how a trout sees the world above the water is imperative if you want to catch fish.

When a trout looks up through the water it sees the outside world through a round window. This window encompasses much of the world above the water, except near the edges of the window where things start getting fuzzy.

The trout’s window is actually a cone that extends above its head at an angle of 97 degrees. It’s important for you to stay low while fly fishing. You’ll want to stay in the magic 10-degree angle beneath the trout’s window.

Keep in mind that the concept of the window is based on the assumption that the trout is in glass-smooth water. Riffles and rapids that break up the water’s surface fragment this window. You can get much closer to trout in “broken” water than in the smooth stuff.

As you’ll notice from the diagram, a trout’s window expands and contracts depending on its depth in the water.

The Advantage of Wading

Wading gives you an advantage when stalking trout, because you are automatically decreasing how far you stick out above the water. When fishing from the bank it’s important to kneel or crouch to stay out of the trout’s view.

Beware of Shadows

Few things will spook a trout faster than a shadow moving across the water. When making your approach, pay attention to the position of the sun and where your shadow falls.

Beware of Your Rod and Line

Many anglers inadvertently spook fish with their rod and line. Here are a few tips to avoid this problem:

• Cast sidearm. Your fishing rod will not extend as far into the trout’s window if you hold it parallel to the water.
• Keep false casting to an absolute minimum. False casting is the term for casts that stay in the air and never land on the water. The purposes of false casting are usually to measure out line or dry off a fly. These may be useful functions, but keep in mind, the longer your fly is in the air, the greater the likelihood of spooking trout.
• “Lining” a trout is a common mistake made by beginners. Lining means that you’ve cast a bit too far and your fly line plops on the water within the cone of a trout’s vision. This spooks trout instantly. The only thing that should enter the trout’s cone of vision is the “invisible” monofilament leader and scrumptious-looking fly.

Hearing

Trout hear through something called their “lateral line.” The lateral line enables them to pick up vibrations in the water over very long distances. If you want to catch trout, avoid making vibrations. Walk quietly in and around the river.

Keep in mind that slapping your line on the water makes lots of noise, and it’s a guaranteed way to frighten fish.

Taste and Smell

Trout and their seagoing relatives, salmon, have a keen sense of smell. In fact, this is the sense that guides a salmon thousands of miles from the deep ocean back to the exact stream where it was born. Biologists believe that salmon can in fact “smell” the chemical signature of their natal stream.

Trout employ their sense of smell to feed in some instances, like in muddy water where vision is limited. Luckily for fly fishers, however, trout will feed entirely by sight. This fact allows trout to be caught on relatively odorless flies crafted from fur and feathers.

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