How Do Trout See?

How do trout see?
How a trout sees can impact your fly fishing success:

Their vision is uniquely adapted to locate food and detect predators from above, both things that anglers must consider.
After seeing my fair share of trout scatter while trying to approach them I thought it may be a good idea to do a little homework on how a trout’s vision can impact an eager fly fisherman. We know they can see tiny bugs in an on the water. We also know certain colors in fly patterns seem to attract more attention. But how do they actually see their world? Trout are primarily visual feeders. They also rely on their eyes to detect predators. Therefore, beating their eyes for a stealthy approach will put more trout in the net.

Trout actually see a reflection of the riverbed when they peek upwards from down below the water surface.  At the same time the trout can also see a round hole, like an expanding cone directly above his eyes.  This phenomenon is caused by a law in physics referred to as Snell’s Law. This law states that any light waves striking the surface of water at an angle greater than 45 degrees will enter the water and, that any light waves striking the surface at less than 45 degrees will be reflected.

Unless you are in perfectly still water the image the fish would see would be distorted by current and waves.   This is similar to us trying to look at the river bottom and seeing wavy, changing images of things.

“Cone of Vision”

The size of the hole through which a trout sees the surface above will depend on the depth the fish is swimming.  Generally, it is accepted that two and a quarter times the depth at which the fish is swimming is about the size of their viewing area.  This would mean that a trout at a two foot depth would have a four and a half foot diameter hole through which to see out of the water.

Because the shape of this hole extends at a 45 degree angle from either side of a trout’s eye, trout have a 90 degree area that looks like a triangle from the side and a circle from above. This is vertical vision but once the edge of this angle reaches the surface of the water it lowers to 15 degrees.  What this means for the angler is that trout have 150 degrees of eye site.

This is often referred to as the Trout’s “cone of vision”.  Simply put in this case a fly fisherman would have a 15 degree angle from the water surface to be completely hidden.  Since this is an angle, it widens as you get closer to the apex at the water’s surface. It also narrows the farther you get to the apex at the water’s surface.  Taking this into account and establishing an average height fly fisherman is 6 foot if he is wading in 4 feet of water he could get within 15 feet of a trout without spooking it.  If you require a closer presentation you must crawl or hide behind something.

Trout have a 330 degree horizontal vision beneath the water.  This leaves a 30 degree blind spot directly behind them.  If possible, approach the trout from behind so you will not get noticed so easily.

Final Thoughts:

Sometimes when guiding here in the Vail Valley I will notice trout spooking when they possibly cannot see me, what then?  They do have ears but most likely can only here sounds you would not make when fly fishing.  Trout, like most fish have what is called a “Lateral Line.” This consists of a bunch of highly sensitive nerves. These nerves enable fish to feel pressure differences in the water.  So getting seen by the fish is not the only way they will spook. You still need to be stealthy in approach and presentation.

Bill Perry
Guide and Content Writer