Updated: Oct 19
What is the Trout's Blind Spot?
Trout can see almost 360 degrees around themselves underwater, but are limited in what they can view above the surface. The blind spot is an area above the water's surface that trout cannot see clearly from underwater. Humans and other predators are able to hide in this blind spot without being detected by trout.
Staying in the trout’s blind spot can be tough. Have you ever been stalking a rising trout and after taking your time to get into position they just seem to stop feeding? Or maybe they don't stop, but instead start feeding either up or down stream from you? Or the worst feeling, you see them spook and head for the cover of deeper water? These are all signals you've been detected. So how do you stay hidden from trout?
In this article we will discuss:
What the trout's window is
The size of the window
What creates the blind spot
4 Strategies to stay hidden from trout
What is the Trout's Window?
In 1621, a biologist by the name Willebrod Snell van Royen made an interesting discovery which is now known as Snell’s Window.
Here's how it works:
Trout can see objects below the water’s surface fairly well. Maybe not what we would call 2020 vision, but well enough to distinguish the difference between a rock and a nymph (most of the time). However, when they look up, the entire bottom side of the surface is one big mirror except for a small circular window through which they see the outside world.
Size of the Trout's Window
Because trout aren’t stationary fish, the size of their window is ever changing. It has a direct correlation to their depth in the water. The diameter of the window is 2.26 times the depth of the fish. For an easy calculation, think of it this way: The fish’s window compared to its depth is roughly a 2:1 ratio. Thus, if a trout is holding in 2 feet of water, its window’s diameter is roughly 4 feet. This means the fish’s window extends 2 feet in front, 2 feet behind, and 2 feet to the left and right of it.
Notice the ratio of the fish's depth to the size of its window. The deeper the fish is holding, the larger the window is.
What Creates the Trout's Blind Spot?
Because trout are looking through water, a unique phenomenon is created known as refraction (I’m not going to go in depth about about refraction, ask a science professor if you want to hear the nerdy details).
But here's the important part for us as anglers:
While looking at the outside world through this window, refraction causes images to "bend" as they pass through the water and reach the trout's eyes. The greater amount of bending that occurs, the blurrier the object appears to the trout.
The closer an object is to the horizon, the more bent/blurry it appears to a trout. This means it’s very difficult for fish to clearly see the outline of objects close to the horizon, thus creating a hiding spot for anglers.
Therefore, in order to stay hidden in the trout's blind spot, we must keep ourselves and our fly rods low.
[To learn about the exact height of the blind spot for various trout holding depths, click here or see the table at the end of this article]
Strategies to stay hidden from Trout
In terms of stalking trout, it’s helpful to know how much blurriness is enough to hide us from their cautious eyes. Without actually asking a trout, we can’t be 100% certain, but most who’ve studied this closely agree that any object below the 10° horizon line is too blurry for a fish to distinguish its outline. So anything below this 10° line we will refer to as the trout's blind spot. The further away from a trout an object is, the taller/larger the blind spot is.