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?
Snell’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.
The image below is of an angler making a back cast from land using a 9ft rod. The shaded gray area represents the trout's blind spot.
We will use this as a ”baseline” when discussing strategies on how to stay hidden from trout.
It's clear that when an angler is casting from land at 20 ft away, the entire upper half of their body is visible. When standing at 30 ft, their entire head is still visible.
So what strategies could be used to improve the angler's odds of staying hidden from trout?
Strategy #1 for staying hidden in the trout's blind spot: When possible, cast from standing in the water instead of from the bank.
As mentioned earlier, trout can see fairly well below the water’s surface, however, they don't seem to feel threatened by many things underwater. In a trout’s mind, they're much more likely to be attacked from above than from below. Bears, birds, and fisherman all come from above. Therefore, a wader’s legs below the surface seem to cause little if any concern to them.
Below is an image of an angler casting while standing in 2 ft of water.
At 20 ft away, only the top half of the angler's head is visible, and at 30 ft, their entire body is in the blind spot.
By wading in 2ft of water, the angler has now been able to reduce their overall height above the surface by 2ft. This allows for roughly 1/3 more of their body to be hidden in the blind spot (a higher percentage if they are shorter than 6ft).
Strategy #2 for staying hidden in the trout's blind spot: Use a side arm cast anytime you can to keep your rod tip low.
Both of the examples mentioned earlier show that even when standing 30ft away from a trout, a 9ft fly rod is still visible with a vertical back cast. Movement of a rod tip above the 10° line sticks out like a hair on a peanut butter sandwich!
Let’s see what it would look like if the angler was standing in 2 ft of water and used a side arm back cast:
Standing at 20 ft or beyond, the rod tip on the back cast is now completely in the blind spot and out of the trout's field of vision.
Strategy #3 for staying hidden in the trout's blind spot: Use the shortest rod you can.
What about the rod tip on the forward cast? That distance has to be factored in as well! Using a shorter rod makes it easier to avoid creating commotion above the blind spot during the forward cast.
*But even with the shorter rod, without a side arm cast, the rod tip is still visible when the angler is standing 30ft away from the fish!
Strategy #4 for staying hidden in the trout's blind spot: Position yourself with faster moving water between you and the trout.
All of the examples given so far assume that the trout is holding in fairly calm water with little if any surface disturbance between the angler and the fish. The image below depicts what the trout's window (or lack there of) looks like in faster moving water.
In faster water, it's nearly impossible for a trout to spot an angler. This also holds true if there is disturbance of the surface between the angler and the fish. Any time you can, use this disturbance to your advantage!
Bonus Data: Exactly how low do we have to be to stay the trout's blind spot?
It’s all about geometry. At each distance, there’s a height above the surface which matches the blind spot angle. For those who would like to know the height of a trout's blind spot a various distances, I've created the following table. It can be used to let you know how low you must be in order to hide from trout. Remember, this table should be used in relation to how close your ROD TIP is to the fish after you finish your forward cast, NOT from where you are standing.
Example: If: A trout is holding in 2ft of water and your rod tip is 25' from the trout at the end of your forward cast, you and the rod tip must stay lower than 4.05' in order to stay hidden in the blind spot.
Conclusion
Trout see the outside world through a small “window“ on the water’s surface. Any object above water located below 10 degrees of the horizon is too blurry for a trout to see. To stay hidden from their cautious eyes:
Cast from in the water
Make a low cast
Use a shorter rod
Stalk trout in broken water
Noses Up!
