What Do Fish See?

By Greg LaBonte

I’m currently teaching a 300 level Ichthyology course at a college in Maine and while reviewing our textbook, I stumbled upon the section covering fish vision. This inspired me to write about my knowledge regarding exactly what a brook trout can and cannot see. Depending on what kind of fish you are, you see the world differently, but for this article, we will focus strictly on the brook trout (Salvelinus  fontinalis). 


Before we dive into the trout, you must have a basic understanding of how vision works. Visible light to humans, a part of the electromagnetic spectrum, bounces around in the form of a wave. The largest visible waves in the spectrum at 700nm, appear red to humans, whereas the smallest waves (400 nm) appear blue.

Various wavelengths of light penetrate the eye and are focused onto the retina. On our retinas, we have photoreceptors called rods and cones, just as brook trout do. Rods register light in black and white, whereas our cones register color (or different wavelengths of light). 

Humans have 3 different cones, red, green, and blue. Peak absorption for humans occurs at 565nm for red cones, 535nm for green cones, and 440nm for blue cones.What is peak absorption? Peak absorption is where the cone is most sensitive. Our blue cones can actually see light between 400-550nm, which is violet, blue, green, and a little yellow. So, the names of the cones are kind of a misnomer, as there is tremendous overlap in all three. However, all three cones come together to give us our vision, our perception of the world. Trout are no different, they use rods and cones to perceive various wavelengths of light!  

Brook trout though, they 1-up us. They have four color receptors that pick up light wavelengths, peaking at 600 nm (red), 530 nm (green), 440 nm (blue) and 355 nm (ultra-violet). Ah, there it is… yes brook trout can see ultra-violet light, sometimes…. So, let’s compare:

Red cones:

Ours are peak at 565nm, brook trout peak at 600 nm. What does this mean? This means brook trout can see deeper reds, more shades of red. Where we see a dark red color, trout are seeing a much more definite and vibrant color of red. This allows trout to see much better in conditions of low light, where light entering our planet hits our atmosphere and creates a lot of reddish wavelengths. We know it as the colors of a sunset or sunrise. The beautiful oranges and reds that dance across our sky. Visually appealing to us, visually bright to a brook trout. As long as brookies are within the top 3-5’ of the water column, as red light can’t penetrate much deeper than 5’. Red light is a welcomed sight for a brook trout.  Brook trout’s ability to see red light, could this be a possible explanation for their affinity for feeding during early morning and late in the evening? Perhaps.


Green and Blue Cones:

There isn’t much to say here, brook trout green and blue cones and our green and blue cones are virtually identical. What you see is what they see here, kind of cool to think about.


UV Cones:

Okay, so yes, brook trout can see UV, however, it’s been shown that trout lose the ability to see UV light after they are older than 2. Some studies have shown that the cones associated with UV can simply be dormant and regain use if certain thyroidal hormones are triggered (Browman and Hawryshyn 1993). Thyroidal hormones are commonly active during spawning, which is why some scientists believe trout gearing up to spawn regain the ability to see UV light. A possible and logical explanation for why bright colored streamers (often having UV signatures) create reactionary strikes. Why do Montreal whores, which have tremendous UV signatures, not have an all-black variant? Perhaps, it is the UV. We can’t even imagine what this looks like, we can’t see UV, we can’t really imagine it, but we can take advantage of it in our flies.


Now, who is to say that a 3 year old brook trout hasn’t retained their ability to see UV? Natural variation does exist, gene regulation is a thing. Over-active thyroids are also a thing. The fact that brook trout have the ability to see UV at some points in their lives encourages me enough to add UV hotspots in some of my flies. Perhaps I run into a 6 year old brook trout with an overactive thyroid gland. What’s the harm in adding UV hotspots? Some aquatic invertebrates even have UV signatures and what more are anglers than mere con-artists.


What really strikes me as fascinating is the water in which trout live in. Outside of UV, trout vision is fairly similar to ours, however their environment is just wacky. We always have crystal clear conditions, where wavelengths travel vast distances. We can see a red light from hundreds of yards away. The water messes with light.  Water quickly disperses blue and UV light. The shorter the wavelength the quicker water disperses it. This dispersal is what makes water appear blue, it’s what makes our sky appear blue too. Although dispersed, blue light will penetrate water the deepest. Blue things will appear blue for a long way down, like 100’ deep. Perhaps this is why brook trout can discern more shades of blue compared to any other color in the spectrum. So, in crystal clear water, your blue streamer will appear blue to a brook trout regardless of where they see it. On the other end of the spectrum, red light barely penetrates water. At around 10’, a red object becomes black to a fish, just a silhouette. If fishing in a deeper pond, a brook trout who notices your red streamer from 20’ away, will be quite surprised as it approaches. Your streamer will morph from black to red in the swipe of a fin or whatever is the fish comparison to blinking an eye. In rivers, mountainous streams, and most streams where Brook Trout reside in Maine, the water depth doesn’t exceed 5 feet. 50% of red light remains 3’ down in clear water. So, to a fish living 3’ down, your red fly appears quite dull.


Let’s get technical, skip to the next paragraph if you don’t want to get technical. Red copper johns are quite common. A red copper john appears quite different to a fish at a depth of 5’ than a fish at a depth of 2’. What does it look like at a depth of 5’? Well, it looks basically black or an extremely dull red. So, you think you’re fishing a red copper john, but to the fish, you’re fishing a black copper john. Should you have copper johns in black and red, the black ones you fish at deeper depths, the red ones you fish at shallower depths? Does it matter because they appear visually the same basically? This theoretical concept really only applies to the color red, so don’t worry about your other colored nymphs! Okay, technicality over.


Most of these scenarios are under the conditions of crystal-clear water, which is almost never the case. Turbidity, water impurities, tannins all impact the ability of trout vision. Water turbidity and any color impurities will negatively impact a trout’s ability to discern color. One color that is visible usually no matter what the conditions, is white. Reflecting the entire color spectrum, brook trout can usually see white even at distances of 15-20’. The flash of tinsel, on your fly usually reflects “white light” or the entire visual spectrum. Thus, a flashy fly is likely to be seen by a brook trout at greater distances compared to flies that have colors closer to the red end of the spectrum. That doesn’t mean they’ll eat it, your flies still suck. Nevertheless, it is probably a good idea to try flashy flies in turbid water.


Of course, this all depends on where the fish are looking. A fish looking up in a stream at a dry fly can’t see any color at all. Silhouette, surface tension patterns, and posture are much more important. In fact, the only color a trout can see when looking up is the lack of color, black. All your dry flies could be black, because they just cast a silhouette, color really doesn’t matter for dry flies.


All in all, trout have some pretty incredible eyes that morph throughout the day, the year, and their lifetime. Brook trout can see all that we can and then some. Deeper reds, UV, and probably more precise shades of blue. Visionary adaptations are all in an effort to increase brook trout fitness, that Is ecological fitness, which is the ability to compete and pass on your genes successfully. I wonder what variation exists in populations regarding vision, are there trout who have diminished vision or expanded vision? Most likely, after all, evolution can’t occur after the disaster, it’s already occurred awaiting disaster.

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