What Trout See

Trout are visual predators that use definitive cues to decide what to eat and what not to eat, including our flies.

[by Jason Randall]

THE FIRST TIME I DONNED SCUBA GEAR AND JUMPED IN A RIVER, I was amazed at the maelstrom of debris swirling in the water. With all the bits of leaves, clumps of algae, and junk stirred up from the bottom, it’s a wonder trout can tell what to eat from what’s inedible. Feeding in the current must be like peeking around a tree to snatch a cheeseburger from a passing tornado.

But trout have adapted to life in current, and their sense of vision has evolved to work within their underwater world. A fish’s eye, the quality of light that reaches it, and its brain, where the information is processed, determine what it sees. A trout’s sense of vision affects every aspect of angling—from fly design and presentation to the way we approach the water to how we should handle fish after they’re caught.

The Trout Eye

One difference between a trout and human eye is the shape; trout have an elliptically shaped orbit as opposed to ours, which is spherical. The elliptical shape allows for two simultaneous focal lengths, one to the front and one to the side.

A sharp, forward-looking field of vision is useful for feeding right in front of a fish’s nose. A notch in the forward edge of a trout’s pupil allows it to better see across the bridge of its nose. At the same time, the second focal length keeps the lateral, far field of vision in perfect focus, which helps with things like the detection of predators. That would be like having your peripheral vision in perfect focus at all times. You might say trout are nearsighted to the front and farsighted to the side.

The notch in the forward aspect of the pupil allows the trout to see over the ‘bridge of his nose’ to inspect bits of food. (photo by Tim Romano)

This sort of split vision makes it pretty hard to sneak up on a trout without getting noticed, so the best way to avoid spooking fish is to make your approach unthreatening. Since trout have a natural fear of aerial predators like eagles, herons, and ospreys, they don’t like objects or shadows overhead. Underwater, I’ve gotten very close to trout without spooking them, but it’s our “abovewater profile” that triggers alarm. The key to approaching trout successfully is minimizing your abovewater profile. Members of Fly Fishing Team USA, our international competitive fly fishing team, have described crawling in the water to avoid spooking trout. While that might be a little beyond what my arthritic knees can handle, crouching helps tremendously because your profile is lower than a trout’s vision window. Be quiet, too; a fish’s sense of hearing is pretty acute to vibrations made by heavy footsteps on rock.

Trout lack eyelids to protect and moisten the eye, but because they live in the water, dry eyes are not an issue. But when an angler scoops a fish into a net, lifts it out of the water, and holds it to remove the hook or for photos, its eyes are susceptible to injury. Abrasions from the net or our hands can damage the cornea (the outermost part of their eye). The risk of eye injury increases if we lay a fish in the grass or potentially drop it in the bottom of the boat. When handling fish, be careful of their eyes and gills—air begins drying out a fish’s eye less than 30 seconds after it’s removed from water. Also you can turn trout away from the sun or hold them in your shadow while releasing—its pupils don’t constrict in bright light. That’s just one more reason I like to see water dripping from fish in photos: because it indicates that the fish was just lifted from the water. Trout have the same photoreceptors in their retina as humans: cones and rods. Rods are extremely sensitive to light and provide excellent nocturnal vision, while each of the three types of cones are “tuned” to color—red, green, or blue, to be specific.

This series of photos, taken by the author and his wife, illustrate a trout’s vision window. Photo #1 is a color board from the perspective of a trout if it were one foot below the water’s surface and six feet away. Photo #2 is at the same depth, but 12 feet away. Photo #3 is at the same depth, but 20 feet away. The author’s wife is easily visible in the first photo, compressed but visible in the second, and nearly invisible in the third. The lesson is a crouching angler is easy for a trout to see from 12 feet away and probably unrecognizable from over 20, but can still spook a fish with a sloppy cast or other visual abnormality.

Unlike humans, trout use only one type of receptor or the other most of the time, typically either the cones or the rods, although there’s a brief period of overlap at dawn and dusk, when fish use both receptors. The cones are used during the day for incredibly sharp, full-color vision, but the ultrasensitive rods are disengaged to protect them from bright daylight. At night, the cones disengage, and trout exclusively use the rods to see in the dark.

The downside (or upside if you’re an angler) is that trout cannot see color at night. Instead, fish rely on seeing contrast between objects. Think of it as “night vision for trout,” and when you’re night fishing, select flies that have a defined silhouette and definite contrast to the natural world. If it helps, photograph the flies you plan to fish at night and convert the image to black and white. Then compare the fly’s appearance to a black-and-white photo of the natural prey species you’re imitating. It should show you that shades of gray are more important than color.

The Great Debate

UV or not UV; that is the question. Do trout really see ultraviolet (UV) light? Is it important to their feeding activities? These are questions anglers and tiers alike have hotly debated for many years. And not without just cause, since until recently, conflicting research has fostered a lack of consensus even among the scientific community.

UV light is rays of electromagnetic radiation that’s just beyond the deepest shade of violet. The largest natural source comes from the sun, and it’s what likely caused that sunburn you got from a long day on the water last summer. Unfortunately, ultraviolet light is largely invisible to humans, so we lack an appreciation for what it actually looks like.

The ovoid shape of the trout eye allows for two simultaneous focal lengths—one to the front for near vision and to the side for far fi eld vision. Both are in clear focus at the same allowing trout to eat with discrimination and remain on the lookout for predators or anglers. (Thomas Barnett/Courtesy of Stackpole books)

The latest research indicates that while all trout have UV-light-sensitive cones up until roughly the parr stage, which is when a fish is just a few inches long, trout beyond that stage (juvenile and adult trout) no longer have UV-dedicated cones. By then, the UV-sensitive cones have switched from detecting ultraviolet light to a wavelength of blue light in the visual spectrum. The bottom line is that by the time a trout is old enough to have any interest in a fly, it has lost its UVlight-specifi c cones and almost all its ultraviolet-light sensitivity.

When I was working on my book, Trout Sense, Dr. Iñigo Novales Flamarique, one of the world’s leading researchers on trout vision, told me, “As for the use of ultraviolet vision in young salmonids (with UV cones), the UV cones enhance the contrast of zooplankton prey and improve the fish’s foraging performance. Nothing is known about the function of UV vision in salmonid fishes at later stages in nature, and it is doubtful that it serves any ecological purpose once the UV cones are gone.”

Does that mean that I’ve discarded all my UV-light tying material? Not at all! I love the stuff; even discounting its UV-light characteristics, it’s as iridescent as a drake mallard’s head, which can appear black, blue, or green, depending on the angle of light. The wings and other features of insects have a natural “shimmer” that’s imitated by UV material.

Water Quality

Like an overly optimistic person’s figurative rose-colored glasses, a trout’s eye filters the light that reaches it, which is already strongly influenced by the water it has passed through. The color stain of the water filters the light that reaches a trout’s eye, eliminating or attenuating certain wavelengths of light while enhancing others and shifting the color spectrum trout perceive. For instance, in the blue water of the open ocean, red turns to muddy brown, while the water’s stain might enhance other colors—yellow stands out dramatically in the fresh water of many rivers and streams, whereas green-stained water favors shades of green.