Sometimes it's easy to see trout, easy to image they see the bait the way the angler intends it to be seen.
Not so. For decades it's been known that trout eyes contain the same type of rod and cone structures that enable humans to distinguish among colors and shades of light. During periods of a trout's life, it sees a broader spectrum of color than humans can perceive, and new research suggests that some colors, unseen by older fish, can be used to attract young hatchery trout.
Understanding what trout actually see in and out of the water can give anglers a practical advantage.
"Trout eyes are somewhat similar to human eyes," said Mike Depew, a fisheries biologist for the state Fish and Boat Commission and fly angler. "But there are differences in what they see that you should take into consideration when choosing a bait or moving around near the water."
Human eyes have three types of color receptors. Specialized cones responding to light frequencies of 565 nanometers allow us to see reds. Other cones set for 535 let us see greens, and blues are detected at 440 nanometers.
Trout have four color receptors. They see the reds, greens and blues seen by humans but with some differences -- what an angler sees as a dark red lure is perceived as bright red by the trout. But with the fourth set of color receptors, trout can sometimes detect ultra-violet frequencies as low as 355 nanometers, below the spectrum visible to humans.
"Those ultra-violet cones are only active when the trout is young and during spawning runs. Science doesn't quite understand that," said Depew. "Maybe it helps them track really small prey when they're young, up to about 2 years old. It reappears when trout mature. It's unclear if that occurs [throughout the trout family], but it seems to be there in salmon, steelhead and possibly rainbow trout."
Pennsylvania stocked trout grow quickly, leaving the hatcheries when they're 1 year old. Depew said it's believed that stocked trout still possess the ultra-violet cones enabling them to see low color frequencies, while older hatchery-raised trophy trout, holdovers and pre-spawn native trout of the same size do not see those colors.
With that in mind, color selection can be crucial to anglers. Colors that seem unnaturally bright to native and older trout can spook them, both in and out of the water -- a big yellow lure, an orange hat. But the new research suggests that flecks of vivid colors in the fluorescent range can pique a young trout's interest.
"Small bits of fluorescent color don't imitate anything natural," said Depew, "but they can create a hot spot. A bit of fluorescent orange, yellow or pink absorbs UV rays and projects that back into the visible spectrum. It's not really visible to them on the surface or just under the water, but it gets more visible the deeper you get."
Sparkle materials in flies, and gold or silver spinner blades, can have a similar effect, sending glimmers of hot light through the water, often triggering an instinctive strike response among stocked trout.
With the optical rod structures, trout are believed to see degrees of light about the same as humans. A broad change in light at dusk and dawn, or even cloud cover, can trigger feeding. A a narrow change of brightness -- a shadow passing overhead -- can send a trout scurrying.
Trout have both monocular and binocular vision. With eyes on each side of its head it can look out to one side with a single eye, or focus both eyes on a single point above or in front. Optimal focus is at about 2 inches, which might explain why they mistake hooks for legs.
"Essentially, the fish can be facing forward in the current and still see slightly behind and to the side," said Depew. "They see through the surface of the water in a sort of inverted cone-shaped area, with the point of the cone at the fish's eyes. The deeper it is in the water, the broader the base of the cone and the more they can see above the surface."
Anglers can take advantage of a 30 degree blind spot behind the fish. But when a trout breaks its forward-facing position and turns slightly, it may be looking back at you.
Conditions change what trout can see. When cloudy water obscures vision, larger whiter presentations can help the fish to notice the bait. A choppy surface from wind or riffles refracts light, altering colors, brightness and even the lure's position. If possible, go deep beneath the chop to give the trout a better opportunity to size up the bait.
Some lures stimulate multiple trout senses. Sparkle materials in paste baits can trigger visual and smell responses. Flies and spinners of particular colors and shapes can cause vibrations in the water that pique a trout's visual and lateral-line senses, which detect vibration.
Dress to blend into the surroundings, and move slowly and quietly. Be aware of the position of the sun relative to you and the spot where you're fishing -- don't be conspicuous on the horizon. Avoid casting a shadow across the water -- sometimes even the passing of a fly line can spook fish.
And when you see a trout well enough that you're looking at its head and eyes, you're busted -- it's probably looking right back at you.
John Hayes: firstname.lastname@example.org.