Peripheral Vision
The reason for placement of the eyes at the sides of the head in fish is to increase the animal’s peripheral vision—that area seen only by one eye. The fish’s peripheral vision field is 330 degrees; ours is 179 degrees. This means that fish see objects behind them, which is very good for a prey animal. On paper, a drawing like the one below would suggest that there’s a blind spot behind the fish of 30 degrees and a tiny one in front of the fish’s nose where it cannot see with either binocular or peripheral vision. As we shall see, such is not really the case—but in a later post. Today, I want to focus (pun) on the fish’s peripheral vision.
This portion of vision is highly sensitive to contrast and motion; in other words, it’s the predator detection zone that allows the fish to notice critters trying to get up on its back side and grab it. When the fish is looking at the currents trying to find food and something suddenly enters the peripheral zone to the side or behind , the fish takes off for cover—at least the ones that survive to pass their genes on head for cover. The rest of them go down the gullet of a bird, mink, otter, etc. This has great import for the fly fisher, who for some reason, insists on tossing that fluorescent line and shiny leader high in the air so that all fish can have an equal chance of seeing it—perhaps they want to give the fish a sporting chance.
The fish's peripheral zone is 330 degrees, it's binocular zone is 30 degrees. Peripheral vision is especially enhanced to detect contrast and motion--it's the predator detection zone. Artwork by Jason Borger