If you've ever spooked a trout while working a wild stream, you were probably seen. Trout are vigilant, and they can detect danger from almost any direction.
Water makes perfect vision difficult because it distorts the path of light; objects are never exactly where they appear. Add a little silt and sediment, and the problem magnifies. Under the best of circumstances, keen underwater vision for fish is limited to short distances.
Experienced anglers appreciate trout vision. They recognize that while brighter conditions may make seeing underwater easier, they also realize that well-lit waters make fish more vulnerable to predators from both above the water and within it. This is one reason many fish, including trout, feed most actively during the lower light levels of early and late in the day.
Flowing water presents additional problems. Even clean streams carry many tiny suspended particles that scatter or absorb light. These particles create a veil of diffuse light that makes vision even more difficult for trout.
Furthermore, ripples on stream surfaces caused by wind or riffles act as liquid lenses that focus sunlight at various depths below the surface.
Fortunately, trout vision counteracts these problems. They see colors well, especially reds, which are important during spawning periods. And trout have an exceptional field of vision. Their eyes are positioned on the side of the body giving them an almost 180 degree field of view of each side of the body.
Fish can also see through the surface of the water into the air above. But because light scatters at the surface, the field of view above the water is only 97 degrees. Within this field, trout can see aerial predators such as kingfishers and human anglers.
This is why trout are so wary when approached by inexperienced anglers. When they see movement overhead, they swim to cover.
On the other hand, trout must stay near a source of shady cover, especially when feeding on drifting prey. This limits the area they can inhabit and their total foraging time. The constraints of vision require trout to strike a balance between finding and eating prey and becoming prey themselves.
Successful anglers remember that trout, especially those in small streams, are always watching. The best approach is a stealthy one.
Biologist, author and broadcaster Scott Shalaway can be heard 9-11 a.m. Saturdays on 1370 WVLY-AM (Wheeling) and noon-2 p.m. Sundays on 1360 WMNY-AM (Pittsburgh). He can be reached at http://scottshalaway.googlepages.com and 2222 Fish Ridge Road, Cameron, WV 26033.