THE EYE  

Like the human eye, the trout's retina contains nerve endings (rods) that are extremely sensitive to changes and low levels of light. Thus, the fish is extremely good at detecting movement over its' whole 180 degree field of vision. In practice, when close to a fish, avoid movement whenever possible.
 
The other nerve endings in the retina, the cones, distinguish color. The trout's retina is most sensitive to red and orange and least sensitive to blues. At least in practice, flies, particularly wet flies, will be most visible to the trout when the color red it is used.

The lens in many fish offers a 180-degree field of vision. Objects from infinity to two feet can be in focus at the same time because the short focal length of the lens gives a great depth of field. In practice, a downstream cast is less likely to be noticed if the trout is feeding close to the surface.
 

SIGHT

The water that fish live in has its own peculiar optical qualities. This, and the fact that fish have specially adapted eyes give it a view of the world very different from ours. Fish hunt by sight. Due to the location of the eyes the fish has the ability to see an object with both eyes, limited however, to a narrow field in front (binocular vision). Consequently, the fly fisher must try to place his or her fly within this narrow range to have the best chance of receiving a strike.

THE PERCEPTION

Fish see the outside world through a large round window with a diameter a little over twice its' depth in the water. It is sometimes thought that a trout deeper in the water has a larger window and thus can see most of the outside world above. This is misleading.

GREATER DEPTHS = LARGER WINDOW

As a fish doubles its depth, it doubles the window diameter, but it is twice as far from the window. The whole window becomes dimmer as the particles in the water absorb more light. In other words the fish has the same problem seeing out of deep water as you have to seeing into deep water. When the surface is rippled, detection by the fish is more difficult, but not impossible. In practice, keep low and avoid fishing directly over flat, calm clear water.

KEEP LOW AND AVOID DETECTION

 
HEARING

Occurs when the fish detects the vibrations in two different ways. The lateral lines that run down on both sides of the body and across the head of the fish are sensitive to low frequency vibration from heavy footfall or a swimming fish. In practice, avoid heavy movement and footfall particularly when wading. Scraping studs and loose stones scare fish. Also remember that sound travels much better in water than in air.

High-frequency vibrations are picked up by ears deep within the fish. However, they are not sensitive to airborne sounds. In practice, chat as much as you like.

SMELL  

is provided by a pair of pits in the nasal area connected not to the mouth, but to the outer skin. Some bottom dwellers also have smelling organs in their tails. Detecting food at a distance and probably enemies or companions, is likely the main purpose of smell for fish.

TASTE BUDS  

Exist on the tongues of fish, however, similar organs also exist on the barbells and bellies of some fish. We know that most fish can distinguish between sweet, salty, sour and bitter taste. However, substances in water quickly become dissolved making it difficult to use the sensation of both smell and taste