Understanding how a Trout sees it’s food on the surface can help increase catch rates when casting to fish. Although this area is quite complicated, an understanding of the basics will certainly help.
We refer to the area of surface water the trout can see through as it’s “window”. At a very basic level, the size of this window is determined by how close the fish is to the surface of the water, the higher up in the water the smaller the window, so the less area of surface it can see. Outside of this window all the trout sees is a reflection of the bottom of the river, we refer to this as the “mirror”.
To make sense of this, next time you are at the side of the lake or river, look in to the water in front of you, you will see the gravel and weed in front of you but if you look further out you will see just a reflection of the sky, clouds and trees. For the Trout this is the other way round, he will see the sky, clouds and trees through his window but if he looks further a field he will see reflections of the gravel and weed.
In The Trout W.E Frost and M. E. Brown describe this better than I can – ” The fish lives, as it were, in a room with a ceiling made of mirrors except for a round skylight in the middle, through which the outside world is visible though distorted round the edge”
The Trouts eye is its prime source of information of the world around it whether it be a fly drifting into it’s window or the silhouette of an angler against the skyline!