Trout are members of the salmanoid family, all of which are distinctive because of their second dorsal fin - the adipose fin. In salmonoids this fin is almost non-existent and serves no real purpose and yet for some reason evolution has not erased it. Salmonoids have been around for 10 million years and are descendants of the teleosts, or bony fish, that go back 70 million years.
Although most folk who don't fish (and I know that can only be one or two of you) will say "rainbow" if asked to name a trout, our only indigenous trout is the brown trout, salmo trutta. These live in places where the air temperature does not go below zero degrees Fahrenheit or above 80.
The rainbow trout was introduced to this country some years ago - they grow fast and survive well - although they occur naturally only in the rivers of the west coast of America and from Alaska down to the Gulf of California. Rainbows used to be known as salmo gairdneri (salmo relates to European trout/salmon) but are now recognised as belonging to the Pacific salmon group of fish and hence have the hideously tricky name of oncorhynchus mykiss (oncorhynchus identifies Pacific fish).
Brown trout spawn in November and December and the egg hatches after 30 to 50 days into an alevin, an embryonic fish with a yolk sac attached from which it feeds. Alevin are quite naked, with no scales; they only start to grow them when they become fry. The scales grow from the outside and these are called "growth rings". They can tell you lot about a fish - when it ate loads and grew, when it was spawning etc.
When this food source dries up the fish wriggles its way out of the gravel in which it was born and rests on the bottom of the river to eat any suitable morsels of food. At this stage in its life it looks like a tiny trout and becomes a fry (which instantly - I know it shouldn't - makes me think of tiny, delicious, fried fish). Frys then turn into parr - the teenage stage of a fish's life - and they remain so for up to three years.
Trout will either stay in the fresh water where they were born or the more rebellious of them will go to sea. At the adolescent stage of their lives, just as in humans, there is no way of telling which way they will turn out. (A brown trout that goes to sea will become a sea trout, whereas a migratory rainbow is called a steelhead, although the sea-going urge doesn't generally happen to rainbows outside their native waters.) When they become fully fledged adults they will lose their parr markings and either start to take on the colours of the freshwater fish, or turn silver and move downstream. If this happens they will have become smolts - fish that have decided they want to go to sea.
Trout have 180-degree vision with a crossover of 30 degrees at the front, as this is where they most often attack. But on the riverbank, the lower down an object is the more difficult it is for them to see it. Hence the need for skulking along banks and learning to cast sideways - which is a good idea for particularly shy fish.
Just like us, a trout has rods and cones - light-sensitive bits of the retina - although a trout has different levels and configurations of them. Rods are very sensitive to light and cones help the trout distinguish colours - they are particularly partial to reds, least partial to blue.
So remember this when you're casting away for your first trout of the year! Also remember that although the trout's tail isn't great for sustained speed it can, momentarily, exert a force six times that of gravity. And it will do just that when you first strike - which is why you need to keep that rod tip up to absorb the pressure. Good hunting.