The male steelhead is the one that looks like a rainbow, the female a bright silver. Steelhead are of the trout family, born in a stream and then head out into the ocean for adult hood only returning to spawn. The rainbow trout, of course, never leaves the stream! There is a term for this for which I'm not sure! Hope that answered your question!
Ever heard about placing a goldfish in a small bowl? How it won't grow bigger than it's habitat, so if you place it in a small bowl it will stay small, but if you put it in a large bowl it will grow bigger! Bigger enviornment, more food, larger food and different food make for big fish!
It's good to see a young inquisitive mind, exspecially in my line of work! Keep it up kid, and you'll outfish us all soon!