I work at a hatchery and yes fish preffer cold water, the warm water in the rivers kills them slowly but their spawning so they will die anyways, river fish who spend their whole live in the rivers, like trout, live shorter lives, and ocean fish survive by swimming deeper, its colder down there, I live in a valley and we have a lake up the mountains and its filled with trout and salmon fry, summer chinook use it for spawning every year.