I think the act of going out in to the larger bodies of water (for rainbows) is called smolting. The juvenile fish adapt to salt water if they move in to the ocean. The other thing that occurs as they smolt and while they are in the lake or ocean is they loose the rainbow colors and become silver on the sides and steel gray on the top and on the head. That is why the rainbow variety is called steelhead. I've been studying up for my first steelhead trip this fall or winter here in Indiana or over in Ohio. So I'm no expert.