There is really no such thing as a steelhead. Seriously. It is just a nickname we give rainbow trout that go to the ocean. The only reason they get big is there is more food out there but they are still a rainbow trout. If they stayed home, they'd be normal size.
If you took two pigs from the same litter and gave one an unlimited amount of food, it would be huge. If you fed the other one a starvation diet, it would be small. But in the end, they are both pigs. You could call the big one a (insert macho sounding name of choice), but it is still a pig. Same thing with rainbows and steelhead. They are identical. One goes searching for more grub out in the ocean and comes back fat. One stays home and doesn't get enough food to grow that big. No matter the size, they are rainbows. We just choose to call the ones that went to the ocean a steelhead. If they do not go, they can't really be called a steelhead. But no matter what we call them, they are still just plain old rainbow trout.