Steelhead are an anadromous strain of rainbow trout. That is, they spend most of there life in the ocean or in the Great Lakes and return to tributary streams to spawn.
Rainbow trout are the landlocked form, so to speak, as they do not have a tendency to migrate from one body of water to another. The Pennsylvania Fish and Boat Commission plants steelhead rather than rainbow trout in the Lake Erie drainage.
There isn’t an easy way to tell one from the other, because they are the same species,Oncorhynchus mykiss. If you fish tributaries to the Great Lakes you are catching steelhead.
Steelhead can be separated from similar-looking coho and chinook salmon by looking at the inside of the mouth. The mouth is completely white in the steelhead. In the salmons, the mouth has some gray or black. Steelhead and other deepwater, big-lake rainbows are more silvery than stream fish, with less of a side stripe.
Their genetic background traces back to ocean run (migratory) rainbow trout brought here from the Pacific Northwest. Our landlocked rainbows have also come from the West, but probably did not come from ocean running strains.
An even bigger debate among steelheaders is the genetic variety or strain of steelhead caught. Some fish come from stock from the Skamania River, maybe you’ve heard anglers say they catch these steelhead. Often they are more sleek, not as wide at the ‘shoulders’ but fight like mad.
Landlocked rainbows will migrate and take on some color characteristics you will see in steelhead, but don’t grow as large.