Oncorhynchus mykiss has two life-history forms rather than two species. Fish that remain in fresh water their whole lives are called rainbow trout; fish that migrate downstream to the ocean, feed there for one to three years, then return upstream to spawn are called steelhead. Steelhead typically grow larger and take on a more silvery colour at sea, reverting to a rosier stripe once they re-enter fresh water.
The two forms are not fixed by lineage. Studies of wild populations, including long-term work by US fisheries agencies, have shown that resident rainbow trout and migratory steelhead can occur in the same stream, interbreed, and each produce offspring of either form. Because of this shared gene pool, some legally distinct population segments of steelhead are protected under the US Endangered Species Act even though genetically identical resident rainbow trout in the same river are not.
What triggers an individual fish to migrate or stay is not fully settled. Researchers point to a mix of genetic predisposition, growth rate, and local environmental conditions such as water temperature and food supply, but no single factor reliably predicts which path any one fish will take.