Complex sentence: The steelhead is a sea-run rainbow trout (anadromous) usually returning to freshwater to spawn after two to three years at sea; rainbow trout and steelhead trout are the same species.
Split into simpler sentences: The steelhead is a sea run rainbow trout (anadromous) usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species.