The Steelhead (Oncorhynchus mykiss) is the anadromous form of the Rainbow Trout. Unlike their freshwater counterparts, Steelhead migrate to the ocean and return to freshwater to spawn. ... The Steelhead (Oncorhynchus mykiss) is the anadromous form of the Rainbow Trout. Unlike their freshwater counterparts, Steelhead migrate to the ocean and return to freshwater to spawn. They can grow larger than pure freshwater Rainbow Trout, often exceeding 40 inches and 15 pounds. Steelhead are known for their incredible stamina and strength, making them highly sought after by sport fishermen for their challenging fights and acrobatic jumps.Click here to read more about Steelhead.