While most avid fly fishermen are aware that there is a difference between Steelhead and Rainbow Trout, not all fly fishermen are able to define the difference between the two beyond stating their behavioral differences and noting their distinct difference in size. Thus, while Rainbow Trout are always thought of as freshwater species, Steelhead are generally thought of as a saltwater species that only migrate to freshwater to spawn. However, while this is an accurate description of their behavior, it does not really do much to define the differences between the two.
Thus, perhaps the best way to describe the difference between a Steelhead and a Rainbow Trout is to say that they are both sub-species of Oncorhynchus mykiss which indicates that while they are both members of the same genus and species, they each also display distinct anatomical, physiological, and/or behavioral differences that enable them to be classified as different sub-species. But, while that is the answer that a biologist would give you when asked what the difference is between a Steelhead and Rainbow Trout, the way that most fly fishermen distinguish them is that, although both sub-species are born in freshwater, Rainbow Trout remain in freshwater throughout their lives whereas, Steelhead only spend their first several months of their lives maturing in freshwater.
Then, when they reach a particular stage of growth, their genes trigger a physical metamorphosis in the cells of their body that both causes them to migrate downstream where they enter the sea, and which also enables them to survive in their new saltwater environment which a Rainbow Trout simply cannot do. Then, once again at a particular stage in their growth cycle, their genes trigger the second metamorphosis in the cells of their body that causes them to return to the particular stream where they were born and then travel upstream to the very place where they were born where they then spawn; thus creating the next generation.
Furthermore, there is the often a very distinct difference in size between the two sub-species. Thus, while it is theoretically possible that a Rainbow Trout living in a particularly rich freshwater environment could reach the average size of Steelhead Trout since a fish never stops growing, the fact is that the saltwater environment enables Steelhead to grow much faster than most Rainbow Trout and thus, most Steelhead Trout are distinctly larger than most Rainbow Trout. In fact, most fly fishermen would consider a 21 or 22 inch Rainbow Trout to be a magnificent trophy whereas a Steelhead Trout would not achieve the same status unless it was at least 36 inches since the present world record Steelhead measures 44 inches and weighs 29 ½ lbs.
So, as you can see, there are actually several different ways to define the difference between a Steelhead and Rainbow Trout raging from the biologist’s perspective to the fly fisherman’s perspective and each of them is correct in at least one aspect. However, now that you understand both points of view, the next time someone asks you what the difference between the two sub-species is, you can choose the answer or answers that you like best from among the ones presented above and be able to explain them precisely what the difference is.
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