Among anglers and fisheries biologists alike, few questions spark more debate than the relationship between rainbow trout and steelhead. When exactly does a trout become a steelhead? Are steelhead simply rainbow trout that migrate to the ocean? And within a single population of rainbow trout, why do some fish leave their natal streams while others remain residents for life?
These questions were explored in depth in an episode of the The Orvis Fly Fishing Podcast, hosted and released by Orvis. In the episode titled “Is It a Steelhead or a Rainbow?”, fisheries biologist Katie Kobayashi joins the conversation to discuss the science behind these fish. Beginning around the 32:13 mark, Kobayashi dives into the genetic discoveries that are helping scientists better understand one of the most fascinating life-history strategies in freshwater fish.
Traditionally, the difference between a rainbow trout and a steelhead has been explained in simple terms. Both are the same species—Oncorhynchus mykiss. The distinction lies in behavior: steelhead migrate, typically moving from freshwater streams to the ocean before returning to spawn, while rainbow trout remain in freshwater their entire lives.
But that explanation has always left an important question unanswered: what determines which path a fish will take?
Recent genetic research has provided a remarkable piece of the puzzle. Scientists have identified what many now refer to as the “steelhead gene”—a specific region of the genome that strongly predicts whether an individual rainbow trout is likely to adopt a migratory life history or remain a resident stream fish.
Rather than migration being purely random or entirely driven by environmental conditions, this discovery shows that genetics plays a major role in shaping a trout’s future. Fish carrying the migratory version of this genetic marker are far more likely to undergo the physiological and behavioral changes necessary to leave freshwater and travel long distances. Those without it tend to remain in their home rivers.
What makes this discovery particularly fascinating is that both life strategies can exist within the same population. In a single watershed, some fish may carry the migratory genetic signature while others do not. This means that a group of rainbow trout living in the same stream may produce offspring that follow entirely different paths—some staying in the river while others eventually migrate.
The existence of this genetic predictor also helps clarify long-standing debates about Great Lakes “steelhead.”Although these fish do not migrate to the ocean, they exhibit the same migratory tendencies and physiological adaptations as their Pacific counterparts. In many cases, they also carry the same genetic traits associated with migration.
In other words, the genetic architecture that drives the steelhead life history appears to function whether the destination is saltwater or the vast freshwater environment of the Great Lakes.
The identification of this “steelhead gene” represents an important step forward in understanding how trout populations function and adapt. It reveals that the distinction between a rainbow trout and a steelhead is not simply a matter of environment or chance, but is deeply rooted in evolutionary biology and genetics.
As research continues, discoveries like the work discussed on the The Orvis Fly Fishing Podcast are reshaping how scientists—and anglers—think about one of the most iconic fish in North America. What once seemed like a simple question—Is it a rainbow or a steelhead?—turns out to have a far more complex and fascinating answer.