The rainbow trout is a global wanderer that reshapes ecosystems wherever it lands

Nature · Published 29 September 2026 Swipe more like this →

In short: Native to the cold Pacific tributaries of North America and Asia, the rainbow trout has been introduced to every continent except Antarctica. While prized for sport and food, this prolific species is now listed among the world's 100 most invasive organisms, often outcompeting or hybridizing with vulnerable native fish populations.

The rainbow trout, Oncorhynchus mykiss, is a complex species with diverse life histories. While resident populations spend their lives in freshwater streams, the anadromous form—known as steelhead—migrates to the ocean for two to three years before returning to its natal grounds to spawn. Unlike many Pacific salmon, steelhead are iteroparous, meaning they can survive to spawn multiple times, though fewer than 10 percent of native adults typically survive between trips.

The species' global footprint began in earnest in 1870 with the first hatchery on San Leandro Creek, California. By 1875, these fish were being shipped across the United States, eventually reaching at least 45 countries. This rapid expansion has had unintended consequences. In many regions, introduced rainbow trout have become vectors for whirling disease, caused by the parasite Myxobolus cerebralis. Because rainbow trout lack innate immunity, they can shed massive quantities of spores, devastating local populations of more resistant species like brown trout.

Beyond disease, the trout's genetic plasticity poses a threat to biodiversity. Through hybridization, they create fertile 'cutbow' offspring when breeding with cutthroat trout, a process that threatens the genomic integrity of native subspecies. Furthermore, the species' intense metabolic activity during growth creates oxidative stress, which researchers study to understand mitochondrial damage. Today, conservation groups like Trout Unlimited and the Wild Salmon Center work to mitigate these impacts, striving to protect the remaining pure stocks of this resilient, yet ecologically disruptive, salmonid.

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