--- title: "Rainbow Trout (Oncorhynchus mykiss) | AnimalTracks.wiki" description: "A stream fish whose sex is set by a Y-chromosome gene unrelated to any known sex-differentiating gene." image: "https://animaltracks.wiki/images/animals/rainbow-trout-1.webp" published: "2026-10-10" updated: "2026-10-11" --- # Rainbow Trout (*Oncorhynchus mykiss*) > **Status**: Checked against 6 sources, listed under Sources below. > **Tagline**: A stream fish whose sex is set by a Y-chromosome gene unrelated to any known sex-differentiating gene. > **Soundtrack Vibe**: Classic Hard Rock & Southern Blues // Fleetwood Mac, "Go Your Own Way" > **Soundtrack Links**: [Spotify](https://open.spotify.com/track/07GvNcU1WdyZJq3XxP0kZa) > **Vibe Match**: Some rainbow trout stay in fresh water all their lives, while steelhead migrate to the ocean and then return to spawn. Go Your Own Way is a song about two lives heading off in different directions. ## Biological Superpowers & Stats - **Taxonomy**: *Oncorhynchus mykiss* - **Danger Level**: Low - **Conservation Status**: Least Concern ([IUCN Red List](https://doi.org/10.2305/iucn.uk.2024-2.rlts.t166481a18229476.en)) - **Superpower**: One gene, sdY, decides whether it becomes male or female - **Habitat**: Clear, cold headwaters, creeks, rivers and lakes of the Pacific Slope of North America and Russia's Kamchatka Peninsula, now introduced widely - **Diet**: Aquatic and terrestrial invertebrates and small fishes - **Tags**: fish, freshwater, river, carnivore, invasive ## Natural History & Behavior ## One Gene Decides the Sex A rainbow trout's sex depends on a gene called sdY, short for "sexually dimorphic on the Y chromosome", and it is expressed only in the testis. Researchers who switched it off in males, using a tool called zinc-finger nuclease, got fish that grew ovaries. Researchers who added sdY to females got fish that grew testes. > **Master switch** > The sdY gene is a vertebrate master sex-determining gene not related to any known sex-differentiating gene. ## Cold, Clear Water Rainbow trout live in clear, cold headwaters, creeks, rivers, lakes and intertidal areas. Their native range runs down the Pacific Slope, from the Kuskokwim River drainage in Alaska to the Otay River drainage in California, with another native population on Russia's Kamchatka Peninsula. FishBase gives a common length of about 60 centimeters. FishBase lists a maximum length of 123 centimeters, a maximum published weight of 25.9 kilograms and a maximum reported age of 11 years. They feed on aquatic and terrestrial invertebrates and small fishes. In 2020 a trout of about 30 centimeters was caught in a small French stream. Inside it was a live hatchling grass snake about 20 centimeters long. The authors say the evidence suggests fish eating these snakes is rare. ## Stream Fish or Sea Fish Some rainbow trout stay in fresh water all their lives. Others migrate to the ocean and come back to spawn, and those are called steelhead. NOAA calls steelhead a unique life-history type of rainbow trout, and says the ones that go to sea typically grow larger than the ones that stay. Steelhead can reach about 30 pounds (roughly 14 kilograms) and 40 inches (about a meter), and NOAA says they live up to 11 years, usually 5 to 8. Unlike most other Pacific salmonids, the group that includes salmon and steelhead, they do not necessarily die after spawning. At spawning time the female digs a pit in the gravel while a male courts her or drives other males away. A female makes 700 to 4,000 eggs per spawning event. ## A Fish That Went Global Rainbow trout are often hatched and stocked into rivers and lakes, especially to attract recreational anglers. FishBase lists them as widely introduced and established in Canada and the USA, and elsewhere in the world. It says several countries report adverse ecological impact after introduction. Animal Diversity Web adds that where they are introduced they can outcompete native trout species. The Red List rates the species Least Concern. NOAA adds that along the West Coast, 11 steelhead populations are protected under the Endangered Species Act. ## Frequently Asked Questions ### How big do rainbow trout get? FishBase gives 60 centimeters as a common length. Its record is 123 centimeters, and the heaviest published weight is 25.9 kilograms. *(Source: [FishBase]())* ### What is a steelhead? A steelhead is a rainbow trout that heads out to sea. Trout that stay in fresh water all their lives keep the name rainbow trout, and the sea-going ones typically grow bigger. *(Source: [NOAA Fisheries]())* ### Do rainbow trout really eat snakes? An angler in France gutted a trout of about 30 centimeters and found a live hatchling grass snake inside it. The paper calls this unusual: what evidence there is suggests fish rarely eat barred grass snakes, and only small ones. *(Source: [Thomas, Allain & Owen 2021]())* ## Sources 1. [Hammerson & Bogutskaya (2024). Oncorhynchus mykiss. The IUCN Red List of Threatened Species 2024: e.T166481A18229476.]() 2. [Yano et al. (2012). An Immune-Related Gene Evolved into the Master Sex-Determining Gene in Rainbow Trout, Oncorhynchus mykiss. Current Biology.]() 3. [Thomas, Allain & Owen (2021). Predation and ingestion of a barred grass snake Natrix helvetica by a rainbow trout Oncorhynchus mykiss in Lot, France. Herpetological Bulletin.]() 4. [Oncorhynchus mykiss (Walbaum, 1792), Rainbow trout. FishBase.]() 5. [Steelhead Trout. NOAA Fisheries.]() 6. [Oncorhynchus mykiss, Coast rainbow trout. Animal Diversity Web, University of Michigan Museum of Zoology.]() *Published October 10, 2026. 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