Fish
Trout
Salmo, Oncorhynchus, and Salvelinus · also called trout
“Trout” is one of those words that sounds like it names a fish and actually names a habit. The trout are the cold-water members of the salmon family, scattered across three genera, and a rainbow trout is more closely related to a sockeye salmon than to a brown trout. What they have in common is a need for cold, clean, oxygen-rich water — and a long history of being carried far outside the places that provide it.
Overview
Trout are cool-water fish of the family Salmonidae, in three genera: the true trout (Salmo — the brown trout), the Pacific trout (Oncorhynchus — rainbow and cutthroat), and the chars (Salvelinus — brook and lake trout). All are carnivores, native to the Northern Hemisphere, and now introduced worldwide.
Two lives
Many trout have both a resident freshwater form and a larger sea-run form that silvers up and returns to rivers to spawn — the sea-run rainbow is a steelhead, the sea-run brown a sea trout. These forms can appear in the same population.
Cold-water specialists
Trout are stressed above about 20 degrees C and need high oxygen and clean gravel for their eggs, which incubate through winter in a tail-dug nest called a redd with no parental care. Because they react fast to warming, low flows, and silt, biologists treat trout as an indicator of river health.
Prized, and a problem
Rainbow trout is farmed in dozens of countries, and brown and rainbow trout have been stocked for sport across every continent but Antarctica. Outside their native range, introduced trout have driven declines and extinctions among native fish, frogs, and invertebrates, and removing stocked trout from mountain streams and lakes is now a conservation tool.
Status
Trout as a group are Not Evaluated. Farmed and stocked trout are abundant, but many native trout are threatened in their home ranges — several western North American cutthroat subspecies, several Mexican and Asian trout, and localised native brook trout lost to warming, dams, and introduced rainbows.
Trout: common questions
What is the difference between trout and salmon?
Less than the names imply, because they are close relatives in the same family, Salmonidae, and the split is not clean. In broad terms, the fish called salmon make long feeding migrations to the open ocean and grow large, while the fish called trout more often stay in fresh water and stay smaller, but there are many exceptions. The rainbow trout and the steelhead are the same species with resident and sea-going forms. Brown trout also have sea-run populations. On the family tree, the Pacific salmon and the rainbow and cutthroat trout are all in the genus Oncorhynchus and are more closely related to each other than the rainbow is to the brown trout, which sits with the Atlantic salmon in the genus Salmo.
Why are trout so sensitive to water quality?
Trout evolved in cold, fast, well-oxygenated mountain and spring-fed streams, and their physiology is tuned to those conditions. They have a high oxygen demand and little tolerance for warm water, becoming stressed above about 20 degrees Celsius and dying in the mid to high 20s, temperatures that many warm-water fish handle easily. Their eggs need to incubate for weeks or months in clean gravel with water flowing through it, so silt that clogs the spaces between stones suffocates a whole year class. Because trout react quickly to warming, low flows, sediment, and pollution, biologists use their presence and breeding success as a shorthand measure of the health of a river.
Are stocked trout the same as wild trout?
They are the same species but often not the same fish in behaviour or genetics. Hatchery trout are bred for fast growth and tolerance of crowding, and fish raised in tanks tend to be bolder, less wary of predators, and poorly prepared for finding natural food, so many die within weeks of release. Where hatchery fish do survive and breed, they can dilute the locally adapted genetics of a wild population and lower its fitness. Many fisheries now distinguish put-and-take stocking, where trout are added purely to be caught, from the management of self-sustaining wild populations, which depends on habitat and light or no stocking.
How can trout be both prized and a pest?
It depends entirely on where they are. In their native ranges, wild trout are valued fish that indicate a healthy river, and several native forms are themselves conservation priorities. Outside those ranges, brown and rainbow trout were introduced across the world from the 1800s onward for sport and food, and in many places they have thrived at the expense of native wildlife, eating and out-competing endemic fish, galaxiids, frogs, and large invertebrates that evolved with no such predator. In parts of New Zealand, the western United States, and the Andes, removing introduced trout from streams and high lakes is now used to help native species recover.