Rainbow Trout Vs. Steelhead – The Difference

Genetically, Rainbow Trout and Steelhead are the same species, Oncorhynchus mykiss (O, mykiss), but both fish live vastly different lives. The most basic difference between both fish is quite simple; rainbow trout reside and always stay in freshwater, while steelhead are anadromous, meaning that they migrate to the ocean.

To make this a little more confusing, these trout are also often considered steelhead when they migrate from lakes to rivers to spawn. The best example of this is in the great lakes where the “steelhead” are completely freshwater fish, but they live in the lake, and spawn in the connecting rivers. (All, if not nearly all of the successful reproduction of these fish is due to fisheries, and not natural spawning).

Scientists are still puzzled to this day as to what causes these fish to be anadromous vs freshwater dwelling. Some theories and correlations have arisen:

  • Steelhead have higher metabolic rates
  • Food availability seems to be a factor. Areas with high food availability will have less anadromous fish as there is no need to migrate.
  • When water is cold, summer water flows stay high, and food availability is high, there tends to be less migrating fish.

There are known different strains of rainbow trout, some of which appear to have a better likelihood of being anadromous.

Physically, steelhead get much larger than rainbow trout. This is shown in more detail below.

Rainbow Trout


Adult freshwater stream rainbow trout average between 1 and 5 lb (0.5 and 2.3 kg)

Rainbow Trout

Rainbow Trout from South Western Montana



Adults average between 8-11 pounds (3.6-5.0 kg), but can get as large as 40 pounds (18.1 kg)


During the spawning season from January through April, female steelhead build redds (gravel nests) at the top of a riffle and lay their eggs. Generally, the young fish will stay in the streams for years, grow large enough, and head out to sea. From this point, the trout spend 1-4 years growing in the sea before returning to spawn.

Steelhead are very closely related to salmon, but they are able to spawn, and continue spawning, unlike salmon, which spawn and die.

Rainbow Trout Head

Looking to get into fly fishing? Check out our post about the best beginner fly rod and reel combos.