Rainbow Trout - Oncorhynchus Mykiss

About the Rainbow Trout.
The rainbow trout is a freshwater fish that was imported to Europe in 1884 but is native to the USA. It rarely spawns in the wild in the UK, yet nowadays is found all across the UK and in fact all over the world due to their easiness to farm. The rainbow trout is a hard-fighting fish, which makes it one of the angler's favourite. Many rivers, fisheries, reservoirs and still waters have been well stocked with rainbow trout numbers, specifically for sporting purposes.

Rainbow Trout Identification

There are several features that make the rainbow trout easy to distinguish from other species of fish, the main one being the lovely pink - purplish stripe, similar to the colour of petrol in a water puddle, along its sides.

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In addition, the rainbow trout has many black spots dotted randomly across its body, head, dorsal, adipose and caudal fins. It is the only trout species throughout the whole of Europe that has spots present on its tail.

As with all the other members of the salmon family, a small ray-less adipose fin is also present near the tail of the rainbow trout. The caudal fin is ever so slightly forked.

The body is large, covered with small scales and is torpedo-shaped, similar to that of the brown trout and the Atlantic salmon. The body colour can vary depending on habitat location and can range from a blue - green to a greenish - yellow - brown coloured back with lighter shading on the sides and a white underbelly.

The head is small but the rainbow trout has a large mouth that opens up wide to be able to engulf its prey and it has numerous small teeth located inside.

Rainbow Trout Habitat

The rainbow trout species is native to North America and has been introduced to countries all over the world in practically every continent.

In the UK it is found in most fisheries and is more often than not farm-reared, although there are some areas where it does spawn naturally.

Unlike the brown trout it is able to withstand waters with higher pollution levels and also higher temperatures, although their preference is still cool, clean stillwaters such as reservoirs and lakes. It is found in some rivers in the UK.

The migratory form of the rainbow trout that journey to the ocean and then back to freshwater to spawn are known as steelheads, due to their more silvery appearance.

Natural Rainbow Trout Food

Rainbow trout have similar feeding patterns to the brown trout and the Atlantic salmon in that newly born alevins start off their life by feeding on the nutrients contained within their yolk sac, whilst they are still buried in their "redd".

From there the juvenile trout begin to feed on plankton and they quickly begin to grow.

Insects and insect larvae then become a staple diet for the rainbow trout with nymphs, mayflies and caddis flies high up on their list of favourite foods.

As the rainbow trout get older, they become more predatory and in addition to all types of insects, they begin to eat small fish such as minnows, stickleback and mackerel, as well as crustaceans, molluscs and fish eggs, particularly salmon eggs and salmon fry.

Rainbow Trout Reproduction

For some reason many rainbow trout that have been imported in to the UK do not spawn. Numerous deaths are produced this way due to the fact that the eggs or sperm are retained within the fish's body and end up killing it. Recently, rainbow trout have been genetically modified so that they are unable to spawn thus decreasing the number of deaths in this way.

Rainbow trout that have been reared in hatcheries are bred from eggs and milt (male sperm) that have been taken from the spawning species. The fish are then released into fisheries, lakes and rivers when they reach a suitable size.

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European rainbow trout tend to spawn in the late autumn and early winter and their eggs hatch in early spring, whereas the imported rainbow trout that are able to spawn do so when the water temperatures are slightly higher, from May - August, which is consistent with their native rivers in the USA.

Rainbow trout have the same breeding patterns as the Atlantic salmon and the brown trout and deposit their eggs in specifically made holes in the gravel bed called "redds".

The eggs are medium-sized and can reach up to 0.2 inches (5mm) in diameter. Those that are not hidden away safely enough will be eaten by predators including the crayfish and various predatory fish such as the pike.

For more detailed information on the reproduction practice of the rainbow trout please see our Atlantic salmon article.

The Rainbow Trout's Vital Statistics

  • Average weight: 3lb - 7lb (1.4 - 3.2kg)
  • Average length: 15 - 20 inches (38 - 51cm)
  • Specimen weight: 5lb (2.3kg) and over depending on location
  • Life-span: 8 - 10 years in the wild and up to 5 years in a fishery
  • Current UK natural record: 34lb 12oz (Loch Earn, Perthshire, Scotland)
  • Current UK cultivated record: 36lb 14oz (16.71kg) - (Denver Springs Fishery, Hampshire)
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