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Brown Trout (Salmo trutta)

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The brown trout, Salmo trutta is a cold freshwater fish, which resides in Britains lakes, rivers and streams. Its native distribution spreads all across Europe and parts of western Asia from as far north as the Arctic Circle to the Atlas Mountains of Morocco in Africa. This geographic distribution has been shaped by the retreat of the ice caps after the end of the last ice age leaving populations only in locations where water temperatures never exceed about 20 degrees Celsius. Above this temperature brown trout cease to grow and reproduce.

The brown trout has a particularly variable life history depending upon the habitat in which it lives and various different forms or morphs are recognised from each of the different habitats.

Stream dwelling forms tend to be small individuals, which are primarily insectivorous as adults feeding upon both aquatic insects and terrestrial/aerial insects which land on the water surface. However some individuals living in streams and rivers can become migratory and leave freshwater for the sea where they feed on other fish. These “sea trout” only return to the river to spawn and can attain a much larger size than their less nomadic relatives. Sea trout always return to the same river that they were born in to spawn (this is called philopatry).

Within Static bodies of freshwater there are also different forms of brown trout. Lake trout such as the ferrox trout can be very large carnivorous individuals with strongly curved toothed jaws for hunting their prey, which may even include smaller brown trout! Alternatively they can also take a much smaller form as an adult and become more of a zooplanktivorous/insectivorous form. Both forms may inhabit the same body of water. It is unclear whether these different sympatric morphs represent genetically different populations and hence potentially different species or simply represent an adaptive environmental response of developing individuals to make the most of available niches.

Brown trout typically reproduce between October and February in Britain. Females excavate a nest in the stream or lakebed by flapping the side of their bodies violently against the substrate. This creates a depression (called a redd) in which the eggs are to be laid. The female is then “courted” by a male and eggs are released into the nest by the female. Once deposited the male quickly fertilises the eggs with the sperm.












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