fly fishing

Fly Fishing - The Lifecyle of a Brown Trout

Author: TG Contributor
Date: 2020-01-13

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A detailed overview of the lifecyle of wild brown trout and sea trout.  Describes how brown trout mate, spawn, hatch and grow to adulthood.


Before hatching, the brown trout ova will have lain for between four and twelve weeks, depending on temperature, within the covered hollow of a clean gravel bed known as a redd.  The water will be well oxygenated and quite cool (usually less than 16° C) and most likely upstream from the trout’s usual haunts.  The parent hen trout creates the hollow during mating by quickly sweeping her tail from side to side across the gravel creating a nest (the design of the nests provide a percolating effect to the ova and keep them well oxygenated and free from fungal growths).  Into it, she will then lay several hundred of the 4000 or so ova she will lay in total, before moving slightly further upstream to repeat the process, simultaneously covering the previous hollow with the gravel excavated by her tail.  The ova are fertilized by the male trout which, positioned beside and slightly upstream of her, will secrete milt into the current, some of which will make contact with the ova.

During this process some of the brown trout ova will be lost to the current.  More will be eaten by other fish and others will be lost to environmental factors such as poorly oxygenated (particularly polluted) water.  The oxygen content of water above 24° C will be deadly to the ova and indeed trout in general.  It has been estimated that only 1% of the ova laid by the female will survive for more than a year.  The mating process also weakens the trout and it can take as much as three months for them to regain fitness.  Closed seasons for brown trout fishing were introduced in the UK to accommodate this period of recuperation.

The beginning of the mating period starts in winter and is governed by day length.  The actual calendar month therefore will be reliant on latitude.  In the UK the period is generally between November and late February.


The ova can take from four to twelve weeks to hatch.   The length of this period being dictated by the water temperature, warmer water generally producing quicker incubation times.  The ova develop into the ‘eyed egg’ stage, so called because two black spots are visible through the ova membrane.  Following this, the ova hatch into yolk-sac fry or alevins.  The yolk-sac is the alevin’s food supply as their digestive system is developing.  The strange looking creature becomes more recognisable as fry as the yolk-sac, which hangs from the alevins throat, is consumed.  At this stage, a period of between 12 and 20 days, they emerge from the darkened safety of the gravel bed to spread downstream and start feeding.

Brown Trout Fry

The fry will tend to shoal where the water flows less quickly, normally near to the river bank, where they feed on tiny invertebrates and plankton.  By now they are beginning to develop the marking of their species and become identifiable as brown trout fry.  This is a dangerous stage for the young trout and they will be fed upon by all manner of creatures including other fish, mink and birds.  Once they have reached roughly three to four months in age (10 – 15cm) they are known as fingerlings.  A 95% mortality rate has been estimated for fish reaching this period.

The Adult Brown Trout

A brown trout is mature between one and two years of age and theoretically can live for up to twelve years, spending most of their time avoiding predation and saving as much energy as possible.  They tend to be territorially individualistic and will seek out lies, the bigger fish usually securing the best, which offer them an energy efficient source of oxygen and a good supply of food.  They also require a nearby bolt hole for protection from the many predators found in rivers and lakes.  They will eat almost any creature of the right size (typically insects, small fish, frogs and even crabs) and will not move far, maybe as much a metre onlyComputer Technology Articles, from their lie to find it.

Sea Trout

Sea Trout and Brown Trout are the same species (Salmon trutta).  The parent fish mate in the same manner as the brown trout but they have migrated from salt water to the river rather than simply moving upstream.  They are generally indistinguishable from brown trout in looks and behaviour although once they move from the fry stage they are known as a parr rather than a fingerling.

At about three years old they start to become visibly more silvery than a brown trout.  Now known as a smolt their gills begin to develop in a manner that will enable them to process salt water.  Once they reach about a pound in weight they will migrate to the sea where they will feed with gusto and acquire more weight.   Their return journey back to fresh water to spawn can happen as early as that same year or in some circumstances they may wait for up to four years.

Vist the Fly Fishing Journal online to search through hundreds of locations to catch brown trout.

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Chris Whittaker is the owner and developer behind the Fly Fishing Journal a comprehensive resource of fly fishing locations, tips & techniques and much more.

By Chris Whittaker

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