Brown Trout (salmo trutta)

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  Brown Trout
Photo by Malcolm Goddard


The Brown Trout Salmo trutta is one of the few native fish species found in Scotland. The brown trout belongs to the Salmonidae family which includes the Atlantic salmon, the charr group, grayling and the oncorhynchus group (rainbow trout and North American "salmon" types. All these fish have an adipose fin. This is a tiny fin between the tail (caudal) and the back (dorsal) fins. Nobody is sure what this fin does - breeding or streamlining? This does open to question the method of clipping off this fin to mark fish!


adipose fin


The brown trout is much more closely related to the Atlantic salmon than with either the charr or its US cousins. Indeed trout can (but only very rarely do) cross breed with Atlantic salmon. Scientists now agree that sub categories of brown trout (Ferox, Sea Trout, Bull trout, Alpine, Atlas and Marbled etc.) all belong to the same species.


The natural range of the brown trout was originally Europe. But many introductions have taken place (for sport) around the world from Australia to America. Even within the UK brown trout have been moved about so much that very few distinct populations can now be identified. The brown trout lives in both still and flowing water, but it is demanding in terms of water quality. Perthshire offers many hundreds of miles of trout fishing organised through angling clubs.

Water Quality

Oxygen levels, temperature and sediments are probably the most important factors. Trout need 6 ppm of dissolved oxygen for good growth, which is high. Temperatures above 20-21oC are lethal to trout (and warm water contains less oxygen). Trout will tolerate acidity levels of a pH down to 4.5. In the UK there are very few waters with such acidity, but in Scandinavia thousands of lochs and streams are now devoid of trout due to acid rain.


Inspite of the strict requirements the brown trout is still one of the most adaptable species we have. It lives in fresh water, estuaries and the sea. Like the salmon the brown trout can migrate to the sea (anadromous) The species re-colonised the land after the last ice age (along with salmon and sticklebacks).

Sea Trout

Unlike the salmon not all brown trout bother to migrate, the majority live out their whole life cycle in fresh water. Only in the last century was it realised that the Brown Trout and the Sea Trout were the same species. Just why some brownies become sea trout is unclear. Certainly the rivers of the west coast of Scotland produce more sea trout. This may be because they are generally poorer in terms of nutrient (spate rivers). So the trout may be looking for more food. Rivers which produce a lot of juveniles may also stimulate a return to the sea since adult trout are territorial. Poorer specimens are forced ever more downstream until they reach the sea and richer feeding. Their brothers and sisters must get quite a shock when these fish return as monsters!

Ferox Trout

A ferox trout is is a trout which has turned cannibal. Juvenile trout live on insects and grubs, but when a brown trout reaches about 12" in length it can change diets. At that size it can catch and eat other fish up to half its own length. Such a sizeable meal is rich in protein and may require less effort to obtain. As a result trout which discover this food source soon pack on the weight. Growth curves which may have been gentle can now explode. Not very many trout take this option, perhaps due to a lack of prey fish. Some lochs such as Rannoch are famous for their ferox, these lochs often have good heads of charr and perch as a food source. Loch Tay has given up ferox to 16lb in recent years. See more about ferox trout fishing.

Bull or Slob trout

Bull trout are fish which never get past the estuary. Like sea trout they migrate back up the river to spawn.

Trout Variety - Marbled Trout, Gillaroos, Sonaghen, Leven etc.

There is a vast variety of strains of brown trout. These are best seen in isolated lochs and lakes. Perhaps the most famous morphs (from Ireland) are the Gillaroo, which is rich in colour, deeply speckled and well built, and the Sonaghen which is bright, bluish and lightly speckled. The former lives around the margins feeding on larger insects while the latter feeds on plankton in the open water. The differences are clearly an adaptation to local environment. Here in Scotland we have our own varieties. The most famous is the Loch Leven trout. These are bright silvery fish with few red spots. Many Leven trout were used to stock Scottish rivers and their native strains submerged. The typical Tay trout is said to have a very buttery yellow belly and a lot of red spots.


All trout return to the rivers and streams to spawn. Timing will depend on latitude (day length), but here on the Tay it takes place in November/December. The trout seek out gravel beds (stones about the size of a pea are best) with good water flows over it to bring in oxygen and to carry away silt. The hen fish cuts a trench (redd) into the gravel with her tail fin. Once dug she is joined in the hole by the male. Hundreds of eggs and milt are squirted into the base of the redd simultaneously. Thus fertilised the eggs absorb water and sink. They are quickly covered up by the female. Inevitably eggs miss the redd and drift downstream. Fish including other trout will eat them, but some will find their way into other cracks and crevices where they may well develop.


Egg development is dependent on water temperature. Here in the Highlands the water is cold and development is slow. This is why spawning takes place relatively early. At a temperature of 3 - 4 degrees C it takes 3-4 months for the eggs to hatch. So if spawned in December they will hatch in March just when food is coming back on-line. Water temperatures gain govern development of the fry and parr. See how to recognize brown trout parr. Highland water being cold slows their growth. It can take 2 years for the parr to turn into trout, and it may be 3-5 years before the fish is if a takeable size.


The wild brown trout is much more suspicious and selective than the reared rainbow trout found in reservoirs and put and take fisheries. Brown trout love cover, even a weed bed affords some protection. Trout will often guard one flank by resting up against an obstacle. Over hanging bank sides and vegetation, even bridges are a favorite. It should be remembered that fish do not have eyelids, so in bright weather fish will seek shelter from the sun. Next in terms of priority comes food. The best lies will bring food to the trout on the current. The best lies thus offer cover and food. Being territorial as adults you can watch a trout feeding at the very same place on the same food for hours.

Even in a stillwater trout will use currents to allow food to drift down towards them. Trout will cruise into the wind along "wind lanes" looking for food trapped in the glassy surface film.


The trout is a predator. Open its mouth and you will find rows of sharp pointed teeth. Along the roof of the mouth are extra teeth along a ridge - called the "vomerine" teeth. These are much more pronounced in the trout than the salmon. The mouth can be opened wide to engulf quite large prey. The fish actually sucks in a large amount of water to catch food and filters it out using gill rakers at the back of the mouth. Prey species will depend on the size of the fish and availability.

Brown trout are particularly well known for their selectivity. If there is a hatch of a certain fly on then you may well find that these are taken exclusively. Perhaps the best well known hatch is the Mayfly on the chalk streams of southern England. In the 1st and second weeks of June fish feed only on these flies. It is not just the species of fly which may be selected. Some brown trout will only seek out certain life stages. For example they may only be eating "emergers" - flies just breaking the water surface or "spinners" - adult flies returning to lay eggs on the surface. Others may be attacking nymphs as they rise from the river bed.

The trout feeds on a wide variety of insects both aquatic and terrestrial. Sedges (caddis flies) and stone flies can be identified by their wings which lie along horizontally the body. Upwing flies as the name suggests have wings which stand upright. Worms such as the blood worm and crustacea such as daphnia will also feature on the menu as will beetles, grasshoppers and grubs which fall onto the water.


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