Life History of a Fish: Brown Trout

By Scotty Kyle

I am a brown trout. I am, or at least was, the cause and prime target of the traditional fly fishing fraternity from early in the 19’th century. I was the species around which fly fishing in Europe developed, and I remain today one of the most sought after and prized species of freshwater fish for sport and, unfortunately, also for food. 

While in Northern Europe only relatively few anglers, mostly affluent, could have an opportunity to go after my cousin the Atlantic salmon, the majority of fly fishing enthusiasts began with me, and this has led to books, stories, poems and even legends about my fighting prowess. The difficulty, but allure, of my capture has led to the development of a multibillion dollar industry which has spread to many corners of the world.

People become confused with the use of the word “trout” and even “brown trout” but the reality is quite clear. Scientists call me “Salmo trutta” and the true trout and Atlantic salmon are all members of what these scientists call the “Genus Salmo”. This fish group is lodged within the Salmonidae, a family that also contains char, grayling, several other salmon, rainbow trout and also some additional “trout”.  Within my genus are the well-known and appreciated Atlantic salmon (Salmo salar) and several trout found around the the Adriatic Sea, Turkey and Armenia. These species are the Adriatic trout (Salmo obtusirostris), flathead trout (S. platycephalus), marble trout (S. marmoratus), Ohrid trout (S. Letnica) and the Sevan trout (S. ischchan) which is restricted to Lake Sevan in Armenia. The word “trout” has become very loosely applied in different parts of the world to many fish species unrelated to true trout. 

a brown trout caught by an angler

All “real” trout are cool water, northern hemisphere, freshwater fish. My arch rival in many areas, the rainbow trout (Oncorhynchus mykiss), is actually not of the true trout line but a member of a closely related group, the Oncorhynchus Genus. Other close family members are found in the Salvelinus Genus and these include the Dolly Varden (Salvelinus malma), brook trout (S. fontinalis) and lake trout (S. namaycush). 

Early Europeans took my species to most suitable areas of the world in which they decided to settle and, in many, they now have natural breeding populations and also fish farming operations. In Australia and New Zealand brown trout were introduced from 1864, in South Africa from 1890 and Kenya from about 1909. In the Americas, both in Canada and the USA, introductions began in about 1883 and continued until about 1933. In some areas we have been accused of pushing out indigenous fish species ,but the “bottom line” is that once we are breeding naturally is it almost impossible to remove us from a river system without destroying its ecology.  

Most fish that are called “trout” have some similarities to my form as, basically, I was the first thing called a trout, in 1775. I was described by a gentleman called Carl Linnaeus in Europe and I was one of the first species ever properly described. My species can vary a lot within our natural range depending on whether they live in rivers or lakes and if the waters are nutrient rich or lack good feeding. Some areas produce fish that are almost silver in colour, others can have the most magnificent and intricate colourful patterns, while others again can be so dark that there seems to be little pattern at all. A well-known feature of our species and many other “trout”, is our ability to change colour rapidly and markedly. 

Where we live we are often the top fish predators and we have large eyes, big mouths and fearsome teeth from the perspective of our prey. Our scales are small and we have a thin layer of protective “slime” over our body. Our large body fins enable us to maintain our position in running water or “hover” in still water. Our tail fin is relatively big, allowing us to dart forward to gulp in prey and our specific colours are often suitable for camouflage in our natural habitat. Complex bright colour patterns are often found in clear fast flowing mountain streams to blend in with the many multi-coloured rocks usually found there. 

Brown trout that annually travel downstream and into oceans are called “anadromous”, as opposed to those that remain in a river (riverine) and lake forms (lacustrine) which are usually more robust (that is, they can gain more fat). They are all the same species and yet the anadromous form can often return from the ocean as a bright silver colour and it may have grown much faster than those that remained in the rivers. The feeding is often much better in the ocean and, if you are lucky, the fishing and predation pressures can also be less. Some other anadromous individuals (sea trout) travel far from their home rivers in search of good feeding, but, all of us when we are mature, travel back upstream to spawn. 

Trout are generally referred to as “cold water” fish in that we carry on feeding and moving throughout the year, even in the depths of winter. We also often spawn in ice cold water usually on fairly shallow gravelly banks.  Smaller males and females sometimes look similar, to the eyes of humans, but we are really very different. Large males can have huge heads and mouths and the lower jaw can have a “kipe”, an upward hook at the front. The colours in most trout become more evident around spawning and mating times. We thrash the water during actual mating and also to move the gravel to try and cover our eggs and give them protection from predators, often other trout, and prevent them from being washed away.  Our ladies usually produce about 2 000 eggs per kilogram of body weight (900 eggs per lb.) and, very sadly, many trout particularly the males, die after spawning. We generally attain maturity at two to four years of age but this can be delayed in low nutrient environments. 

Our eggs develop fairly rapidly and hatch into larval trout called alevin while they absorb their yolk sacks. They then progressively become fry, parr and then, when about two years old, are called smolt. Young trout feed and grow rapidly and generally slowly gravitate downstream. Small trout feed on insects and invertebrates and quickly graduate to eating small fish, including other trout. As they grow, trout will feed on almost anything suitable that they come across, including frogs and even small mice.

Growth rates vary markedly depending on available feeding, temperature and also the size of the water body in which we are living. Generally lake trout can grow larger than those found in small rivers, but exciting “cannibal” trout can occasionally be found in deep pools. The “standard” brown trout, like me, has been known to grow to 1 meter long (39 in.) and about 20 kilograms (44 lbs) but, our friends in the deep lakes, can attain 1.4 meters (55 in.) and a massive 27 kilograms (60 lbs.). In our home range, however, we often live in very low nutrient waters where a river fish of one kilogram (2.2 lbs.) can be regarded as a “good-sized” fish. 

brown trout small

I hatched from an egg on a freezing cold day in late autumn, in a small crystal clear mountain stream in the Scottish Highlands. I did not know my parents, parental care not being a strong point of my clan. I grew rapidly as I was slowly washed downstream to a small hill loch where I stayed for several years. Most of my fellow cohorts were taken by herons, otters, and other fish and, in the loch, other birds like great northern divers had to be avoided. My rate of growth slowed as, in these areas, food was scarce, so I left the loch and moved much further downstream to a large dam in a farming area. Here the water is enriched and much more productive, the food more plentiful, but the fishermen can be a problem.

I grew more rapidly and I am now over ten years old and weigh about 1.5 kilograms (3.3 lbs.). My cousins, the rainbow trout, would all be dead by now as they usually only live for about 3 to 5 years and a known maximum of 11 years but my species can live much longer, sometimes over 20 years. My cousins, however, can grow much faster in their shorter lives and attain as much as 24 kilograms (53 lbs.). This is why many human “fisheries”, both for food and fishing, prefer rainbow trout, as we ‘brownies” grow too slowly and do not convert food as efficiently. 

As we grow in size our natural predators become less numerous and less threatening. Birds cannot catch us, otters tend to take smaller fish and other trout are not a problem. In some areas, but thankfully not this dam, the northern pike can be a serious challenge as they lurk in cover and kill many unwary trout. My greatest fear and threat is now human fishing and they use an incredible variety of ways to try and catch an unwary trout. In my dam only fly fishing is permitted but there is now such a vast variety of fly patterns and sizes that it is often very difficult to spot the artificial fly among a natural “hatch”. 

While some anglers are real amateurs and we can “hear” them and spot them from a distance, others are much more careful and have spent time perfecting their artificial flies that closely resemble the real thing. They sneak up on us and present the “fly” in a natural way that is very appealing. In the “old days” almost all reasonable sized trout were killed on capture but now things have changed in many areas. In my dam anglers usually carefully land and release large brown trout while they are often less kind towards my rainbow cousins. If we are landed fairly rapidly, handled reasonably carefully and returned to the water as soon as possible, there is an excellent chance of our survival. I really would like to reach my 20 years!

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