THE commonest quarry of the fly-fisher is the brown trout (Salmo trutta), an indigenous fish found in streams, rivers, lakes and lochs all over the British Isles. The size and weight it may reach in its lifetime largely depend upon the chemical and physical factors of its environment. On these ultimately rest the amounts and kinds of food available for the fish. Under the right conditions, trout live for many years and attain a very large size. For example, trout have been kept alive in research stations for more than sixteen years and have reached weights of over 1 lb, while the record wild fish for Britain presently stands at 39£ lb.

The average sport-producing fish, however, are very much smaller. They range from about 6 oz in streams and burns to approximately a pound in such famous angling waters as Loch Leven and the River Don. Larger fish, up to six pounds and over, are, of course, occasionally taken from favourable environments such as chalk streams and from such well-known reservoirs as Chew and Blagdon. Rainbow trout (Salmo gairdnerii), an introduced species of trout and easily distinguished by its remarkable iridescent coloration, are sometimes used to stock both rivers and lakes. Like brown trout, they are fine fighting fish and possess some advantages over the former in growth rates.

The fishing season for trout officially extends from the beginning of March to the end of September but fish tend to lose condition during the winter and at the approach of the spawning season. For this reason, many clubs further restrict the fishing dates to suit local conditions.

Life History

In the autumn, in response to falling water temperatures and a decrease in the number of daylight hours, the pituitary gland, a tiny pea-shaped organ attached to the base of the brain, releases into the trout’s bloodstream a number of chemical messengers known as hormones. These hormones affect many tissues of the body and stimulate the ovaries of the female fish and the testes of the male. These organs, in turn, produce sex hormones of their own. Under their combined influence, eggs ripen in the female and the testes enlarge and produce sperm, or ‘milt’ as it is called in the male fish. Colour changes occur in the skin and the fish shed some of their silvery lustre to become much darker, almost black in appearance. It is believed that hormones are also responsible for triggering off the behaviour patterns which occur at spawning time.

There is strong evidence to suggest that trout, like salmon, tend to ‘home’ – that is, to return to the stream where they were hatched. As spawning time approaches, loch fish tend to return to the feeder streams where they started life, and river trout to migrate upstream. The female fish selects a suitable breeding site or redd and, by sweeping movements of her tail and body, cuts out a groove in the gravel bed. The male contents himself at this time by chasing off other males in the vicinity, provided, of course, they are smaller than himself. Finally, the female settles into the groove she has cut in the gravel and the male moves to her side. As the eggs are shed, in batches of about 200, they are fertilised by the milt from the male. When this has been accomplished, the female proceeds to cover up the fertilised eggs by further sweeping movements of her tail. The female fish is now ‘spent’ and gradually falls back downstream. The male may pair again with another female before he, too, is spent.

During the pairing period, fish lose a good deal of weight, and recovery during the winter months depends on temperature. If the winter is mild, more food is available and the fish may be in good condition by the time the fishing season opens in March. In severe conditions, however, recovery may be slow and the trout may still be in poor shape in the spring.

Meanwhile, the fertilised eggs, under their protective layer of gravel, continue to develop. The greatest danger to them at this time is that they may be swept away by floods and spates. The rate at which the eggs develop appears to be dependent on water temperature, and the first signs of growth are the appearance of a faint streak which represents the future backbone, and development of the dark pigment of the eyes. At this stage, Ihey are known as ‘eyed ova’. Finally in February or March, the egg hatches to release the young fish which is known as an ‘alevin’. Attached to the lower side of each alevin is a large yolk sac, at first globular in shape.

After a day or two, the alevin starts to move upwards through the gravel. During this time, the contents of the yolk sac are used as food and gradually diminish. When the alevin eventually emerges into open water, most of the yolk has been absorbed and the young fish are then known as ‘fry’.

The young fry must now fend for themselves and, fortunately, their emergence coincides with that of other freshwater organisms, such as water fleas and young Mayfly nymphs, which are also small and serve as food. The behaviour of the young fry is interesting and gives an early indication of the habits of the fully-developed fish. Instead of shoaling, they lie away from each other. As they grow in size, they disperse individually from the redds to find a territory of their own. The best territories go to the biggest and strongest fry and they will defend them from encroachment by other fry. This habit continues as the fry grow. Adult fish show this same tendency to want ‘a place of their own’ and if a particularly good spot is found, it may be patrolled and other fish chased away. When a fish is taken from such a good lie in the river, the lie is frequently occupied by another fish within a very short time.

Knowledge of the whereabouts of good lies is, therefore, of considerable importance to the angler.

Growth and Food

In general, it takes about three years for the young fish to reach maturity. The size they finally attain depends, of course, on their inherited constitution and on the availability of suitable food. Whilst the young fry feed at first on small insects, they soon seek a much wider diet. As well as insect larvae, pupae and nymphs, shrimps and other small crustaceans are freely taken, along with snails, bivalves, worms, tadpoles, frogs and also smaller fish, including those of their own species. In fact, they will eat almost anything which is present in the water and which is of a suitable size.

Very little accurate information exists about the periodicity of feeding. It is common to find trout coming on to feed simultaneously with the appearance of a hatch of flies and then going ‘off’ with a similar spontaneity. Perhaps hunger has been satiated and there is no immediate desire to continue feeding, even though the supply of food is still plentiful. During the day, however, there are sometimes sudden surges of activity when fish after fish may be taken in a short period for no apparent reason and when no flies can be seen hatching. Whatever the physiological explanation for this group behaviour, it is welcomed by the angler. With the possible exception of the evening rise as the sun goes down, he cannot predict with certainty when trout will start to move, nor even, indeed, what they will accept. I have taken fish on fly at all hours of the day and night – often to discover that their stomachs were bulging with freshwater snails or mussels. No matter what the weather conditions nor the time of day, there is always hope, and it is this chronic optimism which keeps the angler happily engaged at the waterside. The observant beginner should be continually noting not only where and when fish are rising, but the nature of the exact spot where a fish is seen to move. As his experience grows, the better will he be able to select the likely spots to cast his fly when fish are not obviously feeding.

As the young fish grows in size, other changes are taking place in its tissues. As well as the special mucous glands on the surface of the body which form the slippery, slimy, protective secretion familiar to all fishermen, other cells, known as the chromatophores, develop in the skin. These contain black, white, orange, yellow and red pigments, and it is the amount and distribution of these coloured substances which produce the trout’s beautiful, spotted appearance. The shining, silvery white of the belly is due to the presence of small crystals which possess both reflective and iridescent properties, as well as to the absence of pigment cells. The wide colour variations often seen in individual fish are due to the numbers and the spacing of both crystals and pigments. It is believed that food is the source of some of the pigments, and this may explain why the differences shown by trout from various localities are sometimes so great as to mislead anglers into imagining that there is more than one species of brown trout.

As it is illegal to take young salmon (parr), these must be distinguished from small trout. The easiest way is to look at the mouth. In trout, the mouth usually extends beyond an imaginary line drawn vertically downwards from the tailward edge of the eye; in young salmon, the mouth extends only to about the middle of the eye.

The scales of the fish – thin, transparent discs overlapping like slates on a roof – form an outside ‘skeleton’ with a protective and supporting function. As they are living structures, they continue to grow throughout the life of the fish. The surface of the scale carries a number of small ridges, and these increase in number as the fish continues to grow. In times of quick development, as when food is plentiful in the summer, these ridges are relatively widely separated; when growth is slow during the winter, they lie much closer to each other. There thus appears with each successive winter and summer, a number of bands which indicate the age of the fish. At spawning time, the outer edge of the scale may show an eroded area where some of the last formed ridges have been destroyed. This spawning mark is less clear in trout than it is in salmon and sea trout, but it still gives some information as to how often an individual fish may have spawned. Scale-reading yields a considerable amount of information about the age of a fish and its rate of growth, but it is a job for an expert. Some of the fishing magazines offer the services of scientists who will give a scale-reading of any particularly interesting fish caught by readers. The scales should be carefully scraped off with a knife and placed in a cellophane envelope before despatch.


Besides the angler, trout have to contend with numerous other predators, and although the number of eggs produced at spawning appears to be very large, only a few will survive to reach maturity under wild conditions.

The eggs and young alevins are attacked by the larvae of the caddis fly and the nymphs of the stonefly, as well as by eels, perch and other fish. As the survivors of these initial onslaughts grow larger, pike, and such fish-eating birds as the heron and the cormorant, also take their toll.

Trout, however, are not without means of protection. Normally they are illuminated from above, which has the effect of seeming to lighten the tone of the upper parts of the body and darkening the lower side which is in the shade. As this would make their shape too clear-cut, Nature counters it by making the skin of the back darker and that on the belly lighter, so reducing the solid effect and giving an illusion of flatness. The shape of the fish is difficult to detect, too, because of the coloured spots. Patches of colour tend to catch the eye and distract attention from the outline on which they are placed. Trout are well camouflaged in both these ways and some practice is needed before they can be spotted easily in their natural surroundings. Polaroid spectacles are very helpful in cutting down reflected light and enabling the angler to see fish more clearly below the surface of the water.

As well as camouflage, trout rely on speed to avoid impending danger, though many anglers tend to over-estimate the rate at which they can move. The evidence suggests that the maxi- mum speed of a nine-inch trout is around 5 – 8 m.p.h. What fish can do, however, is to produce a very large amount of ‘explosive’ energy very quickly and to reach their maximum speed in less than a second from rest.

Their swimming muscles are arranged in blocks in ‘herring bone’ fashion along either side of their backbone. Alternating contractions and relaxations of these large muscle groups sweep the tail through the water in side to side movements. Water is displaced sideways and backwards , and in this way the fish is propelled forwards. The problems of water resistance have been reduced by the evolution of a smooth, streamlined shape. A straight course is achieved by the use of vertical and horizontal fins and these are also brought into play in banking, turning and diving movements. Directional changes towards an item of food, or away from a source of danger, are the results of information received in the brain of the fish from one or other of its special sense organs for sight, sound, touch, taste and smell.


Trout, as the beginner will soon discover, possess vision of a high quality. Like Man himself, they are capable of detecting movement and appreciating form. They have cells which react to different wavelengths of light and are thus believed to have some degree of colour vision. Just how clearly they can see in terms of exact appreciation of structure is uncertain, but the accuracy with which they can rise to take minute flies suggests that the sharpness of their vision approaches that of land animals. In trout, the lens of the eye is spherical in shape and bulges through the pupil. It can receive light from above and below as well as from the front and sides. This affords it some of the properties of a periscope and it is even thought that the eye can see behind, except where the fish’s own body blocks out the visual field.

When it comes to the perception of fine detail and colour, we are on difficult ground. On occasions, trout are highly selective and will refuse almost everything but the natural fly hatching at that particular moment. This has given rise to the ‘exact imitation’ school of thought, whose adherents go to elaborate lengths in attempting to dress flies which resemble the delicate hues and colours of the natural insect. This theory presupposes that trout see colour in the same way as man does, whereas the special cone cells which respond to colour in a fish’s eye are not, in fact, arranged in quite the same manner as in the human eye. It thus seems unlikely that the theory is entirely accurate, though it is certainly true that when trout are feeding exclusively on a specific insect, the closer the artificial approximates to the natural, the greater is the likelihood of success.

At other times, fish can be tempted by gross caricatures of their natural food, and even by flies which do not resemble anything in particular. It would, therefore, seem that mere clarity of vision and colour discrimination are not the only factors determining whether or not a fish will accept an artificial fly. Since they do not possess a very advanced brain, their ‘intelligence’ and capacity for learning is small, and their behavioural reactions are likely to be largely instinctive. If the fly triggers off a feeding response either by movement, shape or colour, or by a combination of all three, the fish will snatch it. When it steadfastly refuses to do so, we are at a loss for an explanation. Science cannot produce an answer and we can only fall back on speculation.

Whatever the trout’s reaction to a particular fly, the sudden appearance of an angler in its visual field is likely to cause it to move off swiftly, or to ‘put it down’ if it has been rising. A cautious approach and the use of any cover available is sound advice. Fish rising close to the bank or in open water can often be reached, unseen, by resorting to a kneeling position and by careful casting. But at all times it is sensible to assume that if you can see the fish, it is likely to be able to see you.


When hearing is mentioned, Man is inclined to think of the ear on the outside of the head, whereas it is the drum at the inner end of this which is set vibrating by sound waves. A chain of small bones in the middle ear magnifies these vibrations and transmits them to sensitive auditory cells in the inner ear. The outer and middle ear structures are absent in trout but the basic auditory cells are present inside the head. In addition, the lateral line which runs along either side of the body from head to tail, contains special ‘hair’ processes which pick up underwater vibrations and transmit information about them to the brain.

Sound travels in waves sent out by objects vibrating in air or water. Aerial sounds, however, are reflected or ‘bounced off’ when they strike a liquid surface. For example, a man swimming under water cannot hear a whistle being blown in the air above him – nor apparently can a fish. But both man and fish can detect and respond to the vibrations of a submerged bell. In practice, this means that while conversations above the water cannot be heard by a fish, they can pick up any vibrations transmitted through the ground by a heavy footfall, by knocking on the bottom of a boat, or by clumsy wading. Such vibrations may not necessarily be interpreted as danger signals, and fish may sometimes rise almost in the swirl of an oar, or within a rod’s length of the wading angler. The lateral line organs, which provide them with ‘touch or pressure at a distance’ information, are used to supplement vision, to locate distant objects and to enable them to orientate themselves in the direction of the current flow. Trout are thus very sensitive and it pays to move quietly both on the bank and in the water.


The sense of smell is more developed in trout than is usually appreciated. The ‘nose’ is represented by two small pits on top of the head. A current of water passes continually through them and brings small amounts of chemical substances in solution into contact with special receptor organs so fish can scent food even at a distance. As one-sixth of the brain’s area receives this olfactory information, the sense of smell may play an important part in the selection of one species of fly.