Understanding Fish: Fishing Basic Knowledge

Externally, fishes look much alike – the shapes being fundamentally spindle-like, with various modifications according to the particular species and its way of life. The EEL, for example, is elongated and cylindrical, but the PLAICE is compressed or flattened from above, while the BREAM is flattened from the side. Further minor modifications are seen in the bottom-feeding BARBEL, which is flattened lightly along the belly, with the head depressed from the medial line to give it its typical wedge shape. The opposite occurs in the surface-feeding BLEAK, which has its dorsal surface less curved than the belly, and the mouth and head tilting strongly upwards.


The basic colour pattern is almost universal, with the back and upper body shades dark, those of the belly bright, and the flanks shading intermediately between the two extremes. This much alone gives effective camouflage from above and below, irrespective of the colours involved. Obviously such protective coloration is considerably improved if the predominant hues match the environment in some way, either by obliterative colouring, or by direct matching. To this end some fish are laterally striped (MULLET and GRAYLING), and others are transversely barred (PERCH). Some are marbled or mottled (father lasher or goby), and some bear large or small spots (SALMON, TROUT, BASS, shad). Beneath the basic colour pattern the colours themselves are surprisingly restricted in most fishes of temperate zones, the great majority being predominantly silvery, brassy or golden, green or brown.

With so much restriction in shape and colour, the differences between the various species are small, and the angler’s attention is often focused on details, such as the head, mouth, eyes, fins, tail, and scales, which have a variety of character sufficient for recognition.

The Head

This extends to the edges of the gill-covers and bears the major sense organs : eyes, nose, and mouth. The mouth may be oblique (RUDD and BLEAK), or almost horizontal (ROACH and BREAM). It may be superior, with the lower jaw extending beyond the upper (RUDD and PIKE), or the opposite, with the lower jaw well under-slung (BARBEL, Sturgeon). The lips may be fat (CHUB), thin, or absent, and may bear feelers known as barbules or barbels (CARP). Barbels may also be attached to the corners of the mouth (BARBEL and CARP), or to the snout (Burbot, Catfish).

In some fishes the scales extend on to the cheeks (BASS and PIKE-PERCH) and even on to the snout between the eyes (MULLET), or they may be absent from the head altogether (ROACH, RUDD, DACE, CHUB, etc). The mouth may lack teeth, as in CARP-family fishes, or bear only small bristle-like teeth (PERCH, Shad), or may have many rows of sharp strong teeth (SALMON, PIKE, COD).

In some fishes the maxillary (jaw) bone is easily observable along the lower jaw line; it may be short or long according to species. The nostrils are not normally conspicuous, and the eyes, although varying in size and position, are set either on the side of the head or towards the upper surface.

The Trunk

In the interests of streamlining there is no neck, the head being joined directly to the trunk. The body is normally covered completely from head to tail with a layer of separate overlapping scales which give protection without impairing body movement. The scales may be fairly soft as in the CARP family of fishes (Cyprinidae), hard and rough-textured as in the PERCHES, or large, projecting, and bony as in sturgeon and SKATE. A few fish are scaleless, and the skin is then tough and leathery to compensate (bullhead).

In most fishes a lateral line is visible along the centre row of scales on the flanks. This consists of a series of tiny pores found only on centre flank scales, one to each. This line may extend to the head as in PIKE and TENCH.


All true fishes have fins, the paired fins representing the forerunners of the limbs in other animals, and the unpaired ones lying on the medial line and being extensions of the vertebral processes.

The dorsal fins are set sail-like on the ridge of the back, and the anal fin keel-like beneath. There may be one dorsal (CARP), two (PERCH), or even three (cop). In some fishes there are two anal fins on the underside. These fins work in conjuction with each other as stabilizers and also have some function in swimming.

The paired fins, pectoral and pelvic, are attached to the lower body, the pectorals being just behind the gill-covers, on the lower flank. Sometimes the pelvic fins are set on the underside just below the pectorals (PERCH, BASS), but in the CARP family of fishes and the salmonoids they are attached roughly level with the dorsal fin, but on the underside of the body. These are of great importance in starting, stopping, reversing, turning and maintaining a position.

All fins consist of a fairly thin membrane supported by bones or rays. In PERCH or BASS the first dorsal fin bears stout spines, but the second bears mostly finer flexible branched rays similar to those which support the fins of the CARP family of fishes. In the bullhead, the rays are also flexible, being jointed and unbranched. A further kind of fin is found in salmonoid fishes. This is simply a fatty finger of flesh protruding from the back near the tail, and totally unsupported by bones of any kind. It is called the adipose and its function is not really known.

The tail fin is perhaps the most obvious and variable. It may be steeply forked and symmetrical as in BLEAK and PERCH, or asymmetrical, with the lower lobe rounded (BARBEL and BREAM). In TENCH and adult SALMON, the tail fin is unforked, being almost square-ended, or very lightly concave at the trailing-edge.

A description of the trunk would be incomplete without mention of the smooth layer of slime which covers the bodies of fish. This slime in fact protects the fine layer of live skin cells superimposed over the scales and acts as a barrier to infection by bacteria and parasites, as well as lubricating the surface and preventing water from soaking through the body of the fish.

The Senses and Sense Organs

Sight The eyes of fish are lidless and have a muscular control within a limited range which permits them to move up and down or sideways within the orbit. The pupil is not adjustable, and the sight is normally of short range. Long sight would be of no advantage to a creature living even in clear water, which itself considerably restricts the range of vision. Scientists are not entirely agreed as to whether fish have colour vision or not, but the eye is known to be associated with colour changes in the appearance of the fish, which is continually adjusted to camou- flage with changes of surroundings. Blind fish lose this capacity and become evenly dark coloured.

Shoal fishes which are non-predatory bear eyes set to the sides of the head, giving a very wide radius of simple monocular vision on both sides of the body, with only a very narrow radius `blind’ in the region of the tail. Binocular vision is of little importance to these fishes and is possible only over a very small radius in front of the head. All-round visual alertness to attack is their primary need. Predatory fishes, on the other hand, have eyes set forward and on the upper surfaces of the head to give them a good overall view of the surface above them, and a wide radius of binocular vision which is essential for judging distances, so that they may accurately grasp their fleeing prey. The radius of monocular vision on either side is correspondingly limited.

The angler is often concerned with how far, and how well, the fish is able to see objects above the surface. Vision is modified by refraction, familiar enough to the fisherman who sees his landing-net handle appear to bend when he puts it in the water.

If the handle is held vertically, no distortion is apparent, but as it is tilted away from the vertical the amount of bending increases. If the angler sighted along his net handle at an angle of about seventy degrees he would find himself unable to see through the surface because of dazzle from reflection. Anyone studying fish below the surface knows that his line of sight must be close to the vertical to prevent dazzle obscuring the view.

From the fishes’ viewpoint much the same is true, since there comes a point at which total reflection occurs because the line of sight of the fish reaching the surface at an angle of fifty degrees or more is bent horizontally along the surface itself.

This means that a fish at any given point can see through the surface only so long as the line of sight strikes the water at an angle smaller than fifty degrees to the vertical. The fish, therefore, views the outside world through a circular ‘window’, the size of which is determined by its own depth in the water.

This can best be represented by the roof of an inverted cone, the apex angle being about 100 degrees, and resting upon the eye of the fish. The cone ‘widens’ above the surface because rays of light coming from above are bent towards the vertical. The fish’s ‘window’ is thus filled with images of the objects outside the water which actually occupy the visible horizon, but which appear compressed into the space of the ‘window’. In many respects this would account for the shyness of the fish when bank objects are moved suddenly, because these always appear to be more directly over its head than they are in fact.

Obviously too, the size of the window varies with the depth of the fish. A fish lying sunning itself in 15 cm (6 ins) of water would have only a small window about 30 cm (1 ft) or so in diameter. Conversely, a fish in 3 m (10 ft) of water would have a window of 6 m (20 ft) across. Other factors also operate here, otherwise a fish really deep in the water would be practically unapproachable. First, the colour and nature of the water tend to obscure images as the light diminishes with increasing depth, and secondly, the near-sightedness of the fish prevents really effective vision beyond say 3 m (10 ft). Nevertheless, at this range the fish is still capable of discerning movements and these are sufficient to frighten it. In the last resort, much depends on the water and particularly whether it is at all coloured. The angler has only to recall swimming a few metres below the surface in clouded water to have a very good idea of how limited above-water vision is, and how little light penetrates well below the surface. Fortunately for him, most of these factors operate to the angler’s advantage.


Fishes do not possess external ears, which would interfere with streamlining. They do not require them in any case, since the internal ear structure is in direct contact with the water through the skin and is therefore capable of conveying vibrations and sounds to the central nervous system. The ear is also equipped with semi-circular canals which are responsible for balance and kinetic sense. This is also aided by the functioning of the lateral line, which picks up information about pressure variations necessary for station keeping and body control in swift-surging and variable currents of water. The lateral line is also capable of detecting vibrations of certain frequencies which are transmitted to the brain. The swim-bladder too is associated with hearing in some fishes, being taut like a drum and capable of receiving vibrations of low frequencies. In some fishes this is linked to the ear by means of a series of tiny articulated bones.

Undoubtedly this complex and efficient hearing mechanism is of the utmost importance to fish in detecting the presence of danger as well as in seeking prey, avoiding attackers, and making accurate movements in turbulent waters.

Taste and Smell

Taste and smell are almost indistinguishable in aquatic creatures, which receive impulses of this kind through the agency of the water in which they live and must be capable of detecting minute concentrations of substances in solution. The mouth and barbels bear taste cells, and the nostrils, each of which consists of a pair of openings, are also equipped with sensory organ cells. Water is continually flowing into the first opening through the connecting canal, and out of the other. In doing so it passes over the sensory cells which obtain information from it, detecting food flavours and such things as chemical pollution. The salmon probably homes by detecting the taste-smell of a river estuary while cruising along the coast.

Barbelled fishes are usually bottom-feeders. The barbels serve as additional sense organs in helping them to locate concentrations of micro-organisms in the bottom layers which are often dark or muddy enough to prevent them searching for food by purely visual means.

The Skeleton

Fishes are the earliest of the vertebrates so it is to be expected that the skeleton is less complex than that of land animals. Some fishes, such as the SKATES, SHARKS, and RAYS, have purely cartilaginous (ie. gristly) bones, but the majority (the ‘true fishes’) have bones which contain calcium.

In most animals, the function of the skeleton is bound up with body support as well as articulation and protection. In fishes, support is largely unnecessary, since the fish is borne up chiefly by the water in which it lives and is weightless for all practical purposes. Rigidity is achieved with simple, thin, needle-like bones, which at the same time give protection to the brain, vital organs, and spinal chord. Articulation is comparatively simple since the limbs bear no complicated joints. Body movement is indeed confined to the undulatory movements of the body in swimming, the moving of the paddle-like fins, and movements of the eyes, jaws, and tail.

To this end bones require to have little weight or strength, although the muscular system which operates upon them in movement comprises the greater part of the total body mass.

The Blood System

The function of the circulatory system is to transport food and oxygen to the tissues and remove waste matter for disposal. It is also important in combating bacterial invasions and controlling body temperatures.

In fish, the task is simplified because they are cold-blooded, and, within a degree or two, maintain the temperature of the water around them. The heart is therefore a simple three-chambered muscular organ situated in the throat quite close to the gills where blood is re-oxygenated. Blood flows sluggishly from the heart to the gills; thence to the head and body, collecting food from the digestive system and liver, getting rid of waste via the kidneys and intestines.

The Gills

These are provided with fine, feather-like tissues richly endowed with tiny blood vessels. These are the gill filaments. They have a wide surface area over which the water flows so that the oxygen can be extracted. For protection from gritty particles which might damage these delicate tissues, the gills are supplied with rakers or comb-like appendages which filter off solids in suspension and also serve to collect or strain out tiny food particles, which are directed to the gullet. Oxygen and carbon dioxide are exchanged in the blood stream at the gills. The gills are very delicate and in danger from attack by parasites, as well as being easily damaged by the angler’s hook.

Osmosis and the Slime Barrier

Paradoxically, the fish, which lives in water throughout its life, requires a slime layer to keep its body watertight, because of the process of osmosis. This is essentially a physical phenomenon which operates when two liquids of different concentrations or strengths are separated by a semi-permeable membrane — such as the skin of the fish. The liquid from the weaker solution tends to pass through the separating layer, diluting the stronger solution until equilibrium is achieved.

In freshwater fishes, the blood and vital liquids of the fish are stronger than the fresh water surrounding the body. The water therefore tends to soak through the skin and gills, diluting the body solutions and blood-stream of the fish. Freshwater fishes cope with this continual dilution by not drinking and by the efficient and hard-working action of their kidneys, which continually throw out the excess water. The slime barrier effectively prevents osmosis taking place through the skin to any marked degree, although the gills still absorb considerable amounts of water.

In sea fishes the effect is the opposite, since the salt water in which the fish live is actually stronger than the vital liquids of the fish. The fish is, therefore, continually losing water through the gills and in constant danger of dehydration. To counteract this tendency, sea fishes drink considerable amounts of water, and produce very strong waste liquids which are disposed of through the kidneys. In both sea and freshwater fishes, damage to the slime layer increases the burden of osmotic pressures by increasing the areas through which liquids can pass one way or the other. In freshwater fishes this throws a great burden on the kidneys, and in sea fishes the danger of dehydration is very much increased.

Damage of this sort is probably more serious because of the possibility of bacterial attack or infection occurring at the same time through the damaged areas of skin.

The Digestive System

Non-predatory fishes feed largely upon small articles which require little chewing and absorption by the system. Vegetable matter forms a large part of their diet. Chewing is performed by the pharyngeal teeth, which are attached to the fifth gill arch and set behind the gills so as not to interfere with the ordinary business of respiration. These teeth are often the only certain means of identifying such oddities as possible hybrids. Non-predatory fishes have no separate stomach, the gullet merging into the gut and intestines, which may fold twice or several times within the body cavity before ending at the anus where waste is expelled. Digestion occurs throughout the length of the tract where digestive juices and enzymes break down and extract from the food the substances required by the body. Nourishment is obtained from vegetable substances containing cellulose by the action of bacteria in the lower intestines.

In predatory fishes the teeth in the mouth are used not to chew, but simply to grasp the prey. Once swallowed this is received by a pouch-like stomach where powerful digestive juices act upon it. The rhythmic movements of the stomach assist in breaking it down into a sludge-like consistency capable of being dealt with by the intestines, where it is finally absorbed by enzymes and juices from the pancreas. Undigested food and waste products from the kidneys are passed to the vent where they are expelled.

The Reproductive System

The ‘roes’ of fishes require little description. The hard roes consist of the female ovaries which, during the breeding season, are packed with ripening eggs ready for expulsion through the narrow tract which leads from the ovaries to the vent. In the male fish soft roes are packed with ripe sperms which are shed when the fish mate.

During the breeding season these organs of reproduction take up considerable space within the body cavity and are responsible for the plump appearance of the belly. Once spawning has occurred the fish appear lean and out of condition. Often, too, they are exhausted by the rigours of spawning and make for the streamy well-oxygenated reaches where they can scour and recover. Associated with spawning in many fishes is the formation of tiny wart-like protuberances which appear on the head or scales of the male. These give the flanks a rough, raspy texture. This undoubtedly secondary sexual characteristic, found only in the male fish, has some value in assisting the shedding of spawn when the fish rub against each other. It is probably also significant in shoal fishes, which cannot distinguish each other in the densely packed shoals except by touch, enabling the females to recognize male fish in close proximity and vice versa.

Eggs are shed amongst the weeds, where they may sink or float according to the type. The sinking kinds become attached to weed stems where some of them at least are hidden from the attentions of the innumerable creatures which feast on the many millions of eggs produced by any single shoal. Some fishes cover their eggs with gravel to protect them, but the great majority simply leave them to chance.

The young fry hatch within a few days or several weeks according to species, and take shelter amongst the weeds of stones during early life. Within a year or two, their numbers very much reduced, they make up shoals and undertake the instinctive pattern of life for their particular species. As the years pass the shoals become smaller and smaller until the older and larger fish are few and far between, when they may continue to live socially in small groups or, in some species, adopt solitary habits and become more predatory.

Hybridism, uncommon in the sea, but fairly common in fresh water, occurs when two shoals of different but closely allied species happen to be spawning simultaneously in close proximity to each other. Fishes on the fringes of both shoals tend to intermingle accidentally and eggs from one species are fertilized by sperms from the other. The resulting hybrids are intermediate between the species, and in most cases are quite infertile, although in laboratory experiments it has been shown that one or two species can produce fertile hybrids.

Special Organs

The swim-bladder and lateral line have already received mention in certain contexts, but both have particular functions of great importance. The swim-bladder is undoubtedly of immense value as a hydrostatic organ enabling the fish to make small adjustments to its specific gravity as it moves from deeper waters into shallower zones or vice versa. The lateral line is important, not only as a ‘hearing aid’, but as a gauge of changing pressure on the flanks. It enables the fish to interpret vagaries of current and properly control itself in very turbulent water. It probably also has some function in shoal behaviour, which is little understood but is nevertheless of great importance in the survival of the species, particularly among the non-predatory fishes.

15. July 2011 by admin
Categories: Fish, Livestock | Tags: , , , , , | Comments Off on Understanding Fish: Fishing Basic Knowledge


Get every new post delivered to your Inbox

Join other followers: