In order to live, animals must have access to food, air and water. They cannot survive if one of these essential elements is missing. If an element is not available in the requisite quantity an animal will not be able to fulfill its natural functions of growth and reproduction. Instead, it will become weak and diseased and will die before reaching its natural age limit.
We all know that game, for instance, can live only in places where the climate and soil provide enough vegetation for food, or, in the case of predatory animals, where there are sufficient other animals present for them to prey upon. There must also be enough water for drinking and air to breathe. Exactly the same law of nature applies to fish.
Some types of fish live mainly on water plants just as cattle live on grass; others live mainly on small animals such as snails, worms and insects, like the birds. There are also predatory fish which prey on other fishes, just as the predatory lion feeds on other animals.
Like other animals, the fish needs air to live. Air consists of a mixture of gases, mainly of oxygen (21 percent) and nitrogen (78 percent). Fish take up oxygen from air dissolved in the water by means of gills. Some fishes, such as the lungfish, have proper lungs; other such as the catfish group, of which the most common is Clarias (barbel), have both gills and accessory breathing organs.
The physiological processes of the body of fish are adapted to the condition of the water, which depends, in turn, on dissolved and suspended matter in it. In their natural state, rivers and lakes have fresh water, the sea has salt water.
The natural food of fish is mainly derived from the world of living water organisms, which are dependent directly for life and reproduction on the physical and chemical qualities of the water.
Water is always contained in some kind of hollow bed or basin which is variable in area and depth. It is either stagnant, as in ponds, lakes and reservoirs; through-running, as in river pools and some lakes; or running water as in rivers. All these waters are, in the main, filled by water running off from the catchment area and, to a lesser extent, by water from rain, wells and seepage.
From the point of view of biological value runoff water is best, as on its way it brings down dissolved nutrient salts and organic matter from the ground surface of the catchment area. If the ground is poor in nutrient salts and organic matter, as is the case in areas consisting of rock and sand, the runoff water is also deficient in these. On the other hand, if the catchment area is composed of rich soil, such as fertile arable land, the runoff water is also rich and we call it fertile water. The quality of the water is also influenced by the substance forming its bed as valuable nutrient salts are leached out from the ground.
Nutrient salts, such as those of calcium, potash, phosphorus and other elements, as well as organic matter, are as important in water as organic and artificial fertilizers are to agriculture. The grade of fertility of the water depends on the amount of nutrient salts present. On this, in turn, depends the intensity of biological production of such living organisms as bacteria, algae, plants and fish.
Other important influences in biological production are the sun's rays, temperature and dissolved air. Water temperature depends firstly on the climate of a particular region, and secondly on being warmed directly by the rays of the sun. The sun's rays can only penetrate the water to a certain depth. In deep, stagnant waters, therefore, the upper layer can be noticeably warmer than the lower layers. It is only in shallow waters that the temperature remains even in all parts. This is not only due to a uniform penetration by the sun's rays, but is also because wind helps to even out the temperature by moving the water. In rivers and also, partly, in reservoirs which have through-running water, the temperature is evened out by the flow of water. Warm waters stimulate higher and more intensive biological production.
Dissolved air is carried into the waters by runoff water and rain, but much of this is used up in various chemical and biological processes. Direct contact of the water surface with the atmosphere helps to replace these deficiencies, and keeps the upper layers of the water at saturation point. The deeper parts of a water are only supplied with dissolved air from the saturated surface layer, either by means of the vertical movement of the water, or by the air being “pushed” down through the action of wind and flow.
Apart from warming up the water, the sun's rays play an important part in chemical and biological processes, such as the assimilation of green plants, etc., which react more intensively to higher temperatures.
All the factors mentioned are of major importance for supporting life in a water, both in regard to food for the fish and for the fish themselves.
As stated above, distribution of these factors is not even in stagnant waters. This is chiefly because of limited penetration by the sun's rays and limited action by the wind. In deeper lakes and reservoirs these limitations, and their consequences, can be clearly seen in the formation of typical strata and zones.
The upper portion of the water mass in lakes and reservoirs is distinguished by higher temperatures, higher air content and greater penetration by the sun's rays. This is the zone which has the highest biological activity and it is called epilimnion.
The lower part of the water mass is called hypolimnion. The conditions typical of this zone are the opposite to those in the epilimnion. Water temperature and air content are lower here and the light is dim to dark. Biological activity is always lower here than in the upper parts.
A transitional zone, the metalimnion, is found between these two zones. Here the biological conditions of both the epilimnion and hypolimnion meet.
This stratification, or differentiation into layers, is not necessarily present in all waters. Where it appears it need not be permanent, but may disappear and then reappear. Its appearance, however, gives the water its character. For example, stratification may disappear in a stagnant water through forced water movement. In rivers, where the water flows constantly, a stratification has no chance of becoming established. This is also the case in shallow waters, where the movement of the water caused by winds would extend to the bottom.
Figure 65. Scheme of limnological zonations.
The mass of water lies in a hollow as in a bowl or plate and the horizontal stratification zones of the water mass are in contact with certain areas of that hollow. These areas are directly influenced by the physical, chemical and biological properties of the stratification layers with which they are in contact. So the area of the hollow covered by the epilimnion is the most active in biological production and the most important. This is mainly the inshore area of a water and is called the littoral zone. The bottom area covered by the hypolimnion, mainly the offshore, is called the profundal zone. The profundal zone is biologically less productive than the littoral. The portion of the hollow covered by the metalimnion has no specific characteristics because of the changes occurring in this region and is usually treated as the littoral (Figure 65).
It can be understood that in waters which are unstable, or where no stratification appears, the bottom of the water has all the characteristics of a littoral formation.
The type of biological life in any water is related largely to the horizontal stratification of the water mass and the bottom zones.
The water mass produces plankton, the group name for extremely small freely floating organisms, which when dying or dead sink to the bottom. Plankton consisting of plant organisms is called phytoplankton, which lives on nutrient salts. Plankton consisting of animal organisms is called zooplankton. This, in turn, feeds on phytoplankton. Most plankton is of microscopical size. There are, however, many varieties, especially among the zooplankton, such as the water flea, which can be seen with the naked eye. If plankton grows vigorously in concentrated masses it gives the water a specific coloring: green, yellowish green, bluish green, brownish green, etc. It is said of such cases that the water is “in bloom.” The color depends on the density and variety of plankton. The transparency of a water “in bloom” can be considerably reduced, sometimes to only a few inches. Intensive growth of plankton occurs especially in the epilimnion (Figure 66).
 |  |
Figure 66. Plankton. Left: phytoplankton. Right: zooplankton.
The bottom of a water produces generally bigger (macroscopical) organisms. These organisms, known by the group name benthos, include such animals as insect larvae, oligochaetes, nematodes, snails, certain kinds of crayfish and so on. All these organisms live as a rule on the surface of the bottom or in the bottom mud. They feed normally on decaying plankton and organic matter (Figure 67).
Figure 67. Benthos.
A variety of plants grow in the littoral on the inshore part of the bottom. Some plants, such as reeds, bulrushes, etc., while having their roots in the bottom under the water, grow and bloom above the water surface. Semisubmerged plants, like water lilies, the Potomogeton species and others, have their leaves and flowers floating on the surface of the water. Submerged plants, such as Chara and the Nitella varieties, grow and bloom under the water generally deeper than the semisubmerged plants. These, by vigorous growth, can build a solid mat on the bottom of the water.
A variety of littoral plants are used by some grass-eating fish as food; their main importance, however, is that, together with under-water stones and rocks, they form a base on which specific macroscopical organisms called periphyton live. These organisms, generally algae, insect larvae, etc., are distinct from benthos in that they never use the bottom or mud of a water to live on.
This mass of different living organisms multiplies and grows in different parts of a water, and forms a close association; one kind depending for its living on the other. One member of this association is the fish which depends for its existence on other members for food. Nutrient salts and other chemicals are used by plants, such as the littoral plants and phytoplankton. Plant matter, in turn, is used by animal organisms, such as zooplankton, insect larvae, fish and so on. All these organisms in time die a natural death or are eaten by other organisms. Dead organisms, as well as the excreta of living organisms, sink to the bottom and it is on such matter that the benthos organisms live. In the end all sediments are turned into bottom mud through the activity of bacteria, which split the wastes into more elementary materials. These processes go on and on in an uninterrupted life cycle.
The intensity of these biological processes depends on the harmonious relationship of the main factors. These factors have already been described, but are summarized here again.
Optimal water temperatures.
Sufficient amounts of each of the nutrient salts needed.
Sufficient dissolved air, especially oxygen.
A high degree of penetration by the sun's rays.
The factors leading to biological production in natural waters are not balanced and its operation is determined by the smallest factor. Let us suppose, for instance, that a water contains sufficient amounts of some fertilizers, but only some of the superphosphate needed. In such a case, the plants will thrive as long as the superphosphate lasts, but once it is used up the plants will suffer despite the presence of other fertilizers. In this instance superphosphate is the limiting factor.
No amount of biological production in two natural waters is identical. Waters are divided into the following three main groups in respect of their productivity:
Waters with high biological productivity are called eutrophic.
Waters of medium biological productivity are called mesotrophic.
Waters of low biological productivity are called oligotrophic.
The biological productivity of a water is judged mainly by the pH, by the alkaline reserve, by the oxygen content and by the amount and quality of plankton and benthos production.
The rate of fish production in each of these groups depends firstly on the kind of fish and, secondly, on the existence of the right number of fish in the water for the best utilization of available food resources.
In artificial waters such as fish ponds, it is possible to correct almost all the deficiencies found in natural waters and thereby to bring biological productivity to the highest possible level.
The advantage of fish pond culture over fish production in natural waters is evident. Ponds are shallow and, therefore, easily aerated. On account of this shallowness, the water is easily warmed and the sun's rays penetrate to the bottom. The fertility of a pond is easily controlled and corrected by adding the missing fertilizers.
By raising biological productivity to a maximum, a pond can be stocked with an increased amount of the kind of fish most suitable for higher production on natural food. The production of natural food has its limits, however. Just as a pasture can only produce grass up to a certain limit, so there is also a limit to the production of fish food in the water. Production limitations in ponds can be overcome by adding artificial food to the natural food.
In order to be able to rear fish successfully, it is necessary to know at least a little of how fish live and grow in nature and something of their habits. To recognize and identify correctly the different fish with which he may be dealing, the fish culturist has also to know the terms used in describing fish and the main features of their structure. Anatomy deals with the structure of the fish, the form of its body and internal organs. Biology covers the way of life of the fish, its feeding habits, breeding, growth and how it is able to live under the particular conditions in which it is commonly found. In the following section, some of the fundamentals of fish anatomy are described and points in biology are briefly discussed.
Fins
The basic structure of fish is similar to that of the other backboned animals or vertebrates, but because they spend their lives in water there are differences which, at first glance, appear to be very great. There are paired limbs which aid in movement and these limbs are called fins. In front, at the shoulder, are two paired pectoral fins, while on the ventral, or underside, of the body, either at the back or near the front, is another pair of fins, the pelvic fins. These two pairs of fins correspond to the fore and hind legs of the land vertebrates. The fins are supported by bony spines and soft fin rays. Along the top of the body, or dorsal surface, is the dorsal fin, which again has soft and spiny rays (Figure 68). At the hind end of the body is the tail, or caudal fin, with only soft rays. Just behind the anus, or vent, is another unpaired fin, the anal fin, with soft and spiny rays.
Figure 68. External features of fish. Above: Outline sketch of a Cichlid fish (after Greenwood); note split lateral line and pelvic fin in forward or thoracic position. Below: Outline of fish with single continuous lateral line and pelvic fin in hind or abdominal position.
Movement of the fish through the water is by means of wriggling movements of the body, rather as a snake moves through grass. Although the fins assist in movement, they are used mainly for balance, steering and braking, as can easily be seen by watching fish in an aquarium.
Body
The bodies of most fish are covered with scales. An exception is the catfish family, while eels and some other fish appear to have no scales, but in fact have very small ones. The body is covered by a layer of slime or mucus which forms a protective sheath and assists the fish to move smoothly through the water. If this layer is removed by rough handling, for example, the underlying skin cells may become injured or infected. Care, therefore, should always be taken to handle fish with wet hands and to avoid rough contact with containers, nets, etc. Running along the side of the body, from behind the gill cover to the tail, is a line of scales which have little pits in them. This appears as a line running along the body of the fish and is called the lateral line. In some fish there is a continuous line, in others it is interrupted. Others, again, have two lines, an upper line going backward from the gill cover for about three quarters of the way, and a lower line running forward for a quarter of the way from the center of the tail. The pits are sensory organs and are responsive to changes in pressure in the surrounding water. The number of scales in the lateral line and the way the lateral line lies along the body are important in identification.
Mouth
The form of the mouth of a fish differs according to its feeding habits. For example, some fish have large mouths with sharp teeth for seizing prey, while others have small mouths on the under surface of the head suitable for scraping algae up from the bottom. The strange looking Mormyrus spp., or elephant-snout fish and bottle-nose fish, have mouths which are well adapted for grubbing on the bottom.
Teeth
The teeth of fish also tell us something of their probable habits. Predatory, or hunting fish, such as the tigerfish, are well supplied with sharp teeth. Vegetation-eating fish may have coarse teeth, in the case of the Tilapia melanopleura arranged in bands along the side of the jaws. In the fish family of greatest importance in central African fish culture there is, in addition to the teeth in the jaws, a bony plate bearing teeth in the back of the throat. These are called pharyngeal teeth.
On the snout are either one or two pairs of nostrils. The Cichlid fish — the family to which some of the fish used in pond culture belong — have only one pair. The nostrils are not used for breathing, and, in fact, have no connection with the mouth, but are purely organs of smell.
Gills
At the hind end of the head are two slits, one on each side, called gill openings. These slits are formed by two bony flaps attached to the head and called opercula (singular, operculum) or gill covers. If these flaps are lifted, the gills are seen underneath. There are a number of semibony arches carrying long red filaments or threads on one side and short teethlike, or longer comblike projections on the other. The red part is called the gill filament and the comblike projections are called gill rakers. The number of these gill rakers on the lower part of the first gill arch is important in identifying fish. To count them, one starts at the bottom and counts upward until the gill bends (Figure 69).
The gills are the breathing organs of the fish and, in some fish, the gill rakers are used as a sort of strainer to sieve out food particles from the water.
At the hind end of the body is the anus or vent and slightly in front of this is the genital opening. In most species this is different in the two sexes (Figure 70).
There are also other external differences between the sexes which are noticeable during breeding. Male fish are often more brightly colored and generally more aggressive than females.
| Figure 69. Gills and gill rakers in Tilapia mossambica. Head with gill cover removed to show gill rakers, gill arch, and gill filaments. Note that only the gill rakers on the lower part of the arch are counted. The figure shows a fish with 18 gill rakers. |  |
Oxygen
Like all other animals fish need oxygen, but since they live in water they must get their oxygen there. The amount of oxygen dissolved in water depends on the temperature of the water. Colder water can carry more dissolved oxygen than warm water. Large amounts of decaying matter require oxygen, and by using up what is available in the water, leave smaller quantities for the fish. Waters with large amounts of decaying vegetation, or too great a quantity of manure, may have too little oxygen for healthy fish life. How does the fish get the oxygen from the water? Anyone looking at fish in an aquarium will notice that the gill covers open and close in a regular rhythm, and that the mouth opens and closes. The mouth is opened and water taken in while the gill covers are kept closed. The gill covers then open and the mouth closes and water is forced out through the gill openings over the gills where the oxygen is passed into the blood vessels in the gill filaments. When large numbers of fish are crowded together in small amounts of water they may die of suffocation as the oxygen is used up. Catfish or barbel have, in addition to gills, a special breathing organ above the gill and can live for long periods in poorly oxygenated water, e.g., swamps, or even out of water, provided they are kept damp. Fish vary in the amount of oxygen they require. Some, which are said to be tolerant of low oxygen values, can live in very crowded conditions, or where there is very little oxygen in the water. Others need relatively greater amounts of oxygen and are found only in fast-flowing, well-oxygenated waters. In ponds where the oxygen is temporarily at a low level, fish may often be seen gulping for breath at the surface, taking in the surface water and air as well.
 |  |
Figure 70. Male and female Tilapia mossambica. Diagram of genitalia in adult fish.
Temperature
Fish differ in their temperature preferences. Some, which are found only in colder waters, are unable to withstand warm waters. Others die if exposed to lower temperatures than those found where they normally live. A sudden drop in water temperature or a severe winter often causes deaths among the less hardy species. Within the range of temperatures which they can tolerate, generally fish feed more and grow faster at higher temperatures. Other things being equal, fish production is therefore higher in warmer areas.
Feeding
The feeding habits of different fish often differ widely. The same fish also may show a preference for different types of food as it grows, or at different times of the year. Fish which prey on other fish are called predatory fish. Those living on vegetation are called herbivorous fish, whilst those feeding on the minute plant and animal life floating in the water are called plankton feeders. Fish with a large number of fine gill rakers are usually plankton feeders, sieving out the plankton from the water. Carnivorous fish live on other animal life, while some species are known as omnivorous because they shift readily from plant to animal food or eat both depending on availability. When the preferred food is not available, some species readily turn to other sorts of food, while other species do not.
Breeding
The breeding habits of fish vary greatly and are characteristic of each species. Some breed only once a year, usually at the beginning of the rains. Others may breed many times a year, providing that the temperature is high enough. Just as temperature affects the feeding of fish, so it affects breeding and there are temperatures below which certain fish will not breed. There must also be other favorable conditions, such as a suitable area in which the fish can spawn or lay its eggs.
Very often the age and size at which fish breed in the wild state and the number of times they breed in a year changes very greatly under artificial conditions in ponds. There is also a great deal of variation in the number of eggs laid at a time by females of the same size and species.
Growth
Although the rate of growth of different kinds of fish and even of different individual fish of the same species varies greatly, the way in which they grow is the same. At first fish grow very quickly, but the rate of growth slows down gradually until very little growth occurs at all.
The curves (Figure 71) show the increase in weight in Tilapia mossambica. Once the female tilapias start breeding this growth slows down very greatly. Many different factors affect the rate of growth of fish. It is impossible to tell the age of a fish from an unknown water by its size, since if there has been little food, large numbers of fish and poor living conditions, a fish of 4 oz may be three, four or five years old. Whereas a fish of the same size from a water rich in food and with good living conditions may be only six months old.
Figure 71. Growth of Tilapia. Above: Growth rate of T. mossambica. Below: Diagrammatic representation of growth rate of male and female T. mossambica under different conditions.
POND RECORD SHEET
Pond no. .......... Area .......... Av. depth .......... Year .......... 1. Date filled .................... 2. Stocking Date .......... Total wt. .......... Av. wt. .......... No. .......... Wt./acre .......... Species .......... Wt. or proportion .......... .......... .......... 3. Cropping Date .......... Total wt. .......... Av. wt. .......... No. .......... Wt./acre .......... Remarks on species .......... 4. Net production .......... Net production/acre .......... 5. Food .......... Weight .......... Cost .......... 6. lb food per lb of fish .......... 7. Fertilizer .......... Weight .......... Cost .......... Manure .............. Weight .......... Cost .......... 8. Lime ................ Weight .......... Cost .......... 9. Repairs ............................... Costs ......... 10. Vegetation or weed control ........................... 11. Remarks on diseases, etc. ............................ 12. Total costs ................ 13. Total sales ................ 14. Net profit .................
Chimits, P. 1955 Tilapia and its culture. FAO Fisheries Bulletin, 8 (i): 1–33.
Commission for Technical Cooperation in Africa South of the Sahara. 1961 Harmful aquatic plants in Africa and Madagascar, by H. Wild. Salisbury. CCTA/CSA Publication No. 73.
Conférence piscicole anglo-belge, 1919, 1950 Elisabethville. Comptes rendus. Bruxelles, Direction de l'agriculture du Ministère des colonies et du Gouvernement général du Congo belge.
Hickling, C. F. 1962 Tropical inland fisheries. London, Longmans, Green.
Hickling, C. F. 1963 Fish culture. London, Faber.
Jackson, P.B.N. 1961 Fishes of Northern Rhodesia. Lusaka, Government Printer.
Joint Fisheries Research Organization, Rhodesia and Nyasaland. 1959–62 Annual reports, 1958–1961. Lusaka, Government Printer.
Jubb, R.A. 1961 Illustrated guide to the freshwater fishes of the Zambesi River, Lake Kariba, Pungwe, Sabi, Lundi, and Limpopo Rivers. Bulawayo, Manning.
Lagler, K.F. 1959 Freshwater fishery biology. Dubuque, Iowa, Brown.
Maar, A. 1956 Tilapia culture in farm dams in Southern Rhodesia. Rhodesia Agricultural Journal, 53: 139–151.
Symposium on African Hydrobiology and Inland Fisheries, 1, Entebbe, 1954 1952. Record. Bukavu, Scientific Council for Africa South of the Sahara.
Symposium on Hydrobiology and Inland Fisheries, 2, 1956, Brazzaville. 1957 [Papers.] Louvain, Scientific Council for Africa South of the Sahara.
Symposium on Hydrobiology and Inland Fisheries, 3, 1960, Lusaka. 1960? [Papers.] Problems of major lakes. Louvain, Scientific Council for Africa South of the Sahara.
