Mr. A. Moussa NOUR
Fisheries may be divided into three broad types. The first type of fishery is not agriculture and is comparable to hunting. As with hunting, the yield of food per unit of surface area is low. In the 2nd type of fishery, selected species are stocked in natural waters or impoundment, and fertilisers are applied to stimulate primary productivity. Greater abundance of fish food organisms resulting from higher levels of primary productivity enhances fish yields, this type of fish culture is true agriculture and is similar to the terrestrial practice of fertilising pastures to favour for greater production of livestock. The 3rd type of fishery involved stocking desirable species and supplying feed to increase fish production above that possible in fertilised ponds. The amount of water required to produce a given quality of fish is greatly reduced by this method. However, agricultural land is required for the production of the fish feed. This method of fish culture is analogous to the production of livestock in the feed lots.
Fish culture is principally done in ponds. It permits the supervision and regulation of reproduction, feeding, quantitative growth and control of the size of the fish as well as the stocking and maintenance of the ponds instead of leaving this to nature.
Fish farms may be classified as extensive, semi-intensive and intensive according to weather the farming is based on natural food only, or whether it includes, additionally, more or less complete artificial feeding. Extensive farming means obtaining a quantity of fish corresponding to natural productivity; intensive farming seeds to produced a maximum quantity of fish in minimum water.
Fish cultivation is suitable for only a restricted number of species. In order that a fish be useful for bond cultivation it must;
The desirable fish species will have a rapid growth rate, good productivity per unit volume of water and economic and efficient feed conversion. Common carp (Cyprinus carpio L.) and tilapia are the most suitable fishes for the needs of the modern aquaculturist. The intensification of fish culture through the use of high stocking and feeding rates can lead to severe water quality problems. Therefore, in water quality management, we regulate environmental conditions so that they are within a desirable range for survival and growth of fish. Careful attention to water quality problems is an absolute necessity in intensive fish culture. No matter how much care is taken in constructing ponds, stocking, feeding and parasite and disease control, all will be to no availability if the large crop of fish dies other water quality problems.
Generally, fish with the highest market value are cultured most intensively, using more artificial, or more expensive, facilities and food, whereas species with lower market value are produced under less modified culture conditions with greater dependence on natural foods and, consequently, with lower alttitude per unit of culture space.
This paper deals with the recent advances in fresh water pond fish culture.
II- SITE AND PRECONDITIONS
One of the advantages of fish culture over conventional agriculture crops is ability of the farmer to utilise non-arable soils and water not suitable for irrigation. This is especially important where land and water are in short supply. Thus fishponds can be found on swampy soil as well as on sand dunes. The most suitable soil where seepage is low. When ponds are constructed on sandy soil seepage is usually quite high, often reaching 10 cm/day ans more - This decrease quickly due to blocking of the interstitial pores by organic matter produced in the pond and precipitated. In 1–2 years, the seepage drops to an acceptable rate of 1–2 cm/day. This process of blocking the soil pores can be accelerated by spreading about 10 cm 3 of cattle manure unitl seepage is reduced. In loamy soil the alteration of soil structure through compaction by mechanical means, such as “Sheep foot” or caterpillar tractor, may help in reducing seepage. However, rocky and sandstone soils, as well as pebble beds, are not fit for pond construction. Also areas with a high ground water level may pose severe problems for pond construction, to overcome any soil acidity, it is advisable to lime the soil either with agricultural lime (Ca (OH) 2) or lime stone (Ca CO3) by spreading the lime over the entire pond bottom and usually repeated every year or two.
Modern fish culture is impossible without control of the fish population in the pond. Therefore, the most important consideration in selecting the site are the following:
III- POND CONSTRUCTION
To construct a pond under good conditions it is necessary to have enough water of good quality and it is also necessary that topographical characteristics should be satisfactory. To ensure this the following demands are indispensable:
The principal points which must be considered when constructing a pond are, the choice of ground of the dike, the drainage system, the water inlet, the by-pass and the over flow. Depth will be chosen to avoid the invasion of emergent vegetation but if must be excessive so that emerged plants can grow and develop. It should be possible to drain the pond dry easily, rapidly and completely which is achieved by means of drains and ditches comprising one principal ditch and several secondary ditches. Each pond should be provided with a water inlet, and outlet independent from those of other ponds.
According to their water supply, ponds are divided into:
It is necessary to have a sufficient supply of good quality water for the construction of water. First
of all it is necessary to assure enough water to compensate for losses through seepage infiltration
and evaporation. This same quantity of water must be replaced by an adequate flow to maintain the
level, in addition to enough water to meet the breathing.
requirements of the fish. The water level in fish ponds must be kept constant and the temperature within the limits imposed by the requirements of the species. Loss from infiltration will depend on the care taken when construction the dikes, however, loss through evaporation differs throughout the year and also according to climatic and local conditions. In tropical climates the loss in water ponds due to evaporation can reach 2/5 cm (about one inch) per ha/day and require inflow of 3 litres (6 1/4PT) per second to compensate for the loss. Calculating the necessary quantity of water for intensive fish farm, it is necessary above all to base calculating on the breathing requirements of the fish.
Cost of pond construction will be decreased with increasing the size of ponds. However, more time required for filling and draining the large ponds than the small ones. During that time fish are not fed and normally do not grow. The calculations showed that 4ha pond required 5 days for fill and draining at rates of 300 m3/hr and 1000 m3/hr respectively during the working hours. The pond normally produces at the rate of 30–50 kg/ha/day in 5 days, this means a loss of 250 kg/ha of fish per harvesting. Therefore is not advisable to construct a pond larger than 8–10 ha. Due to the sensitivity of some of the fish in pond to hard conditions in the harvesting sump and the danger of possible fish loss during harvesting, it is desirable to complete harvesting as quickly as possible. It should take no more than one day per : pond. If maximum amount of fish which can be handled by the normal size team and equipment in one day 5–10 tons and the productivity of one ha of the cultivated area is expected to be 1.5 this means that the harvest of 7.5 tons could be obtained from 5 ha pond, which should be considered the optimal size. With increasing intensification of culture and consequent increase the yield per unit area, the tendency is to reduce the size of ponds to 2–3 ha.
The main factors affecting the desired shape of rearing ponds are:
Each pond requires its own separate water supply through pipes or open canals. In open canals, however, it is difficult to prevent wild fish from entering the pond, and canals require more work for maintenance. The diameter of inflow popes will be affected by four main factors.-
The most common pumping units used in fish farms have capacity of 200–300 m3/hr and pipes for
water distribution with a 15–25 (6–10) diameter.
Where water is in short supply and its reuse is essential, a proper integration between supply and drainage system will be ensured.
The minimum pond depth is usually 80 cm at the shallow end. The average pond depth can reach 2.5–3.5 when the water supply is seasonal.
IV- FISH SPECIES
1- Common Carp (Cyprinus carpio)
One of the most important fish cultured in the world today is the common carp. It originated in central Asia and spread to east China and Northwest Europe. There are four different varieties with respect to scale cover ranges, including:-
The last two varieties have a much lower growth rate than the two mentioned first. Common carp is an omnivore that feeds on various foods of plants and animal origin. Young carp in ponds fed on protozoa and zooplankton and when reached 10cm in length it begins feeding on bottom fauna. It burrows in the mud and sucks in insect larvae, warms, molluscs, etc… It accept all supplementary feeds and digest carbohydrates, however, not all of these feeds have the same nutritional value. Common carp spawn readily in captivity when maintained in ponds. It spawn throughout the year in tropical climates, with a peak from January to April.
2- CHINESE CARP
A mostly herbivores group which is native in China;
a - Silver Carp (Hypophtamichthys molitrix)
Fresh water fish, do not breed spontaneously in ponds, however, as do common carp. It feeds mainly on phytoplankton as small as 30–40 cm by filtering these microscopic algae through their gill filter. A 250 g fish can strain 32 l of water/day through its gill, therefore it is suitable for polyculture. It feeds on a trophic niche usually not utilized by other fish. It improves the environmental conditions in the pond by controlling blooming of the algae. It usually prevent the build up of excessive algae concentrations. It can consume leftover finally ground supplementary feeds (offered to carp and tilapia) which may not consumed by other fishes. Therefore, it decreases the load of organic matter and the oxygen demand associated with it.
It has a large number of intermuscular bones and to some the insipid taste of its flesh, therefore, it is not appreciated in some markets.
3 - GRASS CARP (Ctenopharyngoden idelia)
Like all Chinese carp, grass carp do not breed spontaneously in ponds and are reproduced outside China only by induced spawning. It is a very fast growing fish when given proper feed, including grass. It feeds on both soft and hard weeds. Above 20–25° it can eat over 50% of its body weight per day. It eats grains and pellets, however, its feed utilization is less than common carp.
4 - BIGHEAD CARP (Aristichthys nobilis)
Very fast growing fish and reach 1.5 kg during 5 months (June–November) in its first year. It is closely related to silver carp in its feeding habits as a filter feeder. However, it filters large organisms mostly zoo-plankton such as rotifers, copepoda, and clodocerans and large algae such as Aphanizomenon or Microcystis colonies. It is much easier to catch than silver carp because it is heavier and does not leap as high as the later when the pond is seined.
5 - TILAPIA
The tilapia species have become increasingly important in fish culture, especially in warm water climates.
About 16 species of tilapia are cultured, however, only the (Oreochromis mossambicus, O.niloticus and O. aureus) have gained wide distribution.
Tilapias are tropial-warm-water fishes with optimum temp, between 25–30 c. The lethal limit between 9–13 c, depending on species. The biggest drawback of this fish in ponds is its early reproduction. They breed early (2–3 months old) and they have multiple spawning during the year. To overcome this problem it is necessary either to use species that grow fast and reach market size before they breed, or to rear monosex populations.
O. mossambicus originated on the east cost of Africa. It is omnivorous. Fry feed on diatoms planktonic algae and small crustacea. Adults feed chiefly on algae and small crustacea. Adults feed chiefly on algae and zooplankton, worms, insect larvae, insects and detritus. O. niloticus has a lethal low temperature of about 11–12 c. It feeds principally on phytoplankton (either in suspension or from the bottom), of which diatoms are an important item. Its fry also feed on macrophytic detritus, rotifers and other zooplankton, insect larvae and water mites. It digests blue green algae which is usually not digested with other fishes.
Hybrids between O. aureus and O. niloticus seems to feed more on phytoplankton and they caught much more easily by nets.
Control of Tilapia Reproduction
The main drawback to the world-wide culture of tilapia is their excessive recruitment in ponds which results in low yields of harvestable-size fish. Any method recommended for controlling the reproduction and recruitment of tilapia must be easy to apply, effective and economical. The following methods are used for controlling the reproduction of tilapia:-
rational (manual or mechanical separation)
hybrids (production of all male progeny).
sex reversal (steroid hormones such as estrogens, ethnylestradiol, estrone and diethylstilbestrol dosage of 60 mg/kg feed for 6 week at 25 to 29 C).
Use of predators such as Lates niloticus, Clarias lazera etc. at rate of 1 : 10.
Use of high stocking densities
Culture in cages
Use of irradiation, chemo-sterilants and production inhibitors.
It is evident that three methods can be used for the control of tilapia reproduction :-
- monosex culture
- the use of predators
- stocking at high densities;
The use of stock manipulation techniques, irradiation, chemo-sterilants and reproduction inhibitors have shown promise in limiting tilapia reproduction on an experimental scale but commercial scale.
Table (1) represents yields of tilapia from various culture systems including no feed or manure, manure only, feed and manure and intensive feeding without natural foods.
Table 1 : Yields of tilapia from Various Culture Systems
|Culture system||Sex/species||Tilapia stocked per hectars||Inputs||Yield per crop|
|Extensive, ponds||All male, hybrid||5,600||None||288||Lovshin (1982)|
|Extensive, ponds||All male, hybrid||10,000||Cattle manure||1,646||Collis and Smitherman (1978)|
|Semi-intensive, ponds||All male, hybrid||11,250||Castor bean meal||3,276||Lovshin (1982)|
|Semi-intensive,||All male, hybrid||25,000||Inorganic fertilizer, palm nut cake, and cottonseed cake||(1982)|
|Intensive, ponds||All male,|
|50,000||Nutritionally complete high-protein pellected feed||22,500||Liao and Ghen (1983)|
|Intensive, floating cages||All male,|
|1,000,000||Nutritionally complete, high-protein palleted feed||550,000||Liao and Ghen (1983)|
|pigs/fish||Mixed sex, hybrid||210||Pigs fed, fish not||4,560||Ghen and Li (1980)|
6 - GREY MULLET
Mugil cephalus and M. capito fry are caught in nature and is cultured in many tropical and subtropical zones, roughly between latitudes 42 N and 45.5 S where the average monthly water temperature usually does not drop below 16 C and summer temperature are over 18 C. It can be cultured in fresh water with high rates of growth and survival. Young M. cephalus, up to about 35 mm total length, are carnivorous, feeding mainly on microcrustaceans, and larger fish change their feeding habits and consume mainly microalgae and detritus. Diatoms constitute a major part of the diet of young M. cephalus, while the amount of detritus in digestive tract increases with the size of fish. This indicate that adults of M. cephalus feed mainly on the ooze of the bottom of the pond and very little from the water column. The larger bacterial population in mullet guts also enables the fish to utilise non protein nitrogen (NPN) as a source of nitrogen, similar to what can be done by ruminants. Mugil capito feed mainly on plankton. Neither M. capito nor M. cephalus can feed on cereal grains, but they feed readily on protein rich pellets or meals.
In the Mediterranean coasts several species of mullet were found to enter the rivers and steams. The fry are concentrated in schools in the quiet waters of small indentations or lagoons along river banks where they rest before swimming up stream against the current. In these places they can be caught with a small seine of mosquito net mesh. Grey mullet fry is difficult to handle and transport. They should not be touched with the hands since this removes the protective mucus coat and transport. They open the way for various bacterial and fungal infections.
Small ponds were used for preliminary nursing at rates of 30000 fry/ha and nursed for 60–100 days until reached 1–3 g/each and then transferred to lager nursery ponds for further nursing.
As maintained above, mullet are very delicate fish; they suffer and die quickly in muddy water especially when temperatures are higher than 30 C. Therefore, any transfer must be done only in the early morning, quickly with help of well-trained team and have the proper equipment (nets, buckets transport tanks, oxygen, etc…). At the end of will 1st year mullet fingerlings will be transferred directly rearing ponds for the 2nd year growth.
V- WATER QUALITY
Various sources of fresh water are used for culture (Table 2). The temperatures of surface waters (steams, rivers and lakes) fluctuate with ambient air temperatures. Surface waters may vary in quality with the weather or season, they are exposed to pollutants and contaminants, and they usually contain fish and other aquatic organisms that can be reservoirs of disease. Groundwater sources (Springs or Wells) are most commonly used for culture. Groundwater wells requires pumping. Fresh water supplies are often decides into the categories of hard water and soft water. Hard water has high concentrations of dissolved minerals or ions that often allow fish to recover more rapidly from stresses associated with culture and to increase their tolerance to some pollutants, irritants, and toxicant. Alkalinity is a measure of buffering capacity. Important physical characteristics of water include temperature, dissolved gas content and suspended solids. The manager should establish that ion and metal concentrations of the water supply fall within of EPA guideline limits (Table 3).
A- Water quality examination
The most economical method of testing water quality is to obtain 5 to 10 healthy fish and place them in a live box in the prospective waters. Generally if the fish survive for 96 hours, the water is safe and sufficient for raising fish. Although in most instances, particularly if fish are cultured for food, a detailed water analysis should be conducted by a reputable laboratory to determine the water quality. A standard water analysis includes the following parameters (Normal ranges for each will also be listed):
|1-Dissolved Oxygen:||7–10 p.p.m.|
|2-PH:||6.9 to 9.|
|3-Total Hardness:||100–500 ppm. (all combined minerals).|
|4-Carbon Dioxide (CO2):||1–10 ppm.|
|5-Ammonia (NH3):||0.1–0.10 ppm|
|6-Total phosphates:||0.1–0.3 ppm|
|7-Sulfates (Ca or Mg sulfats):||20–30 ppm|
|8-Total Iron:||0.1–0.3 ppm|
|9-Copper:||Less than 0.01 ppm|
|10-Chlorides (as Ca or Mg salts):||20–250 ppm.|
|(fresh uncombined CaCo3 when PH more than 8.5)||10–34 ppm|
|12-Methly Orange Alkalinity combined CaCo3 hardness:||1–10 ppm|
|13-Trace elements:||(lead, silver, copper, zinc, cadmium, molybidnium, cobalt, chromium) will be present in very minute quantities. Small amounts of heavy metals are toxic to aquatic life especially in soft water with low calcium.|
|14-Temperature:||Cold water 50–65°F.|
Warm Water 65–90°F
Table 2. Characteristics of fresh water source for fish culture
|Lakes and reservoirs||1. Large volume available for special or seasonal needs||1. Susceptible to climatic changes and pollution|
|2. Intake at two levels give temperature control||2. Pathogens may be present|
|Sreams of shallow springs||1. Temperatures are usually optimum for native fish||1. Highly variable chemical quality and sediment load due to climatic influences|
|2. Usually have high oxygen content||2. Susceptible to pollution|
|3. Pathogens may be present|
|Deep springs||1. Nearly constant flow. quality, and temperature||1. Oxygen may be low|
|2. Supersaturation of nitrogen|
|3. Little effect of drought|
|Wells||1. Small area needed for development||1. Yield difficult to predict before development|
|2. Advantages similar to those of deep springs||2. Pumping costs; power backup required|
|3. May deplete ground water resources|
|4. Supersaturation of nitrogen|
Source: Adapted from Buss 1979
Table 3. Water quality standards for fish culture
|Alkalinity (as CaCO3)||10–400|
|Cadmium Alkalinity<100 ppm||0.0005|
|Carbon dioxide (CO2)||0–10|
|Alkalinity <100 ppm||0.006|
|Alkalinity >100 ppm||0.03|
|Dissolved oxygen (Do)||5 mg/L to saturation|
|Hydrogen cyanide (HCN)||<0.005|
|Hydrogen sulfide (H2S)||<0.003|
|Nitrogen (N)||<110% total gas pressure|
|<103% as nitrogen gas|
|Nitrite (NO2)||0.1 in soft water|
|PCB (polychlorinated biphenyls)||0.002|
|Total dissolved solids (TDS)||<400|
|Total suspended solids (TSS)||<80|
Source: Wedemeyer 1977; U.S. Environmental Protection Agency 1979–80: Pier et al. 1982.
Note: Values are in milligrams per liter unless otherwise noted.
B) Water temperature
Temperature has an extreme effect on production. Rates and efficiencies of feedings, digestion, and growth depend upon temperature. Each species has a temperature range, bounded by an upper and lower lethal limit, beyond which it cannot survive. There is also an optimum temperature range for growth (Fig 12); Within a species, tolerable temperature range (Fig 1b), the growth rate will gradually reach a maximum level and then decline just before the upper lethal limit is reached. A fish immune response, or its ability toward disease, is best near the optimum tolerance to metabolites decreases and intensification of culture must be reduced; for instance, more water flow is required and feeding must be reduced to reduce metabolites and maintain fish health. Some of the report optimum growth temperatures for fish are given in Table 4. Temperature also affects the concentration of dissolved gases, resulting in less oxygen available for respiration. Every aerobic organism has required minimum oxygen concentration, below which it dies. When water temperature rises, excess gases cannot escape in scantly, and the water becomes supersaturated. Suspended solids can cause gill irritation, provide substrate for pathogens, and increase turbidity, resulting in less feeding and growth.
Figure 1a = Relation fo fish growth to temperature
Figure 1b : The influence of temperature on tilapia
Table 4. Estimated optimum rearing temperatures of cultured species
|COMMON NAME||SCIENTIFIC NAME||OPTIMUM TEMPERATURE RANGE, °C|
|American shed||Alosa sapidissina||7–23|
|Pink salmon||Oncorhynchus gorbuscha||9–17|
|Lake trout||Salvelinus namaycush||10–15|
|sockeye salmon||Oncorhynchus nerka||10–17|
|White bass||Morone chrysops||10–18|
|Brown trout||Salmo trutta||10–18|
|Coho salmon||Oncorhynchus kisutch||11–17|
|Chinook salmon||Oncorhynchus shawyscha||12–17|
|Rainbow trout||Salmo gairdneri||13–21|
|Atlantic salmon||Salmon salar||14–18|
|Brook trout||Salvelinus fontinalis||15–20|
|Golden shiner||Notmigonus crysoleucas||17–24|
|Smallmouth bass||Micropterus dolomieui||18–24|
|Northern pike||Esox lucius||19–26|
|Yellow perch||Perca flavescens||20–27|
|Striped bass||Morone saxatilis||22|
|Fathead minnow||Pimephals promelas||23–29|
|Brown bullhead||Ictalurus nebulosus||23–31|
|Channel catfish||Ictalurus punctatus||25–30|
|Largemouth bass||Micropterus salmoides||27–32|
|Tilapia||T.nilotica or T. mossambica||28–30|
|Common carp||Cyprinus carpio||28–32|
|Atlantic cod||Gadus morhua||1–9|
Source : Bardach et al. 1972; Coutant 1977; McCauley and Casselman 1981; and Jobling 1981.
Fish culturists should be prepared to face low oxygen condition due to hot weather, overcrowding, over feeding and excessive algae and aquatic plant growth and decay, all of which can contribute to oxygen levels exhibit the following symptoms :-
Levels of soluble oxygen below the danger point can be detected by several observable conditions in the water body:
The most common cause of oxygen depletion is excessive algae growth especially phytoplankton. Acute oxygen depletion is a condition which requires immediate attention and treatment to prevent the fish death. Oxygen depletion should be controlled by physical or chemical measures. Aeration is the best method of preventing oxygen depletion by providing additional oxygen during critical periods when dissolved oxygen availability is minimal. Mechanical aeration methods include pumping, surface spraying, agitating and subsurface aeration. Under such condition, chemical applications mist be performed to overcome oxygen deficiencies. Potassium permanganate (Kmno4) is a powerful oxidiser which can almost immediately relieve oxygen depletion by oxidising decaying plant material and other organic matter so that they consume less oxygen, thus relieving oxygen depletion and avoiding a fish kill. It relieves the B.O.D. (biological oxygen Demand). The B.O.D. is the amount of oxygen required by a water body to decompose plant and animal matter. The lower B.O.D. the more DO is available for the fish. Initial dosages of (Kmno4) should never exceed 3.0 ppm for warmwater fish and 1.0 ppm for cold water fish and more increment of 1.0 ppm can be added to 10.0 and 5 ppm respectively in fresh and cold waters.
Mechanical aeration is applicable in the following situation:
Fig 2. Average oxygen concentrations at dawn on different dates in aerated (dashed line) and unaerated (solid line) ponds.
Fig 3. Average dissolved oxygen concentrations for a 24-h period in unaerated (solid line) and aearated (dashed line) ponds.
Table (5a). Summary of fish production in aerated and unaerated ponds that were stocked with 20,000 channel catfish per hectare.
|Average weight of individual fish (%)|
|Harvest weight (kg/ha)||1,400||5,307|
Table (5a). Harvest weight of channel catfish, S-values, and net economic returns for aerated and unaerated ponds.
(a) Total feed applied: net fish production.
Table (5a). Average dissolved oxygen concentration at down in aerated and unaerated ponds stocked with different densities of catfish.
|Dissolved oxygen (mg/1)|
(5d). Channel catfish production data for unaerated and aerated ponds.
|Variable||Treatment and stocking densities (fish/ha)|
|Harvest weight (kg/ha)||2,518||8,544||11,108||12,846|
|Average weight of individual fish (kg)||0.57||0.58||0.57||0.55|
Dissolved materials constitute the ionic or chemical portion of water quality. The ph, directly related to changes in the concentration, of carbon dioxide, ammonia, and carbonate, should generally be between 6 and +9. alkalinity is a measure of the capacity to accept free hydrogen ions, whereas hardness reflects the concentration of calcium and magnesium ions both are measured in terms of calcium carbonate (CaCO3) equivalent concentration. Dissolved heavy metals, such as copper, lead and zinc can cause mortality in relatively low concentration (a few parts per million), especially in soft water.
Aeration is used to increase dissolved oxygen (DO) and to reduce gas supersaturation and concentrations of metals. Oxygen levels decrease in ponds due to biological demand of night-time plant respiration, fish respiration and uptake by benthic or other organisms. Oxygen is a major component of air, comprising 20.95%, but it is sparingly soluble in water. Dissolved oxygen concentrations are greatest at 0°C and decrease with increasing salinity, each 9,000 mg/I increase in salinity reduced the solubility of oxygen by roughly 5% that in pure water. Thus, the influence of salinity may be ignored in freshwater. Prolonged exposure to sublethal low concentrations of dissolved oxygen is harmful to fish. Cold-water species die at considerably higher concentrations of dissolved oxygen than warm water species.
Lethal concentrations of dissolved oxygen as determined in laboratory test, for several warm-water pond fish are summarised in Table 6. A similar practical assessment of the dissolved oxygen requirements of warm-water pond fish as the following:
|>1 mg/ml.||Lethal if exposure lasts longer than a few hours.|
|1–5 mg/1||Fish survive, but reproduction poor and growth slow if exposure is continuous.|
|< 5 mg/1||Fish reproduce and grow normally.|
For cold-water species, critical concentrations of dissolved oxygen should be increased by 2–3 mg/1.
Table (6) : Lethal concentrations of dissolved oxygen for several species of pond fish.
Due to evaporation, salt concentration in ponds increases constantly. Where evaporation is high (5– 7 mm/day), and there is no addition of water to make up for the less, salt concentration can double within 150–200 days. This should be taken into consideration when brackish water is used, since salinity may increase above the tolerance limit of some species. In such causes either the species composition should be chosen to confirm within increasing salinity or the water should be changed, Optimum salinity limits for growth varied greatly among tilapia species. Oreochromis niloticus proved to be the least adaptable since it showed significantly lower survival at higher salinity. (25–32 ppt) and could not adapt as well as O. mossambicus and their hybrids to brackish water and seawater (Table 7). Salinity limits for several species of food fish are presented in Table (8).
Some species of freshwater fish are sensitive to sudden changes in salinity. Fry might be killed by osmotic imbalance if they were suddenly transferred from 1.00 mg/1 to 50 mg/1 salinity. Adult fish is usually more tolerant to salinity changes. As a rule, salinity is not an important factor in fish culture.
Table (7) : Effect of salinity on growth (A.W.G) and survival % of O. mossambicus and its F1 nybrids over 3 months
|Average weight gain (A.W.G||SURVIVAL %|
F1 a ♂ niloticus × ♀ mossambicus
F1 b ♀ niloticus × ♂ mossambicus
Table (8): Highest concentrations of salinity which permit normal survival and growth of some cultured fish food.
|Labeo rohita||Slightly brakish water|
|Catla catla||Silightly brackish water|
Metabolites are the waste products of digestion and catabolism. Fish release carbon dioxide and ammonia into the water. For instance, in salmonids roughly 0.28 kg of CO2 and 0.03 kg of ammonia reproduced for every kg of food consumed. Carbon dioxide released into the water lowers the ph depending on the buffering capacity. Depending on DO, light levels of carbon dioxide. 10 m1/1 or more, can case cessation of feeding and eventual mortality. Ammonia, also released through the fish gills, is a by-product of protein metabolism (deamination). There is an ionic from (NH2), and the highly toxic un-ionised from (NH3). The un-ionised formol ammonia increases about 10-fold for each unit increase in pH, for instance, as the ph increase from 7.0 to 8.0.. High ammonia levels resulting in damage to gills and other tissues, and eventual death.
F) Buffering and toxieity Mediators
Waters of increasing alkalinity and hardness show an increased capacity to buffer (reduce the effects of) PH and ionic changes. Calcium carbonate chemical reactions use up influxes of acidic ions, and sufficient concentrations of calcium carbonate can prevent major changes in pH. Since CO2 and N113 can be harmful to fish, the manager should be aware of measures to reduce their effects. Both can be mediated by high levels (near or above saturation) of oxygen and by salt. During times of acute metabolite stress, increase in oxygen and in freshwater system increases in oxygen and in fresh-water systems increases in salinity up to 10%, reduce damage. Any steps that can be taken to reduce production intensification or temperature (slowly), and to increase flow (and cover), will tend to reduce stress.
VI) FERTILISATION AND ARTIFICIAL FEEDING
Fertilisers for fish ponds are classified as chemical fertilisers (inorganic compounds) or organic fertilisers (manure) Inorganic nutrients in chemical fertilisers stimulate phytoplankton production, thereby favoring greater abundance of fish food organisms and greater yield of fish phosphorus is a necessary ingredient of nearly all fish pond fertilisers because natural concentrations of pH in pond waters are usually too low to faster abundant phytoplankton. Though phosphorus is normally the key nutrient to successful pond fertilisation, N, K and even other plant nutrients are sometimes applied to fish ponds in fertilisers. Organic fertilisers (manure, wastes etc…) beside its effect as chemical fertilisers, it may also serve directly as food for invertebrate fish food organisms and fish. Fertilisers for ponds are similar or identical to those for agricultural corps. Nitrogen, phosphorus and potassium are termed the primary nutrient in fertilisers, however calcium, magnesium and sulphur are called secondary nutrients in fertilisers. Trace elements such as copper, zinc boron, manganese, iron, molybdenum, may also be added in minute amounts to fertilisers the fertiliser sources should be solid (urea, calcium nitrate, sodium nitrate, ammonium sulphate, superphosphate, triple superphosphate, monoammonium phosphate, diammonium phosphate and narrate of potash) in the from of pellets flakes or granules and lacquered fertilisers (liquidated ammonia and urea solutions, phosphoric acid). Many workers have demonstrated that applications of inorganic fertilisers increase phytoplankton productivity. It increases chlorophyll a concentrations (fig 4) and gross phytoplankton productivity. The algae produced by inorganic fertilisation were largely general of the following
Chloropyceae : Scenedesmus, Ankistrodesmus, Chlorella, Staurastrum, Pandorina, Cosmarium, Chlamydomonas, Nannochloris, Pediastrum, and others.
Euglenopyceae : Trachelamonas, Cryloglena, Euglena, and Phascus were also abundant and occasionally dominant.
Dinophyceae : Glenodinium, Hemidinium and Peridinium, were often present but never in large numbers.
Chroococcaceae or blue-greens : Coelospharium and Microcystis abundant for limited periods
Diatoms: Which are so abundant in the marine plankton, were relatively unimportant in these fertilised, fresh water ponds.
Fertilisation of pond as was practised, therefore, would appear to result in production of an unpredictable mixture of algae. It would not appear possible that all these forms of algac are equally desirable for the production of fish foods. Certainly, they are not equal from the standpoint of growth habits, appearance, and odours therefore, there is urgent need for a critical study of algae species to determine those that are most desirable for fish production, and to determine the conditions favouring production of these forms. We need to learn enough about the ecology of various species to seed the ponds and to maintain an abundance of the more desirable forms. It should also be possible to keep many undesirable species under control. Nevertheless, fertilisation is an effective way of increasing phytoplanktop primary production and enhancing fish production. The increase in primary productivity following fertilisation usually results in greater zooplankton abundance.
Fig. 4: chlorophyll a concentrations in fertilized and unfertilized ponds.
The following zooplankters were found in ponds:
|Daphnidae||:||Dophnia ambigua, Dapnia Spp. and Ceriodaphnia Spp.|
|Cycloopidae||:||Mesocyclops edax, Cyclops exills Tropocyclos prasinus|
|Rotifera||:||Platyias quadricornis, P. platulus, Monostyla bula, Bracluonus angularis, B. harvanaensis, Trichocera Spp. Anuraeopsis fissa, Keratella Spp., Polyrathra Sp. Lecane Sp. Filina Sp.|
Peak density of rotifers in a fertilised pond was 136,000 per litre, crustacean abundance reached nearly 1000 per litre, however, in the unfertilised pond the numbers were 10 000 and 100 per litre respectively.
Fertilisation also increase the benthos production which composed primarily of the following organisms:
|Diptera||:||Chironomus tentans, Procladius Sp, Ablabesmyia Sp., Tanydtarsini Sp. Glypotendipes Sp., Microtendipes Sp. Chironomus Sp.|
|Trichoptera||:||Ocetis Sp. and Polycentropis Sp.|
|Ephemeroptera||:||Caenis simulans and Callibaetis ferrugineus.|
|Zygoptera||:||Ishneura Sp. and Enallagma Sp.|
|Anisoptera||:||libelluidae and Gomphida|
Fertilisation with nitrogen and phosphorus resulted in a considerable increase in the benthic biomass. Aquatic macrophytes are often abundant in shallow lakes and ponds with relatively transparent waters, however, many species disappear from aquatic ecosystems as turbidity increases. Since fertilisation increases the abundance of plankton and decreases the light penetration, macrophytes are often not important in fertilised ponds, macrophytes are responsible for serval ecological problems in fish ponds as the following:-
Some common pond weeds are:
For the fish farmer the best index of the efficiency of a fertiliser treatment is not the effect on the plankton, bottom fauna, or on the rooted vegetation, but on the increase in fish crop over and above natural productivity. The natural productivity of water vary greatly, and some unfertilised ponds may be just as productive as fertilised ponds. However, fertilisation will generally cause large increase in fish production.
The following fertilisation program proved effective in increasing fish production in many ponds:-
Quantities of primary nutrients per application for the fertilisation program are g kg/ha of N, g kg/ha of P2O5 and 2.2 kg/ha of K2O. Over a growing season 8–2 applications will be made for a total of 72–108 kg/ha of N and P2O5 and 18–26 kg/ha of K2O.
Fertilisation resulted in large increase in fish yields over those of the controls (Table 9).
Table (9): Net production (kg/ha) of three species of fish in ponds to which different fertilisers were applied.
|T. auraea||(2 week × 45 kg/ha)||651||947||930|
To illustrate the amounts of primary nutrient needed from the fertiliser the following example will be followed:
A. 1.5 pond could be fertilised at rate equivalent to 50 kg/ha of 8–20-0 fertilize at a rate equivalent. to 50 kg/ha of 8–20-0 fertilise using ammonium sulphate (20% N) and ammonium phosphate (11% N and 55% P2O5) instead of mixed fertiliser.
|N||=||50 kg/ha × 0.08 × 1.5 ha = 6 kg|
|P2O5||=||50 kg/ha × 0.2 × 1.5 ha = 15 kg|
The amount of ammonium phosphate required for 15 kg of P2O5 = 15 kg 0.55 = 27.3 kg
The amount of N required from ammonium sulphate = 6–3 = 3 kg N
The amount of ammonium sulfate = 3×100/20 = 15 kg
Therefore the mixture will be 15 kg ammonium sulfate and 27.3kg ammonium phosphate.
A 11.2 ha pond must be fertilised with N at 4 kg/ha and P2O5 at 8 kg/ha using ammonium nitrate (33–5% N) and superphosphate (18% P205), the required amounts of N and P2O5 are:
N = 4kg/ha × 11.2 ha = 44.8 kg N
P2O5 = 8 kg/ha × 11.2 ha = 89.6 kg P2O5
The necessary amounts of fertiliser are: 44.8 kg: 0 (0.335=134 kg ammonium nitrate 89.6 kg (0.18 = 498 kg superphosphate.
Organic fertilisers such as: grass, leaves and reeds; liquid manure from livestock holding facilities, sewage water industrial waster from distilleries, leather and milk factories sugar refineries and fish canning plants and many other waster are used in fish ponds. There by-products may serve as direct sources of food for invertebrate fish food organisms and fish or they may decompose releasing inorganic nutrients that stimulate phytoplankton growth. Organic fertilises are especially efficient in increasing the abundance of zooplankton and benthic organisms. Table 10 show the concentrations of fertiliser constituents in fresh dung of farm animals.
Table (10): Fertiliser constituents in fresh manure of selected farm animals.
|Average composition (%)|
The abundance of the phytoplankton and chiromid larvae in ponds treated with manures are shown in Table (11).
Table (11): Effect of different manure on the abundance of phytoplankton in water and chionomid larvae in mud of ponds.
Organic fertilisers resulted in appreciable increase in blue gill production (Table 12) in ponds.
Table (12): Effects of organic fertiliser on bluegill production in ponds.
|Manure||Manure (kg/ha)||Fish (kg/ha)|
Chicken droppings or liquid cattle manure were applied at rate of 5 kg/ha dry matter 5 days per week.
In many areas; chemical fertilisers are either unavailable or too expensive for use in fish ponds. In these areas, the use of manure should be encouraged because applications of organic materials may be the only available means of increasing fish production. Even in technologically advanced nations, it would be wise to use available organic wastes to increase fish production. This particularly turns the waste into useful production and conserves chemical fertilisers and feeds.
Problems with acid-bases relationships in fish ponds can often be solved by liming. Application of liming materials is not a type of fertilisation. Liming may be best viewed as a remedial procedure, necessary in some ponds, to permit normal responses of fish populations to fertilisation and other management procedures.
There are three basic types of ponds that respond favourably to liming:
Liming materials release ions that contribute to total alkalinity and total hardness in equivalent proportion, therefore, liming usually results in increases in total alkalinity and total hardness, the results in Tables 13, 14 show that liming of tilapia ponds improved tilapia production.
Table (13): Effect of limestone on yields of tilapia reared in pond.
|Limestone (kg/ha)||Yield (kg/ha)|
Table (14). Net production* of T.aurea in limed and unlimed ponds.
* All ponds were fertilised twice monthly (March–Sept.) with 45 kg/ha of 20–20-0.
B) ARTIFICIAL FEEDS AND FEEDING
Artificial feeds and feeding is one of the principal methods of increasing production in fish cultivation. It allows high stocking and, from this, the better use of natural food which itself profits from the unconsumed artificial food and the excrements of a denser population-which in turn acts as a fertiliser. The practice of more or less intensive feeding is simply an economic question. This depends on the cost of feed used and their conversion rate. The rate of food conversion depends not only on the feed distributed but also on a number of other factors such as the density of the stocking, individual weight, age class of the fish, their state of health, the temperature of water, the methods of feeding (quantity, the spreading and frequently of distribution).
The preparation of fresh feed for fish is quite simple. According to each individual case it is ground, soaked, dried or cooked. In many instances, the feed can be distributed without preparation. It is only ground when the grain is large and in so far as it will be distributed to fish of small size. The need for soaking is to stop the ground food floating on the surface when it is distributed, and thus risking loss. Germination of the grain is accompanied by an enrichment in vitamins. Drying of food is not always beneficial. It aids conservation and lowers the conversion rate but this latter advantage only means compensation for a loss of water. Cooking may be necessary to stop the decomposition of certain animal or vegetable materials. These cooked materials are then passed through a meat mincer.
The total amount of feed to be distributed will be as the following:
Amount of feed/ha = growth of fish due to artificial feeds/ha N feed conversion ratio.
Distribution rules to be followed:-
Feed distributed by the following methods:
Generally, fish require the same nutrients, minerals and vitamins which are required by the terrestrial animals for growth and maintenance. Fish obtain their nutrients from the natural for or from the artificial feeds. When the natural food is absent, prepared diets must be complete, contain the necessary nutrients and other additional components such as minerals and vitamins. Deficiency of some of these components cause depression in growth of fish and may lead to diseases.
Natural food does not necessarily supply the various food components such as carbohydrates, proteins or vitamins in the same proportions as they required by fish. With the decrease in the ratio of natural food requirements, the deficit of these components does not have to be the same. As example, common carp require a diet containing 36–40% crude protein and rich in energy. Proteins comprise about 50–60% of the calorific value of natural food. This means that when carp fed only on the natural food, they use part of the protein for energy. When natural food become scare, it is energy and not protein which is first lacking. Supplementary feed should cover this energy deficit first and need contain mainly sources of energy. Cereal grains rich in carbohydrates are usually used as feed at this stage.
With increasing stand crop of carp, a point is reached where natural proteins are no longer sufficient to sustain maximum growth. Supplementary protein should be added to the diet (Fig. 5).
A complete diet, or a rich in protein, is “diluted” with a diet rich in carbohydrates. The most common pellet diets are:
|(1) Cereal mixture||: a mixture of cereals contained 10–111% CP.|
|(2) Diet containing 18% CP||: 80–90 wheat|
|5 – 10% fishmeal|
|5 – 10% soybean meal|
|(3) Diet containing 25% CP in a pelleted form:|
|60 – 70% wheat|
|15 – 25% soybean meal|
|3 – 4% oil|
The feeding regime will be as the following:-
|a - up to 700 kg/ha||sorghum only|
|b - 700–1200 kg/ha||75% sorghum|
25% pellets (25% CP).
|c - 1200=1500 kg/ha||50% sorghum|
50% pellets (25% CP).
|d - 1500–1800 kg/ha||25% sorghum|
75% pellets (25% CP)
|e - Over 1800 kg/ha||pellets only (25% CP).|
Energy content and source are also important. Energy is required to cover the maintenance and growth requirements Carbohydrates usually comprise an appreciable part of a warm water fish diet. Too little energy from carbohydrates or fats in the diet causes the utilisation of the protein for energy rather than for growth. On the other hand too much energy, may lead, especially in carp, to the accumulation of body fat which reduces market quality. At lower dietary protein level, carbohydrate from cereal grains should be supplied the energy (3 kcal ME/g feed). However, with higher dietary protein diets, fats, which contain two and one half-times more energy than carbohydrates should be used to increase the energy content of the diets. Fats also supplying diets with the essential fatty acids (linoleic and linolenic acids).
Pellet diets of carp should be stable for 20–30 minutes because carp is a lazy feeder and usually take 20–30 min to eat their food. The use of demand and automatic feeders means that water stability is less important since it seems that the fish take up the pellets as they fall from the feeder.
High water stability of pellets can be achieved in two ways:
Figure 5. Average growth rate under different feeding and fertilisation treatments Heavy line represents potential physiological growth rate when food is not limiting. Broken lines note limiting. Broken lines note growth rate above the critical standing crops : (1) no fertilisation or feeding; (2) chemical fertilisation, no feeding; (3) chemical fertilisation, feeding with sorghum; (4) chemical fertilisation, feeding with protein-rich pellets.
High temp. With moist mix.. gelatinises part of the starch and turns it into a binder and a partly waterproof coating.
Incorporating very fine particles in the mixture seals the small hollows in the glossy surface, through which water can be absorbed into the pellet, thus increasing stability. Agar or alginates usually used as binders, however, it is not applicable for commercial production. Wheat gluten (5%) is a much more widely used binder. Also ground wheat do the same effect. To achieve good binding pellets, diets should contain at least 60% wheat.
At high densities of fish, natural food plays a lesser role so an increase in the amount of supplemental feed is required. This also causes a larger difference in the feeding level (Table 15), when calculated as a percentage of live weight, between small and large fish. At high densities it is better to calculate feeding level per individual fish and multiply the amount by fish density.
Tableau (15) Feeding chart calculated by the individual weight of fish (g/fish/day)
|Density per Hectare|
|Fish Weight||with Carp||Monoculture||Fish Weight||with Carp||Monoculture|
From Market (1975)
S = sorghum;
P = pellets;
T = total;
% percentage of live body weight.
VII) MANAGEMENT OF POND FISH FARMING
The success of a fish farm is measured by its profitability which depends on the yield and market price on the other hand cost of production in other hand. This will be the following factors:-
Most of these factors are interdependent. Therefore it is obvious that these relationships should be carefully thought out. Ecological and economical aspect should be considered in planning of pond fish production framing.
A) ECOLOGICAL ASPECTS
A. 1). Fish Growth Performance
Fish growth performance is affected with the following factors:
- genetic characteristics
- physiological state
- water quality
- metabolic load
- available oxygen
- available food.
Figure shows that growth rate of carp increased with different feeding and fertilisation methods, however, the absolute potential growth rate does not increase in a direct proportion to the increase in weight of the fish, but at a rather slower rate. This means that relative growth rate (growth per unit weight %), decreases with the increase in the weight of the fish.
The density can be used for regulating the average growth rate of fish and, therefore, the length of the raring period. For a given feeding regime, the size of the fish alone will affect growth rate below the critical standing crop. The larger the fish, the more it will grow and stocking rate will make no difference. However, when stocking rate is increased, the carrying capacity will cause a lower fish weight and the growth rate above the carrying capacity will be reduced. The most economical stocking rate is not necessary that which results in the highest average growth rate, but rather that which results in he highest yield per unit area. Overstocking will lead to the following:
Fish density will depend on the following :
This means that when food is available, increasing density will result in increased fish yield;
Polyculture is a way to utilise better natural productivity with a concurrent increase in density with fishes of different species.
Aeration makes it possible to further increase stocking density. In intensive polyculture where stocking density is about 10.000 to 12.000 fish/ha, auxiliary aeration is used as necessary and only during the critical morning hours. When stocking density reaches 15,000–20,000 fish/ha, either in monoculture of carp or tilapia, or polyculture of these two species. Because of the high densities, aeration is obligatory at least during the greater part of the night and early morning. Yield up 20 ton/ha/year can be obtained by the late method (Fig. 1).
Polyculture is a way to utilise better natural productivity with a concurrent increases in density with fishes of different species. Polyculture systems include 3000–6000 carp, 3000–4000 tilapia, 500– 1500 silver and about 1000 mullet and about 300 grass carp/ha, so that the total stocking density reaches from 7000 to 9500 fish/ha.
In order to achieve increased production, the species stocked must have different feeding habits and occupy different trophic niches in the pond. the synergetic effects of polyculture will be explained as the following:
The ratio among the different species and their stocking densities are of prime importance in polyculture and these can be determined in two ways:
Monoculture is the only method of culture used in running water systems and in cages where the supply of natural food is limited. A problem which occurs in tilapia ponds is the development of filamentous algae, which becomes a nuisance when the fish harvested. In order to overcome these difficulties, number of common carp are introduced into tilapia ponds (up to 20%). Also tilapia introduced to common carp to consume part of accumulated organic matter and thus case the oxygen demand.
Fish density and its control are of prime importance in pond management. Increasing the yield per unit area (and unit volume of water) is possible only by intensification, that is, by increasing stocking density on the one hand and the inputs (fertilisation, feeding, improved diets) per unit in other hand. The interaction among the following factors will determine the desirable stocking density:
(1) Weed control
Three vegetation zones can be identified in fish ponds:
the Embankment Zone : hairly willow herb, (Epilobium hirsutum L), (Inula viscosa, Lycopus europeus, Rubus saguineus and Apium graveoleus.).
the Emergent Vegetation Zone: (plants grow in water depths not exceeding 50 cm.) reed (P. communis), cattail (Typha latifolia) and T. augus and rush (Juncus Sp.).
Aquatic Zone (50–200 cm): hornwort (Ceratophyllum Sp.) water milofoil (Myriophyllum Sp.) pond weed (Potamogeton Sp.) duck weed (Lemna Sp.) broad leaved pond-weed (Potamogeton natans), amphibious bistort (Polygonum amphibium) Filamentous algae (Spirogyra, Cladophora, Pithophora…) Microalgae. Blue green algae etc…
Weeds of the emergent and aquatic (submerge vegetation) zones are the most noxious. The extent of the former zone may, however, be reduced by constructing ponds so that the shallowest pat will be at least 80 cm deep. Three methods of controlling vegetation are practised in fish ponds (1) mechanical, (2) chemical and (3) biological.
(2) Mechanical control
It includes moving, dredging and burning using either simple instruments or mechanical weed cutters. The main drawback of all these methods is their low efficiency in keeping ponds clear of vegetation, for moving and burning have to be repeated several times a year. Also owing to increasing labour costs and the shortage of manpower in many countries they have become uneconomical.
(3) Chemical control
Herbicides used as chemical control for:
Herbicides used to cause sterility of the soil in small ponds, limited areas of embankment, of for spot treatment of relatively clean areas. among the products available:
- CMU (3-(P-chloophenyl)-1, 1-dimethyl urea);
- Simazine (2-chloro-4, 6 bis (ethylaminc, triazine).
- Products containing sodium chlorate.
CMU is very expensive.
The most widely herbicides used in control foliage are:-
- 2,4-D as amine or esters
- 2,45-T (2,4,5-Trichlorophenoxy acetate)
- Dalapon (2,2, Dichloropropionic Acid)
(1,2 & : 1', 11'-c) Pyrazidiin ium Salt)
- Copper sulphate (CuSO4 1–3 ppm).
- Sodium Arsenite (NaAsO2).
- Diorex (1–2 ppm).
There is a trend to reduce the use of herbicides in reducing weeds in fish ponds though the pope management of fish ponds (depth of water, planting of embankments with grass etc..) and biological control.
(4) Biological Control
Weed on embankments can be controlled either by grazing or by preventing their emergence with cover of grass, also through using herbivorous fish such as grass carp and T. endallii. Or by suing heavily fertilised ponds with chemical fertilisers.
B) ECONOMICAL ASPECTS
Production economics of fish pond farming will focus on the nature or costs:
which are important concepts that can be used to analyse and improve returns to production processes. The manager uses resources, which are inputs, to achieve a goal or produce a product, the output. The inputs can be fixed or variable. All culture systems have both fixed and variable inputs. Fixed such as facilities and available water flow or volume of water and variable such as feeds and labour. The least cost, or substitution principal is used for decision making when there is some fixed or required level of production and some degree of substitution among the inputs. The use of fixed and variable costs in decision making, those costs must be considered in terms of time.
The manager should compute interest for the business on equity capital as well as on borrowed money. Computation of interest on the capital allows to see and manage the opportunity cost of that capital.
VIII - RECORDS FOR MANAGERIAL ANALYSIS
Detailed, proper and complete records are essential to fish pond production management. Records summarize operations and allow the manager to analysis the effectiveness for resource use to formulate intelligent plans. But besides measuring success and aiding planning, records are needed to comply with tax reporting laws, to obtain credit, and to make comparisons with current goals. Records provide an organized method of talking a complete inventory. They can be used to show growth, and that is important to creditors. Computer software for record keeping is readily available and useful for all types of production systems, but may especially enhance the management of more complex operations. Computers allow the handling and quick retrieval of great volumes of information. The record system should include:-
A system of complete records provides a tool that helps minimise errors in decision making.
A cash flow statement budget or plan, based on records from previous years is a summary of the culture system's receipts and expenses used to project the operation's capability to meet demands over a specified period of time. It can be used to minimise interest by helping the manager to borrow as needed rather than annually, and it provides essential information for capital investment decisions. A financial statement consists of listing of assist, often shown on the left - hand side of a from or sheet of paper, and a listing of liabilities, often shown beside or on the right hand side of the form, and the “bottom line” difference between assists and liabilities, the net worth. Net worth is the owner's equity, or that which the owner truly owns. Assets usually fall into three categories:
- current (within one year)
- working (within two - five years)
- long term (more than 5 years)
and Liabilities are likewise categorised and include credit for feed and supplies and loans for equipements and mortgages.
IX - SELECTED REFERENCES
1- Aquaculture Management, 1989
ED. MEADE, J.W. AVI BOOK,
Published by Van Nostrand Reinhold, New York, 175 Pages.
2- Commercial fish Farming, 1981
ED. HEPHER, B. AND Y. PRUGININ,
Published by John Wiley & Sons New York, 239 Pages.
3- Principles of Warmwater Aquaculture, 1979,
ED. STICKNEY, R.
Published by John Wiley & Sons. New York, 375 PP.
4- World Fish Farming, 1977,
ED. BROWN, EE. AVI
Publishing Company, Inc.
5- Textbook of Fish Culture-Breeding and Cultivation of fish 2nd Ed. 1986.
ED. HUET, MARCEL.
Published by Fishing News Books Ltd. 439 Pages.
6- Principles of fish Production, 1986
EDT. A.M.NOUR, EGLAL OMAR, F. ABD EL KARIM AND A. MOHAMED.
Univ. Pupl. 634 Pages.
7- Feeds and Nutrition (complete).
EET. M.E. ENSMINGERF AND C.G. OLENTINE, JR. IST, ED.
The Ensminger Publ. Co. 1978. Clovis, Califonis 93612 USA.
8- Fish Nutrition, 1991 2nd Ed.
Academic Press, 9nc - London, 753 pages.