MANAGEMENT OF FISH IN THE POND Introduction In aquaculture management we talk about; Managing the pond to be able to hold both the water and the fish in good condition; and Managing the fish to be able to survive and grow well to reach harvestable size quickly. Pond management involves maintenance of the pond structures; conditions to hold the fish, to maintain the amount and productive condition of the water. 1 Pre-stocking preparation of the pond The pond may be newly constructed and has not held fish yet. It may be a pond from which fish has just been harvest. 2 Preparation and liming of a newly constructed pond a) Final finishing of construction This Section describes the preparation of the pond for stocking. b) Applying liming: The new pond is drained completely if it had been filled with water. Lime is applied to cover the entire pond bottom. Why apply lime; Lime is applied to the new pond for two main reasons: to kill off germs and parasites that may be in the water and to reduce the acid level of the water. When acid levels in the water are high, fish may stop eating and could die. High acid levels prevent the growth of natural food for the fish. White builder s lime is commonly used in ponds. It is available in hardware shops in 25kg bags (see figure below). A 25kg bag of white lime used in ponds. This bag is adequate for a 400m 2 pond. The lime is a white powder. Other limes are not as good.
A bag of lime The methods of applying lime Most often the lime is spread very thinly over the surface of the pond bottom in powder form, see figure The lime used as powder, when applying lime the nose, mouth and hands must be protected. Lime is corresive. The lime can also be mixed in water Applying lime to a pond
Pond banks Collapsing The banks may also be damaged by certain fish that dig into the bank (eg. the Catfish ("mmale") and Carp) The mud at the pond bottom may accumulate to levels that begin to affect the productivity of the pond. Ponds may develop leakages through the dyke due to the growth of tree roots or the digging by some fish. Some eggs or fry of previous harvest may remain in the pond and may contaminate the new stock In the pond mud some disease causing agents and parasites may accumulate that may infect the new fish stocked. For the reasons above the following treatments and repairs are required 3 Draining and repairs The pond should be drained completely (see figure below). A drained pond with farmers removing mud from the bottom pond
Fish has been harvested and the pond has been drained. See the amount of mud accumulated. The Mud may be allowed to dry until it cracks. This makes it lighter and easier to remove. The mud is removed from the pond bottom. The lighter surface mud, rich in manure, should be carried away and used to fertilize gardens (see figure below). The firm part of clay is used be used to repair the broken banks and dykes. Pond mud being spread as manure in gardens Repairs of the broken banks and dykes and of any leakages through the dyke are made (see figure below) Repairing pond dyke Lime is applied to the pond bottom to kill off any remaining fish eggs, fry, parasites or any other disease agents. 4 Filling the ponds with water Fixing pond structures After liming the ponds, the structures are fixed, it can now be filled with water. Attention must be paid to repair pond structures, the inlet and outlet. The emphasis is to screen the mouths of the inlet pipe to prevent entry of unwanted fish. And to screen the outlet pipe to prevent loss of fish. Filling pond with water Water is filled into the pond from the supply channels until the correct level is achieved. 5 Applying fertilizers Fry should never be stocked before sufficient quantities of natural food have developed in the pond. Fertilizers are applied into the pond to make it possible for natural foods of the fish to grow. The natural food of most fish include: The tiny green plants that we cannot see with our naked eyes, except with the aid of a microscope. Their presence is indicated by the green colour of the water. These tiny green plants are known scientifically as "algae"
A wide variety of tiny animals that we cannot see with our naked eyes, except with the aid of a microscope. Later on other larger animals (worms, insects, snails, etc) that feed on these small plants and animals also grow and many of them are food for fish. 5.1. Types of fertilizers Fertilizers' commonly used to fertilize ponds are categorized into two: organic fertilizers (also called "manure") and Inorganic or commercial fertilizers. The sources of organic fertilizers (manure) include: chicken and duck dropping, cow dung and urine, pig dung and urine, goat dropping, compost, sunflower cake (see figure below) Compost is a mixture of rotted plant and animal materials. Plant nutrients are readily available. Sunflower cake is the material left after oil has been squeezed out of sunflower. The material is rich in protein and oil. It is good both as food for fish and as a fertilizer. Sunflower cake Inorganic fertilizers : The inorganic fertilizers are expensive and may only be affordable by the largescale commercial farmers.these fertilizers include: NPK, DAP, TSP, SSP, Urea. All these are in powder (or granular) form. They are available in shops selling Agricultural chemical. 5.2. Application rates of fertilizers 5.2.1. Organic manure: The application rates of manure can be summarized as shown in the table1
Table 1 Application rates of different manure types. Source of manure Application rates for different pond sizes 200m 2 pond 500m 2 pond 1000m 2 pond Chicken and duck manure (when dry) 6 kg/week (1/2 jerry cans/ week) 1 5 kg/week (1 jerry can/week) 30kg/week (2 jerry cans/ week) Pig manure (wet) 6 kg/week 1 5 kg/week (1 jerry 30 kg/week (1/2 jerry cans/ week) can/week) (2 jerry cans/ week) Cow dung 20kg/week (1 jerry 50kg/week 100kg/week 30kg/week can/week) (2V-2 jerry cans/ week) (5 jerry cans/ week) (2 jerry cans/ week) Note the following: Chicken and duck manure is the best and Cow dung is the least effective, in the order shown above. The chicken and duck manure release more nutrients, and therefore more effective, and the cow dung releases nutrients less efficiently. Chicken food is rich in nutrients, yet the birds do not extract all these nutrients. Some of the nutrients are lost in the droppings which can be used by fish directly as food. The wet/moist manure is best 'v. During drying some nutrients (especially Nitrogen) fume out and reducing the effectiveness of the manure Therefore, it is recommended to apply the manure wet or to cover it before use. Inorganic fertilizers : For DAP, apply 300g to a 400m 2 pond daily until the right level is attained (see figure 4). For the SSP or TSP - Urea combination, mix 150g of SSP or TSP with 150g Urea for a 400m 2 pond until the right level is attained.
5.3. Methods of application of fertilizers 5.3.1. Organic manure: The manure is collected fresh. It may be covered under tree shade kept moist by regular watering. It may also be applied into the pond directly. The manure can also be placed in a crib constructed of sticks at the corner of the pond (see figure below) a) Cow dung: It is not necessary to dissolve the manure, like this lady is doing. (See picture below) A woman dissolving cowdung NB it is not neccessary to first dissolve the manure. It is enough just to place manure into a crib at one corner of the pond, where water enters from. Keep adding the manure and turning it over every after about a week. Note how the water is turning green. Do not make it too green. b) Chicken and duck manure: Chicken manure is collected fresh and kept covered under shade.the dropping is applied by spreading it directly all over the pond in wet form using the rates given in Table 1. The chicken manure can also be put together with other materials in the crib. c) Integrating poultry and livestock to the pond Both the poultry (chicken and ducks) and livestock (pigs and cattle) can be integrated into the ponds to make it easy to apply their manure into the pond (see figure below).
The chicken and duck houses can be constructed above the pond. The droppings fall directly into the pond. This method saves labour costs. A poultry house above a pond at Kaburu farm in Tororo. The pig house can also be constructed at the edge of the pond so that the urine flows and the manure is washed directly into the pond. The cow shed can also be constructed at the edge of the pond. d) The rates of stocking animals and birds in the integrated system To avoid adding excessive manure, the number of birds and animals to be stocked in the houses should be the right number. Table 2 gives the right number of animals to stock in the integrated systems. Table 2. Stocking rates of birds and pig Bird /Animal type Chicken Duck Pig Stocking density Up to 1 bird per 1,OOOM 2 of pond Up to 1 bird per 1000M 2 of pond Commonly 6 pigs per 1,OOOM 2 of pond e) Treatment of compost The plant materials (usually grass) are dried. It is then mixed with rotting animal manure, if available, and soil. Adequate quantity of water is added and the mixture is covered to retain the water. It is best to make the compost in a hole in the ground and under tree shade. The mixture is kept moist by sprinkling water over it daily until the process of fermentation is complete, and the compost is no longer producing gases. The entire process takes a month to complete. The compost is placed in the crib in the pond
f) Application of sunflower cake The dry cake is weighed (10kg/500m 2 ) and soaked in little water overnight. The quantity of water should be just enough to make a thick liquid The following day the soaked cake is sprinkled all over the pond surface. The process is followed every week Farmer applying sunflower cake into the pond. 5.3.2. Methods of application of inorganic manure a) Combinations of inorganic fertilizers (see figure 4.15) The fertilizers are intended to supply two main nutrients required for growth of natural food for fish. These are Nitrogen (N) and Phosphorus (P). SSP and TSP are sources of Phosphorus Urea mainly supplies Nitrogen DAP supplies both Phosphorus and Nitrogen Because DAP has both N and P, it is usually added alone into the pond. SSP of TSP for P must be applied together with Urea for N to supply the pond with both nutrients. b) Application rates of inorganic fertilizers Table 1 gives application rates; These are the standard rates but in some cases these rates may be more than enough (or not enough) depending on the nature of the soil or water. What is important is to know when the fertilizer applied is not enough or when it is too much and what to do.
Inorganic fertilisers c) Methods of manure application The right quantity of the fertilizer is calculated according to ponds size The calculated quantity is weighed. The weighed fertilizer is dissolved in just enough water to dissolve it The solution is sprinkled in the pond water (same way sunflower cake was is applied) 6 How to determine and maintain the correct level of fertilization Fertilizers are applied into the pond until the water is green to the right level. The green indicates the amount of natural food (algae) for fish especially Nile Tilapia (see figure 7). However the level of green must be controlled, excessively green water is not good for fish (see figure 8). The little plants may prevent entry sunlight intothe water and up and may reduce fresh air in the water for fish, suffocating the fish to death. ; A well fertilised Pond over fertilized pond
The level of fertilization required is determined by measuring light penetration. The conventional instrument is called the "seiki disc" Seiki Disc The circular metal disc has black and white segments to make it visible. It is held by a string fixed from the centre. The disc is lowered into th water until it can no longer be seen. The distance at which the disc disappears is measured on the string. If this distance is 35 cm the pond is well fertilised. If it is less then the pond is over fertilized, in excess of 35cm, it is under fertilised. But simple methods can work as well; A white piece of paper is held in the palm, the hand is lowered into the pond water until the paper just disappears from sight. The water level on the arm when this occurs is marked (see figure below). Owori Local seiki disc If the paper disappears when the water level is at the elbow, fertilization if is good. Below the elbow, the pond is over fertilized (see figure below), above the elbow it is under fertilized. Once the pond has been fertilized and it has turned to the required green, it is ready for stocking. Over fertilized
7 FEEDING THE FISH Different fish prefer different types of food. Fish in ponds can be fed in many ways shown below i. Feeding by fertilizing the pond ' ii. Feeding the fish on large plant materials iii. iv. Feeding the fish on other fish Feeding fish on artificial feed 7.1. Feeding fish by fertilizing the pond. Pond fertilization is the cheapest way of feeding fish. However, the production of natural food through fertilization is usually limited by the environment. Production of fish by fertilization only is low (maximum 0.3 kg/m 2 per year). This low production can only be fit for subsistence purposes and for household nutrition.
7.2. Feeding fish on larger plant materials Among the farmed fish it is mainly Tilapia zillii that feeds effectively on larger plant materials. However, the plant materials must be soft. The easiest way to feed fish that feeds on larger plant materials is to plant vegetables in the dyke space around the pond (see figure 4.21). The younger, softer plant parts are harvested from the garden and placed in the pond.
Soft plants like dodo etc. A pond with soft plants on the banks 7.3. Feeding fish on other fish This case so far occurs only in the farming of the African catfish ("mmale"), which is a predator. The easiest way to feed such predators is by stocking the predator ("mmale") together with a large quantity of the prey fish (Nile tilapia). The prey fish should be stocked at a larger size than the predator so that they are not eaten. They are then able to continuously reproduce juveniles for the "mmale" to feed on. In that case, the pond should also be fertilized for t Tilapia. 7.4. The use of artificial feed For commercial production of more than 0.5 kg/m 2 per year, the farmer must provide good quality feeds either to supplement the fertilization or as the only source of feed. Feeding is more expensive than fertilizing a fish pond. Therefore, it is only economic to feed fish with expensive feed if the farmer is producing intensively for markets that offer good price for fish. The higher the market price of fish, the more the farmer can afford to feed fish with expensive feed. For middle level of production and fish price, the farmer should use limited quantities of the expensive feed only to supplement fertilization. 8. The nutrients obtained from feed Like all other animals and humans, fish requires the following nutrients for the purposes indicated (see table). 8.1. The different nutrients and their purposes
Nutrient Purpose Protein Body building Fats (lipids) Energy Proper functioning of the body Starch Source of energy Minerals Body structure Normal functioning of the body Vitamins Protect the fish from diseases 8.2. The common sources of these nutrients The feeds must be good quality but at the same time not too expensive. Therefore, the sources of the nutrients should be cheap and easily available. Sources of nutrient for making feeds are called "feed ingredients" The common local sources of important nutrients for fish feeds are mainly agricultural wastes, agricultural by-products, feed processing by-products or by-products from slaughter houses. These include the following. 8.3. The different nutrients and their local sources Nutrient Sources Nutrient Sources Protein Mukene, Dried blood meal, sunflower cake Soya oil cake Cotton seed cake Minerals Wheat bran Crushed egg shells Crushed snail shells Shops selling animal feeds Fats Mukene sunflower cake Simsim cake Cotton seed cake Rice bran Soya oil cake Vitamins Shops selling agricultural supplies Shops selling animal feeds Available in green plants in fertilized ponds 8.4. The nutritional requirements of fish The requirements of each of the nutrients by fish as percentages of the total feed varies from fish type to fish type (see table below), It is also different for the different ages of each fish. In general the nutritional requirements are as follows. The requirements of different nutrients by fish at different growth stages
Nutrient Fry Juvenile Grower Brood stock Protein 42-39% 37% 35% 37% Fats < 8% < 8% 6% 5% Starch 25% 25% 25% 25% Mineral 1-3% 1-3% 1-3% ' 1-3% Vitamin < 1% < 1% <1% <1% 9. Sources of quality fish feeds At the moment the Research Station at Kajjansi is testing the feed it has developed. The feed is promising but the testing is still continuing. Soon the feed will be available in the markets. See figure 2a To the left is the feed made into pellets for the bigger fish and to the right are the crumbs for the smaller fish. For the moment farmers can use the feed as supplementary only. 9.1. How to make the feed Selected feed ingredients are mixed in such proportions as to provide the nutrients close to the percentages shown in Table above. The common feed ingredients; Common ingredients used to make fish feeds. Top: Left to Right: Maize bran full of husks (bad), cassava powder, empty shells of snails. Bottom: Left to Right: "Mukene powder (ground), fine maize bran (sieved), wheat bran (sieved & ground), sunflower cake, Vitamin premix (for chicken). 9.2. How to make home-made fish feed The following food combinations using the above ingredients have produced good growth. One way a fish feed can be made from ingredients
Ingredient The number of cups of each ingredient to make 1 00 cups of Mukene. 40 Maize bran 40 Cassava 9 Sunflower cake 9.5 Vitamin pre-mix (poultry) 0.5 Shells 1 In areas where some of these ingredients are not available, there may be alternatives. E.g. Cotton seed cake may replace sunflower cake, Wheat bran or rice bran can replace maize bran, Where religion is not offended, blood meal can partly replace some of mukene. [Muslims may be offended] 9.3. Preparation of ingredients and making the feeds Some ingredients have bones (eg. mukene) and have too much fibre (eg. maize bran, rice bran, sunflower cake, rice bran, cotton seed cake). The bones and fibre should be removed by sieving the ingredients after the first grounding. Fish is not good at digesting fibre. The ingredients are then re-ground into very fine powder before weighing. The ingredients are weight or measured dry in the proportions given in table above and mixed thoroughly to obtain uniform mixture. For feeding the fry and young fish, the feed may be left in powder form. For the larger growers and brood stocks, the feed may be mixed in hot water into dough (kawunga) on the day of feeding. Alternatively it can be made into pellet using simple machines. 10. Important point to note when feeding fish: All the fish farmed in Uganda feed best during day, when the water temperatures have risen to at least 25 C. When water temperatures are low, even during day (eg. on a windy day after a heavy rain), fish do not feed well. Fish does not feed uniformly throughout the day. There are two feeding peaks: between 10.00am and 12.00noon, then from 3.00pm to 5.00pm. Therefore, for best utilization of feed, these periods of good feeding should be carefully timed.
Smaller fish have very small guts (stomachs). Their guts fill very quickly (with little food) and also empty very quickly. Therefore, they require giving a little food at a time more frequently than the bigger fish. 10.1. The feeding rates: i. Fry is given feed equal to 8-10% of their body weight daily. But as explained above, this feed after weighing for the day the feed is subdivided into 5 to 6 parts which are given 3 times in the morning (10.00 am to 12.00 noon) and another 3 times in the afternoon. The total weight of fish is obtained by multiplying the average weight of the fish with their number. ii. iii. The young growing fish (juveniles) are given 5-8% of their body weight daily, calculated in the same way as above. The weighed feed is subdivided into 4 parts, which are fed twice in the morning and twice in the afternoon. The growers and brood stocks are given food equal to 3 % of their body weight daily, given once in the morning and once in the afternoon. 10.2. Methods of applying feeding i. Some farmers put the food (especially the cooked dough or kawunga) on trays and place the trays at various points in the pond. The larger the pond the larger the number of trays ii. The feed, especially powder and pellets, may be broadcasted over the pond. This method is recommended. 11. General pond management Management of the pond involves the following i. Maintaining the amount of water in the pond
ii. iii. iv. Keeping the quality of water good Prevention of predation Monitoring fish health and growth 11.1. Maintaining the amount of water (the water level in the pond) 11.1.1. Causes of reduced water level are: i. Leakages across the pond dyke ii. Seepage at the bottom of the pond iii. Obstructions of water channels iv. Evaporation coupled with reduction in the stream water during the dry season. v. For community shared water source, increased diversion of the water for agriculture by the up stream communities during dry season. 11.1.2. Effects of reduced pond water level When water levels drop there are two main effects of this: i. The space available for the fish reduces and. fish gets crowded in the little space left. With the reduced volume of water the resources (amount of dissolved air and of natural food) also reduce. It is no longer enough for the fish. ii. The level becomes so low that it exposes the fish for birds to eat them. Birds with long legs (eg. Marabou stocks etc.) are able to wade through the shallow water and eat the fish. 11.1.3. How to maintain the pond water level i. Routinely inspected water channels and pond levels for obstruction or leakage. This is done at the start and at the end of the every day. Clean the channels and repair the leakage.
ii. iii. iv. Prevention of seepage should be done at construction or at pond repairs. Soils rich in clay are packed in the areas of the pond where soils are loose or prove to seepage/leakage. Where the dry season tends to be severe, water reservoirs can be constructed to store water for the dry season. The reservoir should be established on the upper side so that water enters the pond by gravity rather than by pumping. The same precaution can be taken where communities tend to take off a lot of water during dry season. v. If the dry season cannot be managed, the stocking of ponds can be planned in such a way that the fish should have reached harvestable size by the beginning of the dry season. 11.2. Maintenance of water quality The main issues of water quality are:- i. Maintenance of amount of dissolved fresh air (oxygen) ii. iii. iv. vi. Minimizing pollution Maintaining transparency (light penetration) Prevention of acid levels going beyond desired level. Maintaining good levels of natural foods 11.2.1. Effects of poor water quality Fish become stressed when the amount of dissolved air in the water falls below a certain level (see figure below) and when the levels of certain unfavourable chemicals rises beyond a certain level.
Fish on the surface, because of lack of dissolved oxygen., This occurs when the amount of dissolved oxygen goes extremely low or when levels of certain chemicals rise beyond tolerable levels. In extreme cases the fish die. 11.2.2. Effects of obstructed transparency Light penetration into pond water may be obstructed by. i. Excessive silt, like the brown marrum soils after a heavy rain or excessive clay. ii. Excessive green due to over fertilization of pond. 11.2.3. How to maintain good water quality i. When fish is observed to be stressed, a stream of fresh water should be flushed through the pond until the fish settle down in the pond. ii. iii. iv. If the cause is because of excessive feed that may have been given and it is polluting the water, the fish should be transferred to another pond. The accumulated materials are then removed by flushing, using a strong water current to wash it out, or manually scraped away any accumulated materials. If the stress is caused by a source outside the pond and it is even in the stream water, the fish should be transferred to another pond. The stream water should be analyzed for pollutants by a competent laboratory. The farmer should report the case to Kajjansi or FIRRI or NEMA. The farmer may also examine the activities upstream just in case someone if pouring bad chemicals into the stream. 11.2.4. Prevention of high acid levels Lime is used to lower acids levels and maintain them within the desired range. 11.2.5. Managing the fish stocks in the pond i. The fish stock in the pond should be managed with the following aims, Keeping the stocks pure; not contaminated with unwanted fish, ii. iii. Keeping the fish healthy. Healthy fish feeds well and grows well Preventing predators that may eat the fish
iv. Monitoring the growth of fish to ensure it is normal growth 11.3. Prevention of entry of wild fish Stocks are protected from contamination by screening the water inlets (see figure below). The screens should have small holes (meshes) to allow the water through but not fish eggs or fry. A Meshed inlet 11.3.1. Protection from predation a) Types of predators The main predators of pond fish are snakes, birds, monitor lizards and others. Other birds include: Fish eagle, King fisher, black heron, comorant, etc. b) Control of predators A bushy pond Such ponds tend to hide snakes which eat the fish. The higher the bushes the larger the snakes found, including python. The ponds also provide hiding for monitor lizards and others. Other birds include: Fish eagle, King fisher, black heron, comoran, etc. i. Keep grass very short to keep away snakes and monitor lizards. Various grass trimmers and lawn mowers are available in shops. Snakes, monitor lizards have
nowhere to hide. If ever you find a snake, it will be a very tiny snake that cannot eat fish ii. iii. Maintain full pond water level to keep away wading birds (birds that walk in the pond.) In some cases, strings are tied across ponds to prevent flying birds from catching fish. 11.4. Maintaining fish health 11.4.1. Monitoring fish survival and growth (pond sampling) In carrying out all the management measures explained above the farmer is aiming improving the survival and growth of fish in the pond so that the yields are as high as possible. The actual yield of the pond is the combination of two main factors: increase in individual fish weight and the total number of fish. If any of these two factors or if both of them drop, the yield also goes down. For the farmer to know how well the pond is doing he or she must carry out regular inspections by weighing and "counting" the fish in the pond. It is very difficult to take out all the fish to do the things mentioned because to do that may kill many fish. Therefore, only some numbers of fish (samples) the fish are taken out to get the general picture of all the fish in the pond. 12. Methods for sampling fishponds. The methods for removing fish from the pond are described in Section Five. However, the methods for sampling fish should be those that do not affect the survival and growth of the fish when returned into the pond. Fish sampling tools and methods should have the following qualities:- i. Methods that do not kill fish ii. Methods and tools that cause the least injuries e.g exhaustion of fish iii. Methods that catch a large enough sample to give the picture of the whole Pond iv. Methods and tools that do not select only the small or the large fish, but a whole range of fish sizes in the pond. 12.1. The best methods or sampling fish The methods that minimizes hurting, injuring and/or stressing fish and that will collect a large enough sample of fish include the following. i. Seine net ii. Dip net Baskets would be good for smaller ponds but they catch a few fish. Other methods injure, stress and kill fish while others select only certain fish sizes.
12.2. What information to record during sampling The main purposes for sampling are:- i. For the farmer to know whether fish continues to grow well. ii. To know the current weight of fish in order to calculate the amount of feed to supply, and iii. To assess whether the number of fish in the pond is reducing Some information is recorded routinely while other information is recorded at sampling. 12.2.1.1. Information recorded routinely This includes: - i. Amount of feed given at anytime plus transport cost where it is applied, ii. iii. iv. Amount of manure added at any time plus transport where it is applied, The number of fish that may have died since previous sampling Any cost incurred in getting services in fish farming 12.2.1.2. Information to record at sampling The information to record at sampling includes:- i. The date of sampling ii. iii. Number of fish caught during sampling Total weight of all the fish sampled 12.3. Interpretation of the information obtained The information gathered is intended to help the farmer make a decision about the business. Therefore, the information gathered should be made meaningful to the farmer by doing some calculations. The best way to show how this works is by the following examples (see table below). Example of records of monthly sampling data
Date of sampling Number of fish at start of the period Number of fish dead fish recorded Number fish caught Tots! weight of fish Stocking 2000 5 50 250 10/02/2004 35 700 12/03/2004 2 46 4600 11/04/2004 0 55 8250 15/05/2004 0 61 12200 13/06/2004 1 35 10430 11/07/2004 0 35 12250 13/08/2004 0 41 18309 14/09/2004 0 36 18200 How can he/she find out how well the fish grew? This done in two steps (see table below). i. Calculate the average weight of fish at each sampling (yellow column), ii. Then subtract the current average weight from the previous average wt (blue column). The results can be plotted in a graph as shown below Example - calculating trends in of growth of fish Number of Fish sampled Total Unit of all fish sampled Average wt of fish sampled Increase in av. Wt Stocking 50 250 5 0 10/02/2004 35 700 20 15 12/03/2004 46 4600 100 80 11/04/2004 55 8250 150 50 15/05/2004 61 12200 200 50 13/06/2004 350 10430 298 98 11/07/2004 35 12250 350 52 13/08/2004 41 18409 449 99 14/09/2004 36 16200 450 1
The graph below shows the growth pattern of fish, in this example GROWTH OF FISH Figure: Growth of fish expressed graphically The interpretation of the tables and graph is that from the last date of sampling the fish is no longer growing much. A decision must be taken to harvest immediately. Using the same sampling data, the farmer may wish to know by how much to increase the feeding rate (at 5% of body weight of fish daily) after each sampling, (see table below) Example of how to calculate the daily feed requirement from sampling data. Sampling date Number of fish stocked Number fish dead Number remaining Current Average wt (g) Total fish in the pond (kg) Daily feed required (kg) Stocking 2000 0 2000 5 10.0 0.5 10/02/2004 50 1950 20 39.0 2.0 12/03/2004 32 1918 100 191.8 9.6 11/03/2004 19 1899 150 284.9 14.2 15/04/2004 5 1894 200 378.8 18.9 13/06/2004 10 1884 298 561.4 28.1 11/07/2004 3 1881 350 658.4 32.9 13/08/2004 1 1880 449 844.1 42.2 14/09/2004 1 1879 450 845.6 43.2 12.4. How frequently should pond sampling be done?
Pond sampling causes some level of stress or disruption to the normal life of fish, however carefully it is done. The fish needs some time to recover from stress and shock and for injuries to heal before it resumes normal life. Full recovery may take as long as one or two weeks, depending on the method used. When the period between the sampling exercises are too short, the fish is stressed before it has recovered fully and resumed normal growth. When this happens, the result is harvested at smaller size and the pond yields are reduced from what was expected. Longer sampling intervals would be better. However, the farmer also needs to know the weight of the fish in order to adjust the feeding rate without starving the fish. This requires frequent sampling. The accepted compromise is sampling once a month or once in two months.. In Uganda, two sampling regimes are advisable, depending on whether the fish is grown predominantly on natural food or on formulated feed.:- 12.4.1. Sampling program for fish grown exclusively on natural foods. Here the farmer does not need to calculate monthly food requirements. Therefore monthly sampling is not necessary. i. The first sampling is done one month following stocking. This is aimed at determining whether or not the stocked fry survived the process of stocking. If number of fry that survived stocked is too small, restocking is done early enough. iii. No sampling is done for the next five to six months. Fish are just left to grow. iii. From the sixth month, sampling is done every month until harvest. During this period, the fish is expected to be approaching harvesting size. The aim therefore, is to careful check for the right time to harvest the fish without delay. 12.4.2. Sampling program for fish grown predominantly on formulated feeds. In this case the quantity of feed to give fish must be calculated every month to match the requirement as the fish increases in weight to avoid underfeeding or wastage of feed. It is therefore necessary to sample the fish monthly. However, using the experience from a previous harvest, the sampling interval can be increased and feeding rates can be estimated using the previous data Therefore, the sampling program for fish grown predominantly on formulated feeds, it is recommended that:-
i. The first sampling is done one month following stocking for the same reason given above. ii. Subsequent sampling is done every two months for the next six months (feeding rates in between is estimated from previous growth data) iii. Monthly sampling for the next six months until harvesting to decide when to harvest. Information Provided by The Executive Director NAADS Secretariat P.O. Box 25235 Kampala Tel: +256 414 345440, Fax: +256414 347 843 Email: naads@iwayafrica.com www.naads.or.ug