About the Author Philemon Buruzi is an Agricultural specialist whose career in the sector spans over a decade. Having graduated from the University of Zimbabwe with a BSc Honours Degree in Agriculture majoring in Animal Science he went on to acquire postgraduate qualifications from the University of Pretoria in South Africa. Philemon was once the Provincial Livestock Specialist for Midlands Province under the Department Of Livestock Production And Development, a position he held for five years. Philemon then joined the Midlands State University as a lecturer in the Department of Livestock and Wildlife Management where he lectured for three years. In between the years has also been involved with the Zimbabwe Open University as a part-time tutor for livestock modules in the Agriculture department. After leaving the Midlands State University he started to do Agricultural Consultancy for various non-governmental organisations, Corporates, farmer organisations and individual farmers. Philemon areas of interest are:
Dairy Production Beef Production Aquaculture Apiculture Poultry production Pig Production Horticulture
Philemon Buruzi has worked on a number of projects in the SADC region in Botswana, South Africa and recently in Mozambique. This publication comes as the 13th on the list of production manuals that he has produced to date. This does not include short technical articles and research materials that he has published over the years. He can be contacted on:+263775138121, philemonburuzi@gmail.com, www.facebook.com/philemon.buruzi.
Contents Introduction .................................................................................................................................. 4 Production Systems ....................................................................................................................... 4 Tilapia............................................................................................................................................ 4 Pond Siting, Design & Construction ................................................................................................ 6 Pre-stocking Management ............................................................................................................. 8 Techniques of fertilizer application ................................................................................................ 9 Liming ......................................................................................................................................... 10 Stocking Management ................................................................................................................. 11 Post-Stocking Management ......................................................................................................... 12 Harvesting ................................................................................................................................... 13 Common Challenges .................................................................................................................... 14 Pond Maintenance ...................................................................................................................... 15 Stocking Model ............................................................................................................................ 17
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Introduction In Zimbabwe, the term aquaculture is loosely taken to mean the production of fish only. Whilst this is not wrong, it remains important to point out that it does not stand for fish alone but also for the production of all the other aquatic organisms such as crustacean, molluscs, catfish and aquatic plants. The harvesting of naturally occurring aquatic organism as in the case of sea fishing is not aquaculture and the same goes for the growth of land plants using hydroponic techniques. Fish farming is the raising of fish in ponds, tanks, net enclosures, cages, or raceways. Usually the goal is to grow the fish as fast and economically as possible to a harvestable size. Some of the factors that farmers manipulate to influence growth rate include pond environment, type and density of fish, food, fertilizer, water quality, and growth period. Fish farming is not new. People have been raising fish for thousands of years
Production Systems Aquaculture can be broadly grouped into three production systems namely: Extensive – whereby large stagnant ponds that allow only a low stocking density and rely on natural production to feed the animals (i.e. there is no supplemental feeding)are used. Management and skills input are low. Semi-intensive – This is much like extensive culture, however there is a greater degree of intervention either feeding and/or improvement of water quality through aeration and partial water
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exchange. This allows for an increase in the production of livestock when compared to extensive systems. Management and skills input occur at a medium level. Intensive – The fish is grown at high stocking densities and feeding comes solely from introduced commercially compounded feeds. The culture systems tend to be highly technical and rely on electricity to operate. The space required is relatively small and the system is designed to optimize water use and quality. Management and skills input are high. There are various kinds of fish that can be produced using aquaculture methods yielding good profits. Those that quickly come to mind are:
Silver Carp Sharp-tooth catfish Common carp Bighead Carp Tilapia species
Tilapia The name Tilapia plays host to a number of sub-species which are closely related in nature. Tilapia is the most common type of fish the world over owing to its capability to thrive in very different climatic conditions. We are however going to limit our focus on the ones that are commonly cultured here in Zimbabwe which are enshrined below. Tilapia rendalli Oreochromis mossambicus Oreochromis niloticus
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Oreochromis niloticus
Oreochromis mossambicus
Tilapia rendalli
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Tilapia can successfully be cultured in cages or in ponds of various types and makes depending on production intensity and the availability resources. Tilapia are herbivores, with some species eating plants and others eating phytoplankton. The Tilapia do well in very enriched waters (enriched by organic fertilisers). All tilapia have slightly different eating habits, depending on the species. Tilapia species have many possibilities for pond culture. Their fast growth rates, ease of breeding, good taste and disease resistance make them a good choice, particularly for the first time fish farmer.
Pond Siting, Design & Construction
Soil type and quality – soil sampling has to be conducted first on the site meant for pond siting. Soil containing greater than 10-15% clay is desirable for retaining water in ponds and preventing seepage. Avoid sandy soils because these allow high rates of seepage. Avoid peaty soils and shallow soils. Topography - should be almost level or gently sloping. Steep slopes should be avoided at all costs. Gently sloping topography is ideal for fish ponds because it is easier to construct drainable ponds. Ponds can be built in flat areas or hilly areas but construction can be more difficult. Avoid low areas where ponds cannot be drained or are susceptible to flooding or damage during heavy rains. As such the land should be above the flood level
Water supply and quality - A reliable water source during the fish growing season is needed. Water can come from springs, runoff, rivers, or ground water. Water contaminated from pollutants is not suitable since it can cause health problems for both fish and humans.
Siting When siting the fish ponds, a proper site selection exercise needs be conducted in order to eschew future challenges that primarily have to do with pond integrity or lack of it. As such the following factors have to be put into consideration
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Accessibility- of the land with regards to roads is of paramount importance. The optimal location for a pond is near the owner’s house so it is easier to manage and to protect from theft and predators. Climate - Tilapia thrive in warm water of 2530 °C usually found in areas of low elevation in tropical areas. Fish growth and reproduction will be slower at cooler temperatures. Ponds should be located in full
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sunlight to receive maximum solar heating and plankton growth.
Construction and Design
Building pond seems quite simple at face value but there is a right and wrong way to build one. An improperly built pond can leak, have a weakened dam, which is susceptible to breakage, and potentially present many other problems. Considering that a pond can be used for many years it is advisable to take the time and effort to build it properly. Ponds should be built in such a way as to enable filling and draining independently from other ponds. Total drainage is advantageous to prevent fish stunting and disease. When intending to put a large swath of land (e.g 1 ha) under aquaculture, it is advisable to construct a series of ponds (200 to 300 sqm) as compared to one large pond. In this way the timing of stocking and harvesting the different ponds can be staggered allowing farmers to have a year round supply of fish
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Characteristics of a standard pond Area: The area of tilapia pond may be a few square metres to several acres. However from the management point of view, the area should be at least 1 -2 acres. Shape - Starting from fish harvest all other management activities are easier in a rectangular pond. However pond can be of other shapes as well. However, based on the shape of the land available for the pond, there isn’t any problem if pond is of the similar shape. The shape of the water body does not have much impact on the production. Size - Fish grow bigger in larger ponds even when the stocking densities are the same as in small ponds and the management of the ponds is identical. This means that the weight of fish produced per hectare in a 0.5-ha pond may be almost double than that produced in a 0.1-ha pond. The reason for this is that large ponds have a larger surface area and are more often subjected to wind action, which results in more oxygen entering the water and the water being mixed better. Although large ponds are preferable, they are more difficult to fill, drain, harvest and maintain. Therefore, the optimal size and shape of the pond will depend on the practicality and management available to make it large enough to grow fish but small enough to manage properly. The recommended maximum size for ponds for edible fish like tilapia or catfish is 1 hectare. Dyke -The pond dyke should be elevated at such level so that the rain water or the water from other outside sources cannot enter in to the pond. Wide dyke is essential. Because, pond dyke become damaged regularly due to many reasons. A pond of 2.0 – 2.5ha should have at least 1.5m wide dyke. However, whatever the width of a dyke, it should be repaired at few years interval.
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Dyke slope - The slope of pond dyke should be minimum of 2:1. Slope should be built such a way that one can go close to the pond water by walking. As tilapia build nest after digging soil, steep dyke becomes easily damaged. Therefore for tilapia pond, a dyke of appropriate slope is essential. Soil- Generally water bodies of loamy soil are the best for fish culture. Thick layer of mud is built-up in pond of sandy soil within 2-3 years of fish culture. As a result, the water holding capacity of pond increases and ponds become suitable for fish culture. Depth - Pond should be such deep so it can hold 1.5 m of water. In cases where water is supplied to pond from deep well, the pond depth / height including dyke should be 2.5m. On the other hand, if there is no provision of water supply from outside, pond should be such deep so it can hold at least 1m of water in lean season. The bare minimums are that the pond should have a sloped bottom whereby there is a deep end of not less than 1.5m and a shallow end of not less than 0.30m. Water supply and drainage - In improved fish culture, regular water supply and draining out are necessary. Therefore, inlet and outlet pipes / drain are required. Generally, a pipe of 6 inches diameter is sufficient for a pond of 1 acre. Therefore, inlet and outlet pipes / drain are required. Generally, a pipe of 6 inches diameter is sufficient for a pond of 1 acre.
Pre-stocking Management After successful pond construction, it is filled with water and then there is need to create an
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enabling environment for the fingerlings to survive before bringing them in. 1) Water pH has to be above 7.5 2) Water temperature has to be optimal 3) There should be enough feedstock in the water 4) There should be enough dissolved oxygen in the water to enable survival Feedstock is put in by addition of organic or inorganic fertiliser in the pond which enables growth of plankton consumed by the fish. What is plankton? All the microscopic animals (zooplankton), plants (phytoplankton) and bacteria suspended in the water comprise the plankton. Phytoplankton synthesize their food using sunlight, carbon dioxide, organic salts and the water, while the zooplankton feed on the phytoplankton. Bacteria utilize any dead organic matter for food. The plankton can be very important to the culturist, as they provide food and oxygen to the fish. (phytoplankton consume CO2 and produce O2 during the day, while the reverse occurs at night). When there are sufficient quantities of phytoplankton to discolour the water, there is said to be a “bloom” which varies from pale green through dark brown to black depending on the types and relative ratios of phytoplankton present. A good “bloom” is olive green in colour. The density of the plants (and hence their ability to provide food and oxygen to the fish) may be increased through fertilization. Conversely the bloom can be decreased with water exchange, and should be of such a consistency that a secchi disc immersed in the water just disappears from view at a depth of 30 cm. Alternatively, the upturned palm of the hand should disappear from view when the arm is immersed to the elbow. A “bloom” that is too
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thin will provide less-than-optimum food and oxygen, while one that is too dense will suffocate itself at night, depriving the pond of oxygen and causing fish kills. “Blooms” should not be confused with turbidity caused by soil suspension or organic leaching from leaf litter or peaty soils.
Fertilization The productivity of the “bloom” may be increased artificially by the addition of organic or inorganic fertilizers. (a) Organic fertilization - dry poultry manure may be used at a rate of 50 kg/ha/d for a week when the pond is filled, to develop the “bloom”. Liquid fertilizers (e.g. the supernatant from dissolved poultry litter) are not commonly used, but have the advantage of leaving no sludge from insoluble litter on the floor of the pond. Dissolve the litter in drums and decant the liquid into the pond at a rate of 420l/ha/d.
(b) Inorganic fertilization - chemical fertilizers are more dependable than manures. The main nutrient limiting plant growth in freshwater ponds is phosphorus, followed by nitrogen, and fertilizers should contain at least as much phosphate as nitrogen with optimum ratios being 3:1 P2O5:N. Fertilizers should be applied at rates of 5-10 kg P2O5/ha per application at 2-4 week intervals. Liquid and soluble fertilizers are more effective, but cut bags may be placed on underwater platforms for the fertilizer to dissolve slowly and continuously. Urea and ammonium fertilizers are popular and inexpensive, but can cause ammonia toxicity if used at high rates.
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Techniques of fertilizer application After uniformly spreading necessary amount of organic fertilizers in the pond bottoms, the fertilizers need to be mixed well with bottom soil by ploughing. If necessary small amount of mustard oil cake (0.5 kg / l) is mixed with the said dose of organic fertilizers. After filling the pond with water, SSP should be soaked in water for 12- 24 hours and finally should be distributed evenly in the pond after adding urea in the mixture. Water filled pond Single-Super Phosphate(SSP) and cow dung should be soaked in adequate water for 12 – 24 hrs in a bucket or drum. Just before application, urea should be mixed with the cowdung – SSP mixture and should be evenly applied in the entire ponds in a sunny day. The dose of fertilizer depends on the productivity of pond and the experience of fish farmers. The timing of fertilizer application: Preparatory fertilizers should be applied 5 - 6 days after liming and 5 – 7 days before fingerling stoking. Applying fertilizer in a sunny morning before the afternoon is better.
Tips about fertilizer application • Fertilizers are less effective in acidic soil. In the water with high and low pH, phosphorus precipitates quickly at the bottom. • Fertilizers are less effective in turbid water. • The fertilizer activity reduces if there are aquatic plants in the pond water because aquatic plant absorb comparatively more nutrients that that of the phytoplankton.
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• If pond / canal do not hold water more than three weeks, the effectiveness of fertilizer decreases. • In a deep-water pond, phosphorus cannot act well. Here phosphorus becomes unproductive in bottom sediments as precipitation. • In case of the application of mixed fertilizers, adequate water should availed in the mix in order to lessen the concentration • Fertilizers are generally less active if applied in a cloudy or rainy day. • Storing urea in open air reduces its activity. The pH is controlled by liming the soil prior to filling in with water or addition of the same in water-filled pond.
Liming Liming is done primarily to buffer the pH of acidic soils and increase total alkalinity and total hardness in ponds. Common liming materials are agricultural limestone (CaCO3), burnt lime (CaO) and hydrated lime (Ca(OH)2). Applied at rates of 1-2 t/ha to the bottoms of empty ponds they will buffer soils with pH of less than 7 and in the cases of burnt or hydrated lime, will provide a sterilizing action to kill pathogens. Lime application The significance of lime application Liming in ponds is an important activity. The significances of liming are as follows – • Maintain water pH higher than 7.5 • Keep the parasites and germs out of the water during pond preparation • Assist in natural food production through increasing the abundance of nutrient materials
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by enhancing the rate of decomposition of organic matters • Free the phosphorus bound in sediment for plankton growth • Remove the turbidity of water the activeness of fertilizer The dose of lime During pond preparation 1 kg quick lime or 0.5 kg dolomite can be applied per 150sqm.During pond preparation, the required quantity of lime in powder form should be evenly distributed in the entire pond including the slope of dried water bodies. In ponds filled with water, required quantity of lime should be evenly distributed to the entire pond after mixing with water in an earthen pot (hari) or in a tin drum. Timing of lime application: • Lime should be applied 1 - 2 days after irrigation in dry pond or pond with wet bottom. • In case of the water-filled pond, 5 – 6 days before the fertilizer application Cautions in lime application: 1. During mixing lime with water and application in pond, entire face should be covered with a piece of cloth. 2. Lime should not be mixed with water in a plastic bucket. 3. Lime should be added after putting water in the bucket. Before pouring water in the bucket containing lime, its top should be covered with jute made bag. 4. Lime should be applied in the direction of air flow. 5. If the eyes/ body parts get in touch with lime, should be instantly washed with clean water and seek doctor’s advice. Dissolved oxygen requirements
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The most important water quality variable affecting fish is dissolved oxygen. Although able to withstand low concentrations of Dissolved Oxygen for varying periods, tilapia will not perform well in waters containing less than 3-4 mg/L (parts per million, ppm), and concentrations greater than 5 ppm are sought by intensive culturists.
Stocking Management Only fingerling stocking in right quantity cannot ensure good production. To get high yield, good quality fingerling should be stocked along with maintaining proper stocking density. The quality of fingerling is largely influenced by the source of fingerling and their handling. Whatever the reasons responsible for the bad quality of fingerling, fish farmers may lose huge money if those fingerling are stocked. If bad quality fingerlings are stocked – • Mass mortality may occur after stocking • Growth rate may be low • As the fish cannot grow to marketable size in time, fetch low market price. That’s why the quality of fingerling should be ensured before stocking.
Presently in our country the hatchery produced fingerlings are transported in oxygenated polythene bag as seen above. Transporting fingerlings in modern way is much safer. Considering the time and distance of transport, the hatchery owners supply fingerlings in polythene bag maintaining optimum density. Ideally 100 fingerlings are transported 4 litres of water in an oxygenated plastic tube.
Stocking density High stocking density of fingerling results low growth. Therefore, fingerling should not be released at high stocking density in tilapia culture. The optimal stocking density for tilapia is five fingerlings per square metre pond area. This normally gives average weights ranging between 300 – 450 g within 3 - 4 months. Should one desire better weights than the ones stipulated above, they can resort to a lower stocking density of 3 fingerlings per square metre pond area. If greater stocking densities are desired the farmer has to make an effort to install artificial aeration mechanisms. The options that can be put to use are: paddlewheels, spraybars and agitators.
Fingerling transport Timing of fingerling release
Packed fingerlings
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In a cool weather, fingerlings can be released into the pond / canal at any time of the day. Fingerlings should not be released in the bright sun of midday or on a cloudy day or in a extremely humid or rainy weather. Small fingerlings should not be released on a rainy day because oxygen content is low and carbon dioxide content is high in rainwater and as a result fingerlings may die. Heavy rain during the day of fingerling release or in the next day may cause mortality of all
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released fingerlings. The risk is not that severe for large fingerlings. Fingerlings should be released after 5 – 6 days of fertilizer application in grow out ponds.
Testing water suitability Testing water suitability is essential before stocking of the fingerling. For testing water suitability, a cage should be set in the fish pond such a way that the base of cage does not touch the pond bottom. Then 15 – 20 fingerling should be released in the cage for observation for 24 - 48 hours. Survival of all of the released fingerling or 10 – 20 % mortality within this time indicates that pond water is not toxic or polluted. If pond water has any problem, stocking should be delayed and fingerling should only be released after necessary treatment of the water.
The step-wise activities of conditioning of fingerling in new environment are as follows– The transport bag should be kept as floating for 15 – 20 minutes in pond water. After opening the bag with fingerling, water should be exchanged gradually between the pond and the bag and the temperature of the two should be brought in to equilibrium. The difference between temperature of bag and pond should be examined now and then using hand. Temperature should be carefully monitored so the difference of two waters is no more than 1 – 2 o C. When the temperature become similar, the bag with fingerling should be slowly plunged in the pond water and with the help of mild current towards the container, the healthy and strong fingerling will come out from the container against the current. It should be noted that, fingerling should be released closed to the embankment, not in the middle of pond. *This is a delicate procedure which can make or break your enterprise. It could be wise to solicit for the assistance of experienced hands.
Post-Stocking Management
Testing water suitability
Conditioning of fingerlings and release The released fingerling may suffer from mass mortality due to the difference in temperature and oxygen, instantly after stocking. This rate of mortality can largely be reduced if the fingerlings are properly conditioned in the new environment before releasing them in the nursery canal / pond. The acclimatization is nothing but bringing the water temperature of transport container and nursery canal / pond at the equilibrium.
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Feed for tilapia and feeding management: Homemade feed should be applied following the doses described below. The feed with food conversion ratio (FCR) less than 2 should be applied.
Average weight of fish(g) 10-15 15 -30 30 -50 50- 100 100-200 >200
Feed (% body weight) 20 15 10 5 4 3
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Timing of feeding Feed the fish 2- 3 times a day at 10 – 11 am in the morning and 4 – 5 pm in the afternoon. However, farmers can change the feeding time according to his/her convenience. If more feeds than necessary are applied the water quality deteriorates rapidly along with wastage of valuable feed. For that reason, estimating the necessary amount of feed is important. On the other hand, if feed is applied less than the requirement, fish growth becomes slower. Feed should be applied over the wide area of the pond. As a result, many fish of the pond can feed at a time without much competition for feed. The right size of the pellet should be selected based on the size of the mouth of fish so fish can easily grasp the pellet. It should be remembered that application of excess feed than necessary or application of larger feed pellet than what fish can grasp is nothing but wasting money for no reason.
Checking the adequacy of feed To know if the supplied feed for fish is sufficient, the abdomen of some fish can be checked after 1 hour of feed application. Full abdomens of fish look somewhat swollen. If fish do not become full from the supplied feed, they do not easily leave the feed application place rather they continue their movement near the place. As the level of the hunger of fish, so is their excitement for a feeding frenzy. This can be easily understood from the experience.
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Checking fish growth Growth rate of fish should be checked at least twice a month from by way of sampling. How many grams weight increased in how many days should be observed and how much feed was applied for resulting growth should be assessed. The reasons behind fish not growing as expected need to be analyzed and control measures should be taken. If fishes in the pond are more or less of equal size, not many fish need to be weighed during sampling. If sizes of fish vary by far, much more (a few hundred) need to be sampled.
Fertilizer management Tilapia culture is mostly supplementary feed dependant. Part of the applied feed remains unutilized by the fish. The unutilized feed and fish excreta act as fertilizer in the pond. Therefore, usually in most cases, no extra fertilizer is necessary in tilapia culture.
Harvesting
Harvesting using nets
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Under the proper culture management, improved quality fingerlings and complete balanced diet, the average weight of tilapia reach to 300 – 450 g within 3 – 4 months. Nets of various mesh sizes are used for handling and harvesting fry, fingerling and adult tilapia. Generally, large-mesh (2-3 cm) nylon seine nets 2.5-3.0 m deep are used to seine 0.4-1.0 ha ponds, although in large ponds it is often necessary to drain the water partially to ease access and concentrate the fish in the catch basin. This is also necessary since tilapia are adept at escaping seine nets, with only 25-50% of the fish in a pond being caught per net haul. For each meter of pond width 1.5 m of net are needed. The fish should not be fed 24 h before harvesting. The absence of food in their guts will lower their oxygen demand and increase the chances of survival and the overall condition of the fish for processing. Harvesting is best done in the coolest part of the day for this reason, and it may be necessary to arrange aeration or water flow in catch basins and holding nets.
Common Challenges
Massive fish death Control measures If dissolved oxygen is short in the pond, water should be exchanged with new water from deep well or from other sources. This is the best measure. If water exchange is not possible, net should be pulled in the pond or pond water should be stirred. Water from deep tube well is virtually oxygen-free and adding water from any new source to pond creates kind of a new environment for fish. Considering these aspects, pond water should be exchanged over a few days instead of changing at once. During water exchange or immediately after the event, lime should be applied in the pond at mild dose. The use of aerators can help solve this challenge.
a) Shortage of oxygen In the ponds of improved fish culture, shortage of oxygen often occurs. When oxygen content drops in the water, fish start floating and grasping on the surface. If the situation lasts for days, fish become sick, lose appetite, their growth rate decreases and may face mass mortality. This significantly reduces the farm production.
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b) Diseases Common symptoms Fish show different symptoms based on the types of diseases. However, one or more of the following symptoms indicate that fish in the water body are infected by any of the diseases. • Fish reduce or stops feeding completely • Lesions or haemorrhages on the body • Tail and fins start to rot • Gills become damaged • Cotton like fungi observed on fish body • White spots form on the body
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• Black and white spots or cyst present on the gill • Reduced growth rate • Fish lose balance etc. Tilapia is known as a high disease resistant fish species. The fish is highly adaptable in adverse aquatic environment compared to many other fish species. Yet when the physiochemical parameters of water reach beyond the optimum range, the disease resistance ability of fish drops far below the expected level. As a result, fish get easily infected by germs and pathogen present in water. The risk of infection further aggravates due to polluted water caused by higher stocking density and the decomposition of unused feed in closed water body, excretory materials of fish and other waste matters. The risk of fish disease can be reduced following the precautions described below – • Stocking of disease free healthy fingerlings • Disinfecting all the tools used in fish culture and fish farm • Avoid overstocking • Application of balanced diet as necessary • Ensuring the proper care of fish and farm
Several organism can prey on fish especially when it is still at fingerling size. If a predators takes residence in your pond it can decimate your fish significantly to the point of yielding losses. Some of the common predators are: -Catfish -Lizards -Bigger fish -Frogs -otters
c) Predators
Dyke repair and removal of decomposed mud When preparing the pond after harvesting, sometimes dyke repair and removal of decomposed mud are necessary. This is so because damaged dyke and presence of decomposed mud may create a range of difficulties.
Mink –a common predator
Pond Maintenance
If dyke is not repaired – • Predatory fish may enter from outside • Spawn may float away during monsoon or heavy rain • Toxic gas may form at the pond bottom Heron
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• Polluted water from outside may enter in to the ponds If black mud is not removed • Pond water become toxic • Oxygen scarcity is observed in the pond water that increases the spawn mortality • Toxic gas may form at the pond bottom • Fish are easily infected by different diseases that may create an epidemic
Aquatic weeds
• Aquatic weeds absorb nutrient content from soil and water. As a result, primary production decreases. As aquatic weed block the entrance of sun light into the water, photosynthesis is hampered. • Aquatic weeds create barriers in the movement of spawns and the spawn become easy prey of different predatory animals. • Aquatic weeds act as the refuge of predatory fish and animals. • Aquatic weeds make the netting difficult. • Because of excess aquatic weed, spawn may die during night due to lack of oxygen. Aquatic weed control methods
Pond choked by weeds Different plants present in water are known as aquatic weed. The aquatic weeds directly or
indirectly are harmful to the fish culture. Generally, aquatic weeds are plants like water hyacinth, water lettuce, duck weed , water lily etc. Maybe present in the pond. If the aquatic weeds are present in the ponds, they should be removed manually. The harmful effects of aquatic weeds All kind of aquatic weeds have a number of direct or indirect negative impacts in spawn culture. The negative impacts of aquatic weeds in spawn culture are as follows –
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Using manual labour – All the aquatic weeds can be pulled up manually after cutting with chopper/ scissors. Sometimes using a rope, the root of aquatic weed can be pulled up. Biological method – There are many fishes that can control aquatic weed by foraging on them, e.g. grass carp so one may opt for a polyculture in a bid to control aquatic weeds.
Control of predatory fishes
Predatory fish can be removed from the ponds by complete drying out of ponds: This method is very effective in removing unwanted insects and predatory fishes. Shallow pump can be used in this purpose. After drying out the ponds, bottom should be kept exposed in direct sun light for a few days to such an extent that when someone walks on the pond bottom, there will be foot print but
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feet will not be soaked. Both time and money can be saved if this is done during April-July.
Stocking Model (suited for Zimbabwean
Main frame Filler frame
Cycle one DecMar Oct - Jan
Cycle two April July Rest ponds
Cycle three Aug -Nov Feb -May
Climate) ______________________________________________________________________
REFERENCES Brink, D. (2004) Trout Farming Reference Guide – Aquaculture Technical Training Course for Smallscale Farmer Division of Aquaculture. Stellenbosch University, Stellenbosch, South Africa. Chakroff, M. (1976) Freshwater Fish Pond Culture and Management. United States Peace Corps and Volunteers in Technical Assistance, Washington D.C. FAO (1994) Handbook on Small-scale Freshwater Fish Farming. FAO Training Services No. 24. Rome,FAO. Haylor, G. & Muir, J. (1998) A Fish Hatchery Manual for Africa. Stirling, Pisces Press. King, H.R. & Ibrahim, K.H. (1985) Village-level Aquaculture Development in Africa. London, Food Production and Rural Development Division. Kumar, D. (1992) Fish Culture in Undrainable Ponds — A Manual for Extension. FAO Fisheries Technical Paper No. 325. Rome, FAO. Piper, R.G. & McElwain, I.B. (1982) Fish Hatchery Management. Washington, D.C., United States Department of the Interior. Rouhani, Q.A. & Britz, P.J. (2004) Contribution of Aquaculture to Rural Livelihoods in South Africa: A Baseline Study. Department of Ichthyology and Fisheries Science, Rhodes University, Grahamstown, South Africa. Woynarovich, E. & Horváth, L. (1980) The Artificial Propagation of Warm-Water Finfishes – A Manual for Extension. FAO Fisheries Technical Paper No. 201. Rome, FAO
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