Introduction to Water and Soil Management for Sustainable Aquaculture:
The following information is all about Water and Soil Management for Sustainable Aquaculture.
Good bottom soil and water quality are essential ingredients for any successful aquaculture practices. Although such problems are related to site characteristics bottom soils have undesirable properties acid sulfate, high organic & excessive porosity, etc. Similarly, the water may have poor quality, highly acidic, rich in nutrient & organic matter, high in suspended solids or polluted with industrial or agricultural chemicals.
Importance of water in aquaculture:
If aquaculture is the rearing of aquatic organisms, it is very important for an aquaculturist to understand the aquatic medium i.e. Water, in which these organisms inhabit. If the water is “bad,” plants & animals won’t grow or reproduce. Animal stressed because of poor water quality is a prime target for pathogens and parasites. Just as people who work in offices or factories that are stuffy & have smoke or chemical fumes in the air are more apt to be sick, so it is with aquatic organisms developed in poor quality of water. Water is the medium in which fish live, & from which they derive oxygen and nutrients. So the quantity & quality of the water very much affects the prospect of fish culture. As water is the basic part of the fish culture, its specific properties as a cultural medium are naturally great in the productivity of a pond.
Importance of soil in aquaculture:
The soil is the main factor in aquaculture. Most of the pond is built from & in the soil. Many dissolved & suspended substances are derived from contact with soil. Pond soil is a storehouse for many substances that accumulate in the pond ecosystem, chemical and biological processes occurring in the surface layer of pond soil influences water quality and aquaculture. Hence an understanding of soil properties and the reaction and process in the soil can be helpful in pond aquaculture.
The properties of pond soil are of greater significance than is commonly realized. When soil conditions are not good, the production will be limited. The productivity of fishponds depends on the occurrence of appropriate environmental conditions and the abundance of fish food organisms. The first step in the food chain of a fish pond is constituted by main food organisms, e.g. phytoplankton, which derive their nutrients from the pond environment & with the help of solar radiation undergo photosynthetic activities. The amount of these nutrients in pond water & protection of its relevant chemical condition depends largely on the nature and properties of the bottom soil wherein a series of chemical & biochemical reactions continuously take place, resulting in the release of different nutrients in overlying water and also their absorption in the soil mass.
The term “sustainability” became popular in aquaculture development plans and project documents after it was accepted that the potential for aquaculture development was threatened by increasing environmental problems, including serious disease outbreaks, which have exacted heavy economic losses in different aquaculture systems. Sustainable aquaculture was defined as “the wise & productive methods of culturing aquatic animals and plants, using natural resources in a manner that is environmentally non-degrading, technically appropriate, economically viable & socially acceptable, ensuring the attainment & continued satisfaction of critical human needs for the present and future generations”.
The material composing the bottom of streams, lakes, and ponds is called sediment, mud, or soil. The pond bottom is initially made of terrestrial soil and when the pond is filled with water the bottom becomes wet. A mixture of solid materials and water is known as ‘mud’. Solids settle from the pond water & cover the pond bottom is ‘sediment’. The basic purpose of pond soil is an embankment that impounds water and forms a barrier to seepage so that the pond will hold the water. Substances enter from the solid phase of soil from the aqueous phase through ion exchange, adsorption, and precipitation.
Substances that enter the soil may be stored permanently, or they may be transformed into other substances by physical, chemical, or biological means & lost from the pond ecosystem. For example, phosphorus adsorbed by pond soil becomes buried in the sediment and lost from circulation with the pool of available phosphorus. Organic matter deposited on the pond bottom is decomposed to inorganic carbon & released into the water as carbon dioxide. Nitrogen compounds may be denitrified by pond soil microorganisms & lost to the atmosphere as nitrogen gas.
Bacteria, fungi, algae, higher aquatic plants, small invertebrates & other organism know as benthos lives in and on the bottom of the soil. Crustaceans & some species of fishes spend much time on the bottom soil and many fish lay eggs in the nest built into the bottom. Benthos serves as food for aquaculture species. It also involved in gas exchange, primary and secondary productivity, decomposition & nutrient cycling.
Substances stored in the pond soil can be released into the water during ion exchange, dissolution until equilibrium attained between the solid phase & liquid phase. The equilibrium concentration too low for optimal phytoplankton growth or the equilibrium concentration of heavy metal may be too high enough to cause toxicity to an aquatic animal. Microbial decomposition is extremely important because organic matter is oxidized to CO2 and ammonia & other nutrient element is released. Carbon dioxide & ammonia are highly soluble and quickly enter the water.
Optimum Soil characteristics:
The soils with moderately deep texture (sandy clay, sandy clay loam & clay loam), electrical conductivity rate of 4 dS m-1 or more, pH ranging between 6.5 to 7.5, the organic carbon content of 1.5 % to 2% & calcium carbonate content of more than 5% are best suited for shrimp aquaculture.
The soil consists of a weathered mineral organic substance. They are a product of an interaction between parent material, climate & biological activity. It is well known that soil differs from place to place on the earth’s surface & beneath a given site the soil profile consists of horizontal layers that change in individuality with depth given below the land surface. The mainly active fraction of the soil is clay particles, because of electric charge & large surface area and the organic matter, of its biological activity & high chemical reactivity.
Water quality requirements:
Water quality & quantity determines the success or failure of an aquaculture operation. An annual water budget must be calculated for a potential farm site so that the supply is adequate for existing and future needs. Water should be free from pesticides & heavy metals. Maintenance of good water quality is essential for both survival & optimum growth of animals. Water treatment is the main step during pond preparation for the maintenance of good water quality at a later stage.
How can health management support sustainable aquaculture?
It is important to understand the relationship between disease control & health management. Diseases in aquaculture regularly depend on the quality of the environment of the ponds. So, maintaining optimal environmental conditions and providing good health management in the culture unit is important to reduce losses & sustain production levels.
Suitable site selection:
The site used for aquaculture is important in terms of initial start-up & the eventual success of the venture. The criteria for site choice should include assessment of soil quality, water quality, and quantity, land use, infrastructure & economic viability. Guidelines for site selection for different types of aquaculture are available in several countries. Such information should be modified & made suitable for local conditions.
Soil and water management:
- In order to recognize the condition of the pond bottom, soil pH, organic matter & redox-potential (Eh) for oxidized or reduced pond bottom condition have to be monitored regularly. The redox-potential Eh of pond sediment should not exceed -200 mV.
- The water parameters that should be monitored routinely in ponds during the culture stage are temperature, pH, salinity, dissolved oxygen & transparency.
- The pH must be at an optimum level of 7.5 to 8.5 and should not vary more than 0.5 in a day.
- The difference in salinity not exceeding 5 ppt in a day will help in reducing stress on the shrimp.
- The optimum range of transparency is 25 cm-35 cm. Transparency can be measured using a Secchi disc.
- The un-ionized form of ammonia nitrogen must be less than 0.1 ppm.
- Any detectable concentration of hydrogen sulphide is measured as undesirable.
- Periodical water exchange as & when required will help in maintaining the water feature in the optimal range. The use of aerators results in mixing of water at the surface and bottom & breakdowns they do & thermal stratification.
- The use of inputs without proving efficiency must be strictly avoided.
- The discharge water from the shrimp ponds has to be accepted into a treatment system pond before letting it into the environment so that the suspended solids may settle at the bottom.
Fish being aquatic being is more prone to disease & are difficult to control. The equilibrium of disease, environment & fish health are important for any change in the equilibrium leads to “stressed” and becomes vulnerable to disease which has an influence on growth & survival.
It is very difficult to set up a regular program of water management for aquaculture as a whole. Optimal water quality parameters vary from every species of aquatic animal cultured. Management should ensure water quality is kept at a level suitable for optimum growth. Cleaning incoming water and use of flow-through water is generally the ideal option for aquaculture, which is applicable for some species of high market value like trout, salmon or ornamental fish. But, changing water can sometimes introduce disease to the pond.
Physical and chemical parameters of water and soil:
Dissolved oxygen: The best possible dissolved oxygen (DO) content of pond waters is in the range of 5 ppm saturation level. Aeration is a proven method for improving DO availability. Any sort of agitation improves the DO content & among which paddle-wheel, aerators aspirators are most common.
Turbidity: Several factors like suspended soil particles, planktonic organisms & organic matter contribute to turbidity. Measured using Secchi disc the optimum visibility range from 40 cm-60 cm. It can be controlled by the application of organic manure at 500 kg/ha -1000 kg/ha.
Depth: Depth of a pond has an important bearing on the physical & the chemical qualities of water. On it, but changeable with its turbidity, depends the boundary of penetration of sunlight, which in turn, determines the temperature & the circulation patterns of the water and the extent of photosynthetic activity. The ideal depth for different kinds of fish ponds from the point of observation of congenial biological productivity are as follows;
- Nursery Pond: 1 – 1.5 m.
- Rearing Pond: 1.5 – 2.0 m.
- Stocking Pond: 2.0 – 2.5 m.
Ponds shallower than 1m get overheated in tropical summers inhibiting the survival of fish & other organisms. Depths greater than 5 m are also not proper for fish culture. Formation of H2S takes place in a reduced layer of pond mud & in the absence of the oxidizing surface layer, this poisonous gas diffusion into the water. In such ponds, there must be the provisions of plenty of breeze flowing which can keep water circulating of artificial water circulation.
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Temperature: Temperature sets the pace for metabolism & biochemical reaction rates. The optimum temperature range of cold water & warm water fishes is 14C-18C & 24-30C respectively. Temperature can be adjusted for an optimum stage in controlled conditions like hatcheries, but difficult to adjust in large water bodies. Operation of aerator helps in breaking thermal stratification while planting of trees provides shade.
Light: Light is the main factor influencing productivity. Penetration of light depends upon the presented intensity of the incident light, which varies with the geographical locations of the pond & turbidity of water. In shallow ponds, light reaches up to the bottom & causes heavy growth of vegetation. Light controls the flora & the oxygen content of the water of the pond. Shade present by the surrounding vegetation affects the incidence of light on the pond. The advantage of the shading effect is often taken at pisciculture result for the control of algal blooms and submerged weeds.
Pond mud: Pond productivity is increased only when the pond mud is rich in nutrients (phosphorus, nitrogen, organic carbon and etc.). The colloidal content of the soil particularly of the muddy layer on the top is of importance in its capacity to fix or chemically bind nutrient. The productive capacity of the pond bottom has to be preserved by an alternative period of mud formation & mineralization the practice of standard draining of fish ponds.
Ammonia: Fish are very sensitive to unionized ammonia (NH3) & optimum range is 0.02-0.05 ppm in the pond water. The same is reduced in the case of high DO & high CO2. Aeration, a healthy phytoplankton population removes ammonia from the water. The addition of salt 1200-1800 kg/ha reduces toxicity. Formalin is used in certain cases. The biological filter may be used to treat water for converting ammonia to nitrate & then to harmless nitrate through the nitrification process.
Hydrogen sulphide: The freshwater fish pond must be free from hydrogen sulphide because at a concentration of 0.01 ppm fish lose their equilibrium. Frequent exchange & increase of pH through liming can reduce its toxicity.
Nitrogen: About 99% of the combined nitrogen in the soil is contained in the organic matter (humus) in the form of amino acids, peptides & easily decomposed proteins. It may be in the form of inorganic compounds such as NH4+ and NO3 which are utilized by green plants Anaerobic organisms (bacteria) helps in the decomposition of organic matter into simple inorganic forms products such as CO2, water & ammonia which influences directly or indirectly in pond productivity.
The range of available nitrogen 50 – 75 mg/100 gm of soil is moderately more favorable for pond productivity. Though nitrogen is mostly available from organic matter, it can be made available by fixing atmospheric nitrogen into organic nitrogen with the help of nitrogen-fixing bacteria present in the soil & water, blue-green algae, and some micro-organisms.
Phosphorus: Phosphorus has been called “the key to life” because it is directly involved in life processes. It is second only to nitrogen in the frequency of use as a fertilizer factor. One or both of these elements are almost always included when fertilizer is applied. Phosphorus occurs in the soil in both inorganic & organic forms. The inorganic phosphorus is calcium phosphate, aluminum phosphate, iron phosphate & reductant soluble phosphate whereas organic phosphorus may occur as phytin or phytin derivatives, nucleic acids & phospholipids. The organic form constitutes 35% – 40% of the total phosphorus content of the soil. The availability of phosphorus is important to aquatic productivity owing to the fact that PO4 ions in soil form insoluble compounds with iron & aluminum under acidic conditions and with calcium under alkaline state, rendering the phosphorus ion unavailable to the water body.
Total alkalinity: Ideal range from 60-300 ppm as CaCO3 & it can be treated with lime. Less than 20 ppm leads to fluctuation & more than 300 ppm may become unproductive due to the limitation of carbon dioxide availability.
pH: pH is a measure of hydrogen ion concentration in water & indicates how much water is acidic or basic. Water pH affects fish metabolism, physiological process, the toxicity of ammonia, hydrogen sulphides & solubility of nutrient thereby well-being and fertility. PH in the range of 6-9 is best for fish growth & can be increased by the application of lime. Agriculture gypsum is applied to correct alkaline pH.
Texture: The nature and the properties of the parent material forming the soil verify the soil texture. An ideal pond soil should not be too sandy to permit leaching of the nutrients or should not be too clayey to keep all the nutrients absorbed on to it. For sandy soil, a heavy dose of raw or collected farmyard manure varies from 10000 to 15000 kg/ha/year is required.
Soil acidity: The soil must be acidic, alkaline, or neutral, but the ideal range of soil pH is 6-8. Acid ponds do not respond well to fertilization & liming is the only way to improve water quality with acid soil and it is the soil that must be corrected for lasting results, rather than the pH of the water.
Bottom soil oxidation: When the redox potential is low at the soil surface, hydrogen sulphide & other toxic microbial metabolites diffuse into the pond water. Sodium nitrate (NaNO3) can serve as a source of oxygen for microbes in a poorly oxygenated environment in which the redox-potential will not drop low enough for the formation of hydrogen sulphide & other toxic metabolites.
Sustainable Pond Productivity:
Nutrient removal: It is possible to precipitate phosphorus from pond water by relating sources of iron, aluminum, or calcium ions. Alum (aluminum sulphate) or ferric chlorides are commercially obtainable of which the former is cheap and widely used. Alum 20-30 ppm is more fitting in alkaline water (>500 ppm) and gypsum (calcium sulphate) 100-200 ppm is better in low alkaline water.
Plankton removal: Copper sulphate 1/100 of the total alkalinity is recommended for reducing phytoplankton abundance & blue-green algae in particular.
Liming: Liming should be always done depending upon the pH of the water & the soil. As the health of the soil determines the nature of the pond water, the pH of the water can be taken as a reference to verify the appropriate dose of application.
|Soil /Water condition
|The dose of lime (Kg/Ha)
It is possible to disinfect bottom of empty pond & waters in newly filled and unstocked ponds by applying chlorine products 1ppm or more of free chlorine residual. The residuals will detoxify physically in a few days so that ponds can be stock safely.
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