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  • Chapter 5 Water Quality and General Pond Management

    70

    CHAPTER 5:

    WATER QUALITY AND GENERAL POND MANAGEMENT

    Water quality is the first most important limiting factor in pond fish production. It is also the most difficult production factor to understand, predict and manage. Water is not just where the fish live. Its quality directly affects feed efficiency, growth rates, the fishs health and survival. Most fish kills, disease outbreaks, poor growth, poor feed conversion efficiency and similar management problems are directly related to poor water quality. Water quality refers to anything in the water, be it physical, chemical or biological that affects the production of fish. The objective of pond management, is to manage the water quality, so as to provide a relatively stress free environment that meets the physical, chemical and biological standards for the fishes normal health and production performance. Within a pond, water quality is a product of:

    1. the quality of water at the water source, 2. the quality of the pond soils and immediate environment, 3. production technology and management procedures

    employed, notably those associated with feeding, the maintenance of adequate dissolved oxygen as well as any other chemicals or inputs applied.

    5.1. The Water Quality Requirements for Catfish Production and their Relevance to Production

    The key water quality parameters for pond production are temperature, oxygen, pH, alkalinity, hardness and nitrogenous waste. In table 5.1 below, the recommended values for the key parameters are listed. However, due to the dynamics within the pond, they fluctuate daily depending upon photosynthesis of aquatic plants like algae, the production management technology employed and local weather conditions.

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    Table 5.1. Relevance of Water Quality Parameters to Pond Production

    Parameter Relevance to Production Recommended

    Range

    What happens when Consistently below recommended Value

    What happens when consistently above recommended value

    Dissolved Oxygen

    Fish breathe in oxygen for their metabolism.

    Dissolved oxygen is needed to oxidise potentially toxic metabolic wastes into less toxic forms (e.g. ammonia (NH3) to nitrite (NO2-) and then nitrate (NO3-)

    Bacteria in ponds that help transform wastes into less toxic products need oxygen for metabolism.

    Phytoplankton use oxygen at night during respiration.

    From 4 mg/l to saturation for catfish eggs, larvae, fry and fingerlings. As catfish develop, their accessory breathing organs enable them get oxygen by gulping at the water surface when dissolved oxygen levels are low. However, even adults will perform better when the dissolved oxygen levels are adequate.

    0 1.5 mg/l can be lethal especially if exposed for long periods

    1.4 5 mg/l-fish survive, but reduced feed intake higher FCRs, slow growth, stress, and increased susceptibility to disease results. Build up of toxic wastes because they are not broken down (oxidised)

    Gas bubble trauma when the water is supersaturated to levels of 300% and above.

    Temperature Fish are cold blooded animals. Their rate of metabolism is directly influenced by water temperature.

    Rate at which wastes in pond are broken down and chemicals dissolve is faster in warmer waters

    Affects the solubility of

    26 to 32 oC Below 15 oC. Growth stops and death occurs at extremes

    15 to 26 oC. Reduced feed intake and growth rates. Higher FCRs. Fish more stressed at lower temperatures, therefore, more susceptible to disease.

    Lower solubility of oxygen, stress and death at extreme temperatures.

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    Parameter Relevance to Production Recommended Range

    What happens when Consistently below recommended Value

    What happens when consistently above recommended value

    oxygen. Organic matter and other wastes are broken down at a slower rate when temperatures are low with high risk of eutrophication.

    pH Affects the solubility and chemical forms of various compounds some of which can be toxic.

    6.5 to max 9 Below 4, acid death point. 4 6.0. Survive but

    stressed, slow growth, reduced feed intake, higher FCRs.

    Higher proportion of Total Ammonium Nitrogen is in the form of ionized ammonia, which is less toxic for fish.

    Low pH indicates high levels of dissolved carbon-dioxide

    9 11 Stressful for catfish, slow growth rate.

    Above 11 alkaline death point. All life, including bacteria in pond will die at this point.

    Higher proportion of Total Ammonium Nitrogen in the form of unionized ammonia in water, which is more toxic for the fish.

    Alkalinity and

    Hardness In combination, influence

    the buffering capacity of the pond water.

    Hardness is composed mostly of calcium and magnesium, which affect the physiological condition of the fish.

    Alkalinity also controls the amount and form of carbon-dioxide in water.

    Alkalinity > 20 ppm Hardness > 20 ppm Total alkalinity and total hardness above 60 ppm is desirable.

    Extreme fluctuations in pond pH levels during the day which is stressful to the fish.

    Fish are under physiological stress

    Low levels of primary production which results in lower natural sources of food.

    Water will be well buffered and diurnal fluctuations in pH will be less extreme

    Fish will be less stressed physiologically

    Young fish will have more natural food available.

    However, hard water in catfish hatcheries should be avoided.

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    Parameter Relevance to Production Recommended Range

    What happens when Consistently below recommended Value

    What happens when consistently above recommended value

    Total Ammonia Nitrogen (TAN)

    Ammonia by-product of protein breakdown. It occurs in both a toxic form (ammonia) and non-toxic form (ammonium) depending on the pH of the water.

    Not more 0.3 2 mg/l of the toxic form (ammonia). The proportion of TAN in the form of ammonia tends is higher as the pH of the water increases above 7

    Fish are happiest when there is no or little ammonia in water.

    The fish succumb more to attacks by trematodes and other parasites.

    The fish fail to eliminate ammonia from their blood because there is too much ammonia already in the water. Ammonia is excreted by fish as a by-product of protein metabolism primarily through their gills. High concentrations of ammonia in water reduce the ability of the gills to do so.

    Table 5.1: Continued

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    5.2. Managing the Water Quality Parameters 5.2.1. Dissolved Oxygen Most of the dissolved oxygen in non-aerated fish ponds is generated as a result of photosynthesis by phytoplankton (microscopic plants). Some oxygen is also incorporated into the water from the air, especially when the wind blows over the water surface and creates water movement (mixing). Photosynthesis occurs only in the column where sunlight can penetrate because sunlight is a catalyst to the process (see equation 2 below). Sunlight in ponds penetrates to a depth of 30 to 80 cm depending on the levels of water turbidity. Therefore, only the water up to 70 cm to 1 m deep from the surface is oxygenated from photosynthesis in static water ponds. That is why there is no need to dig the pond deeper than the recommended maximum, especially if no additional mechanical aeration is available. Also, shade from trees will reduce sunlight penetration and photosynthesis.

    6CO2 + 6H2O C6H12O6 + 6O2 Carbon dioxide Water Carbohydrate Oxygen

    Equation 2 above illustrates the process of photosynthesis showing the major products, carbohydrate and oxygen.

    During the day, oxygen is generated. At night, however, the phytoplankton within the pond use up the dissolved oxygen in a process called respiration. During respiration, carbon dioxide is produced and it dissolves into the pond water. Because carbon dioxide is a weak acid, it causes the pond pH levels to drop at night.

    6O2 + C6H12O6 6CO2 + 6H2O + Energy Oxygen Carbohydrate Carbon dioxide Water Equation 3 above illustrates the process of respiration whereby the oxygen and carbohydrate produced during the day is used up as nutrients by algae at night. Carbon dioxide, water and energy are consequently produced as by-products during this process.

    Therefore, dissolved oxygen levels in the pond are normally at their highest late in the afternoon and lowest after midnight towards early hours of the morning. Dissolved oxygen levels are reduced on cloudy days due to the reduced intensity of sunlight. The pH levels also fluctuate in a similar fashion to dissolved oxygen. pH is normally being highest late in the afternoon and lowest in the early hours of the morning. This is

    sunlight

    nutrients

    Equation 3

    Equation 2

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    because acidic carbon dioxide is used during the day for photosynthesis and carbon dioxide is produced at night from respiration. The other factors that affect the levels of dissolved oxygen in catfish ponds are water temperature, organic loading and the number and size of fish as well as other aquatic animals in the pond.

    5.2.2. Temperature During the day, pond waters often warm up to temperatures higher than that of the inflowing stream. In the tropics, the temperature ranges generally lie within that for production and the significance of temperature on dissolved oxygen levels is less significant than that of organic loading on dissolved oxygen. Since catfish is a warm water fish, it is preferable to have warm water in the ponds for faster growth. Therefore, pond waters should be managed to maintain warm temperatures, the optimum for catfish being 28-32C. Do not allow water to continuously flow through the pond because the water temperature in flowing streams is often lower than that in a pond under static water management. Constant flow through of water, therefore results into a reduction of pond water temperatures. Remember, the lower the pond water temperature, the less food the fish will consume and the slower their growth rate will be. The higher the pond water temperature is, the lower the solubility of dissolved oxygen in water, but this is important to know for hatcheries and for transport of fry.

    5.2.3. Organic Loading When organic matter is added into ponds it results into an increased demand for the available dissolved oxygen and an increased likelihood of pollution. The likelihood and degree to which pollution might occur becomes reduced if the organic matter added can be broken down into smaller less complex particles that are less toxic. This is what is termed as the break-down and assimilation of organic matter. Oxygen and bacteria are required for the effective break-down and assimilation of organic matter. The oxygen binds with some of the compounds constituting the organic matter to form less toxic compounds. The bacteria on the other hand, are actively involved in breaking down organic matter and transforming it into dissolved waste compounds which when they bind with oxygen become less toxic and polluting. The bacteria

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    that breakdown wastes in the pond also need the oxygen to live, Therefore, dissolved oxygen in ponds is used up not just by the fish, but by organic matter and other life forms. Water temperature influences the rate at which organic matter is broken down and assimilated; the higher the temperature the faster the process shall be. Controlling input levels of organic matter therefore becomes a pre-requisite for managing dissolved oxygen levels as well as other water quality parameters within the pond. However, the major inputs into fish production, are organic matter. In the normal course of catfish production, feed is the major organic input into the pond. During feeding and soon after feeding, the levels of dissolved oxygen in the pond drop. This is because at such moments, the fish need extra oxygen to eat, digest and metabolise the feed that has been given to them. In addition, because the feed is organic matter, as soon as it is enter the water, oxygen is used up as oxidation starts right away. More oxygen is also required for the break-down and assimilation of any left-over feed and the fish faeces. Hence, feeding must be done in a manner that minimises the negative effects of organic loading on water quality. This is effectively done by feeding only the amounts of feed that the fish can consume at each meal and opting to use highly digestible feeds with a high water stability (for more details see sections 6.1. and 6.2.). For this reason, one should not administer more than 20g of feed per m2 per day to catfish ponds receiving no aeration. If one is using a low quality feed that falls apart easily when in water and is not easily digestible by the fish, then he/she should apply even less feed. Remember, once feed has been fed to the fish in a pond, the excess feed cannot be removed. It drops to the bottom of the pond as organic matter. Accumulated wasted feed, subsequently results into pollution instead. Feeding fish by response, is one way of ensuring that the fish are only fed what they can consume.

    5.2.4. Ammonia Ammonia is the by-product from protein metabolism excreted by fish and bacterial decomposition of organic matter. Examples of such organic matter include wasted feed, faeces, dead plankton, etc. Fish excrete ammonia across their gills. Ammonia is toxic to fish. High levels in water affect ammonia excretion, blood pH, enzyme systems, and cause gill damage. This is because high levels of ammonia in water impair the

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    excretion of ammonia from the fish into the water across its gills. Because of the way the gills become affected, the efficiency with which the fish can extract oxygen from the water through their gills also falls. Fish can tolerate 0.01 to 0.05 mg/l of unionised ammonia without a significant negative effect on production as long as levels of dissolved oxygen and water temperature are within the recommended range. They are able to withstand levels of unionised ammonia of up to 0.6 to 2 mg/l for only short periods. Ammonia occurs in two forms depending on the acidity of the water. The unionised form of ammonia (NH3) is more dominant when the water is alkaline and the ionised form, ammonium (NH4+) when the water is acidic. In most cases, both forms occur, hence the term Total Ammonia Nitrogen. Total ammonia nitrogen is the combined measure of its two forms, unionised ammonia (NH3) and ammonium ion (NH4+). The relative occur...

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