WATER QUALITY MANAGEMENT IN TANK FISH CULTURE: A SYSTEMS APPROACH
INTRODUCTION
In the wild, fish naturally swim against water current because water influx is richer in food and dissolved oxygen (DO). In static (stagnant) water culture system there is minimal water current, and within a short time system water quality deteriorates and smells due to the processes of fish metabolism, respiration and decomposition of organic waste all of which use up DO in the water. Fish under culture also requires DO to live. Given this competition it becomes necessary, therefore, that DO level and indeed other important water quality parameters, in the pond water must be regularly tracked and maintained within recommended permissible limits in order to provide desirable water quality for the cultured species. Where this is not the case the result will be poor fish growth or in extreme cases mortality. Poor fish growth or mortality translates to loss of revenue and fish farm business failure. Thus fish under culture live and do well in water of minimum quality else they die. This underscores the importance of water quality management in tank/pond fish culture. It is important to trace and understand the origin of water contaminants in fish ponds and tanks in order to manage or control them cost-effectively.
Sources of Water Contaminants/Pollutants in Fish Tanks/Ponds
Water is a universal solvent; it does not exist in its pure state in nature, devoid of contaminants. When the concentrations of contaminants in fish tanks exceed a certain threshold the results are water pollution and fish mortality.Keep in mind that fish eat, play and excrete in water, the uneaten feeds and wastes decompose and the products of decomposition of waste are toxic to fish.
Factors that adversely affect fish culture system water quality are: feeding rate(amount of feed ration given to fish stock per day), feed composition,fish metabolic rate, quantity of uneaten feed and faecal solids generated in fish tanks per day. The by-products of fish metabolism include carbon dioxide (CO2),ammonia-nitrogen(NH3-N)excreted across the gill membrane, urine and faecal solids. If uneaten feeds and metabolic by-products are not removed from the culture system, they will generate additional CO2, NH3-N,reduce DO content of system water and adversely affect fish growth. NH3-N and Nitrite-nitrogen (NO2-N)are toxic to fish and do cause fish mortality at concentrations less than 0.05mg/L.This is the reason why in static tank/pond fish culture it is recommended that polluted, smelling water is flushed out and renewed every 3 days else the natural capacity of bacteria in water to clean up the mess will be exceeded leading to fish kills. In the oceans, the vast volume of ocean water dilutes the concentration of wastes to the lowest safe levels, and the natural capacity of ocean water to clean up itself (degrade wastes) is seldom exceeded.This is why mass fish kills in oceans is not a common experience despite huge amounts of waste being dumped in the oceans every year.
WATER QUALITY MANAGEMENT: A SYSTEMS APPROACH
A system is any reasonably well-defined entity which is acted upon by certain external influences and as a result produces a specific effect or response. A fish tank/pond is a system; remember static water system (SWS), water flushing system (WFS), flow through system (FTS), and water recirculation system (WRS). Here the inputs into the system (tank/pond) are fish fingerlings, fish feeds, lime/fertilizer, good water quality, etc. The tank/pond accepts the inputs, operates on and transforms them into outputs namely fish biomass (kg), metabolic wastes and reduced system water quality. The principle of water quality system analysis is summarized in Figure 1 below.
Figure 1. System approach in water quality management (Source: Uzukwu (2013)).
Referring to Figure 1 above, the observed water quality output is the result of visual observation and testing of system water parameters, while the desired water quality objective is the water quality standards (the permissible limits) for tank/pond fish culture.
Water quality management can, therefore, be defined as monitoring and controlling (or maintaining) water quality parameters levels within the permissible limits for profitable fish production. In this context, water quality management involves three (3) activities: (i) observing and testing for the levels of system water quality parameters on regular basis during culture, (ii) comparing the observed/test results obtained in (i) above with the values in the water quality standards(Table 1), and (iii) taking no action if the results fall within the permissible limits, or taking corrective measures (or action) if the results fall outside the limits. A water quality manager or controller must know the cost-effective, corrective measures to take when test result for each parameter is outside the permissible limits, and this is the hallmark of water quality management.
There are very many water quality parameters in pond/tank culture (Table 1), but only a few are relevant in fish farming business.For large commercial fish farms they include: Dissolved oxygen(DO), pH, ammonia, nitrite, temperature, total alkalinity, transparency, colour, conductivity/salinity and total hardness while for small holder farms they include: Dissolved oxygen (DO), pH, ammonia, nitrite, temperature, and transparency. However, for the benefit of readers in academics a comprehensive treatment of all parameters is given in this publication.
Table1.Some water quality standards for warm water pond fish culture,Source;Uzukwu (2013)
A good practical knowledge of the procedures for testing water quality parameters,water quality standards for warm water fish species,and the effects of the dynamics of water quality parameters levels on cultured fish,are prerequisites for good water quality management practice. A staff with a good OND or HND or BSc. in Science Laboratory Technology, chemistry option, can handle all tasks in water quality management.
WATER QUALITY PARAMETERS
Water quality parameter is a variable indicator scale for defining quality of water for a specified purpose or use. In this context, the purpose is warm water fish ponds. The water quality parameters to be monitored in fish ponds can be classified into:
1.Physicalparameters: transparency, turbidity, colour, temperature, suspended solids, etc.
2.Chemicalparameters: dissolved oxygen, pH, ammonia, nitrite, total alkalinity, total hardness, etc,
3.Biological parameters: bacteria, phytoplankton, zooplankton, aquatic macrophytes (water weeds), etc.
PHYSICAL PARAMETERS
Temperature
Temperature is a measure of the thermal state of a given material or substance. It is measured using mercury-in-glass thermometer in degrees Celsius. In the tropics surface water temperature of shallow ponds/tanks can reach near 40 degrees Celsius or above in peak dry season. This causes fish kills especially if the depth is less than 0.50 m. As the water temperature increases the fish will require a larger quantity of food due to increase in their metabolic rate, The optimum range of water temperature in warm water fish ponds is 20 to 30 degrees Celsius. Temperature affects all biological and chemical processes, that is, all metabolic activities of the fish, notably their breathing, growth, and reproduction. Fish is poikilothermic, that is, their body temperature varies with that of the water environment. The amount of dissolved oxygen (DO) in water varies inversely with temperature and salinity of pond water. Therefore high temperature and salinity of water body translates to low DO content. The practical way to maintain optimum water temperature in fish ponds is to maintain water level at or above 1m mark. In fish tanks flushing out hot, polluted water and replacing with new, clean water is good remedial measure.
Water clarity (Transparency)
Transparency is a measure of transmission of light into the water. The extent of light penetration depends on the phytoplankton and suspended solid loading of the water. Transparency can be measured using a Secchi disc. A Secchi disc is a weighted metal or wooden disc usually of a diameter between 20 and 30 cm and painted in white or alternate black and white pattern. The upper surface is tied to a line or rope calibrated in centimeters (cm). To measure transparency, the Secchi disc is gradually lowered into the water until it just disappears. The depth (D1) at which it disappeared is noted. The disc is then gradually raised and the depth (D2) at which it just re-appears is noted. The average of D1 and D2 gives the Secchi disc transparency reading of the pond/tank. The recommended transparency range for pond water is 25 to 35 cm; within this range the pond can be manured. When transparency exceeds 35cm the water may not be fertile enough and more manure can be applied. When transparency is less than 25 cm, the water is very fertile (full of planktonic organisms). Little or no manure should be applied to the pond water. Transparency is best measured when the sun is overhead, that is, at noon; but it can still be taken any time. It is to be noted that water clarity can affect fish (stress, reduced growth) which prefer turbid waters e.g. catfish. However some suspended and dissolved materials can cause off-flavour in fish.
Turbidity
Turbidity is a measure of resistance of water to the transmission of light. It is a measure of the opaqueness of the water due to the presence of suspended particles. Turbidity caused by plankton is desirable up to certain extent, but that caused by suspended clay particles is undesirable. This is because light penetration into the pond water is drastically reduced thereby lowering primary productivity of phytoplankton leading to poor fish yield. Besides clay particles damage fish gill structures.
In tank fish culture, the major cause of turbidity is suspended solids arising from uneaten feeds, faecal waste solids and plankton die-offs. These soon undergo aerobic decomposition yielding ammonia (NH3-N), nitrite (NO2-N) and other products which are toxic to fish. In situation like this the polluted water in the tank needs to be exchanged (flushed out and replaced) with clean, clear water. One way to avoid water pollution in tank culture is to avoid overfeeding.Turbidity can be measured in either nephelometer turbidity units (NTU) or formazin turbidity unit (FTU) using nephelometer or spectrophotometer respectively.
Water Colour
The colour of pond/ tank water indicates the type of plankton present in it. When the water is brownish green, it has balanced proportion of plankton.This is good. If the water appears dark green, bright green or lime green it indicates too many green and blue-green algae are present in the water. This is not good for the fish. When this happens, the remedy is to partially flush the pond and top up with new clean water to dilute the plankton concentration.
Water Odour
The odour of pond/ tank water indicates serious water pollution arising from the aerobic and anaerobic decomposition of organics in the water. This should be avoided at all cost. If the measures already advanced such as avoidance of over feeding, and flushing the system when necessary, are adhered to, the incidence of bad odour will be reduced to the barest minimum.
CHEMICAL PARAMETERS
Dissolved Oxygen
Fish need dissolved oxygen (DO) in water for their metabolic activities-respiration, and growth. Oxygen deficiency adversely affects fish growth, and their well-being. The optimum range of dissolved oxygen in fish ponds is 5.0 to 9.5 mg/L. If the DO is below 1mg/L the fish will be under great stress. Symptoms of oxygen deficiency include:
1. Fish come to the surface in an effort to breathe air,
2. Fish group together near the water inlet,
3. Fish killed by asphyxia have raised gill covers and their gills are wide apart.
To ensure sufficient oxygen in earthen pond it is necessary to maintain optimum phytoplankton level in the pond. Aerators (paddle wheel, air pumps, root blowers, boat ride, and water fountain) can be used to arrest oxygen deficiency in fish pond/ tank. In the absence of aerators, water can be drained by one-fifth and then topped up with fresh water. DO can be measured either by titration using chemical reagents (modified Winkler method, Polarographic meter or colorimetric kits.
Hydrogen ion concentration
Hydrogen ion concentration (pH) of water indicates whether the water is acid or alkaline in reaction. pH is measured on a scale from 0 to 14. Water with pH of 7 isneutral. Water with pH less than 7 is acidic and pH above 7 is alkaline. The optimum pH range for fish culture is 6.5 to 8.5. The acid death point is pH 4.0 while the alkaline death point is pH 11.0 (Boyd, 1979).
For fish ponds with low pH level, the pH can be raised by applying agricultural lime or crushed oyster shell (which is rich in calcium carbonate, CaCO3) to the pond water. On the other hand, high pH levels can be lowered by treating with either alum (Aluminium sulphate) or poultry manure. In laboratory experiments Carbonic acid, a weak, mild acid can be used to lower pH of test water. The rate of application of alum is 10 to 20 mg/L. Treatment with lime or alum should be carried out two weeks before stocking the pond.pH of pond/tank water can be measured using pH paper, pH comparator or pH meter.
Carbon Dioxide
Problems associated with carbon dioxide in fish pond are likely to arise only when using groundwater source, transportation of fish in high densities, or in water recirculation systems. At elevated concentration of carbon dioxide causes fish to lose balance and possibly die. Various methods can be used to ensure that carbon dioxide level is kept at safe limits such as aeration or diffusion of oxygen or use of buffer systems. Carbon dioxide values can be tested using the well familiar acid-base titration in the laboratory with methyl orange indicator or the simplified version in Lamotte aquaculture water quality test kits.
Alkalinity
This refers to the amount of carbonate, bicarbonate and hydroxide ions and less frequently by borate, silicate and phosphate ions present in the water. Alkalinity can be measured by titration. The desirable range of alkalinity value for fish culture is 50 to 200 mg/L. If alkalinity falls below 20 mg/L, growth of plankton in the water is hindered. The level of alkalinity in the water can be raised by treatment with lime or calcium carbonate before stocking with fish. The higher the alkalinity value the more stable is the pH. The term total alkalinity is the sum of phenolphthalein alkalinity and methyl orange alkalinity.
Hardness
Hardness comprises the concentration of divalent metallic ions, particularly calcium and magnesium, in water expressed as calcium carbonate equivalent. Calcium and Magnesium are generally associated with carbonate minerals which are the principal sources of alkalinity in water.Therefore the concentrations of calcium and magnesium are often similar in magnitude. Water hardness is classified as follows: 0–75 mg/L (soft); 75 to 150 mg/L (very hard). The part of the total hardness chemically equivalent to the alkalinity is termed the carbonate hardness. Therefore, if the total alkalinity is less than the total hardness the carbonate hardness equals the total alkalinity. When alkalinity is equal to or greater than the total hardness the carbonate hardness equals the total hardness (Boyd, 1979). Calcium and magnesium hardness together make up the so called total hardness.
It has also been observed that when the alkalinity of water exceeds its total hardness (as in coastal waters) some of the HCO,- and Carbonate ion is associated with other cations, such as K+, Na+rather than Ca2+and Mg2+.
Likewise, if the total hardness is greater than the total alkalinity, some of the Ca and Mg is associated with Sulphate ion-, Cl-, SiO3-, or NO3- rather than HCO3- and Carbonate ion. Hardness is determined by EDTA titration in the laboratory with Eriochrome black T indicator or the simplified version in Lamotte aquaculture water quality test kits.
Biochemical Oxygen Demand
The biochemical oxygen demand (BOD) is the amount of dissolved oxygen (mg/L) needed during stabilization of the decomposable organics (wastes) by aerobic bacteria. The higher the BOD the higher the organic waste loading of the water body (Abowei and Sikoki, 2005).Horsfall and Spiff (2001) reported a BOD value of 4 as FEPA interim water quality standard for aquatic life. The procedure for BOD determination involves diluting suitable portions of the water sample with water saturated with oxygen and measuring the initial dissolved oxygen content of the mixture and after incubation for 5 days at 20°C in a BOD incubator. Several equipment are used in the process of measurement of BOD such as autoclave, incubator, DO meter, etc.
Ammonia-nitrogen, nitrite-nitrogen and Nitrate-nitrogen
Ammonia is a product of fish metabolism and is secreted across fish gills. Ammonia is also generated as a result of decomposition of fish faeces and uneaten feeds. Fish tank naturally contains ammonia oxidizing bacteria that break down manure into nutrients (Nitrogen, Phosphorus, Potassium-N.P.K), which can be utilized by phytoplankton. The process of organic decomposition requires a lot of oxygen. In situation of sufficient oxygen the process proceeds to the right by aerobic nitrification
In the absence of sufficient oxygen the process goes to the left, and the system contains a mixture of ammonia, nitrite and nitrate. Now unionized ammonia (NH3-N) and nitrite (NO2-N) are toxic to fish. Nitrite combines with haemoglobin in the fish blood and produces a compound called methaemoglobin which reduces the oxygen carrying capacity of the blood. This condition is called methaemoglobinernia. This can lead to mortality of fish. This can be avoided if the system is flushed partially or completely when necessary. Also over feeding should be avoided because it results to excessive production of ammonia and nitrite. Masser at al (1999) stated that nitrite toxicity can be reduced or blocked by addition of chloride ions at a rate of 6 to 10 parts chloride ions to 1 part nitrite-nitrogen.
Hydrogen Sulphide (H2S)
Hydrogen sulphide (H2S) smells like rotten egg. It is toxic to fish at very low concentrations. H2S is more harmful to fish than NH3-N because even in the presence of abundant oxygen, H2S still exerts its toxicity when present at low concentrations. The lethal level of H2S in water is 0.08 mg/L. It combines with the fish blood (haemoglobin) to exert its toxicity. This results in sulphaemoglobinemia or brown blood disease and can lead to mass fish kil1s in the pond. Hydrogen sulphide is measured by method of spectrophotometry.
Methane (CH4) Gas
Methane is commonly present in swamps. It is not very harmful to fish and it can be observed on the water surface as air bubbles coming from the bottom of the pond. CH4 combines with DO as it comes to the surface and releases the DO into the atmosphere as well.
Salinity
Salinity is the total concentration of all dissolved ions in the water expressed in milligrams per litre. The osmotic pressure of water increases with salinity. Fish species differ in their osmotic pressure requirements, so the optimum salinity for fish culture differs to some extent with species (Boyd and Lichtkoppler, 1979).
Fish are highly sensitive to sudden changes in salinity. Boyd (1990) stated that fish living in water of a given salinity should not be suddenly transferred to water of much higher or lower salinity. This is of practical significance in live fish transportation where the source and target water salinities should vary by no more than 5%. The salinity of water also dictates the type of fish species to be cultured in the proposed fish farm. Some fish species are euryhaline (tolerating wide salinity ranges), while others are stenohaline (do not tolerate wide salinity range) in freshwater or marine water. During the planning stage of an aquaculture operation, salinity should be measured to determine whether freshwater, brackish water or marine water fish should be stocked in the pond. Salinity can be measured using conductivity meter, refractometer (salinometer) or argentometric titration.
Conductivity
The conductivity of a solution is a measure of its capacity to conduct an electric current. It is related to the nature and concentration of ionized substances present in the solution and the temperature of the solution. The equipment used to measure conductivity is the conductivity meter. It is very similar to pH meter.
Iron
In the Niger Delta Region of Nigeria especially Bayelsa State many ground waters contain high levels of dissolved iron. When exposed to the air, the iron is oxidized to insoluble brown or reddish clumps of iron. These clumps of iron do settle on fish gills, causing irritation and stress, and most times are the cause of poor hatching in fish hatcheries. Treatment operations for such water supplies are aeration, and filtration of precipitates formed before the water can be supplied to culture systems. Iron content is also measured using spectrophotometry.
Chlorine
During aquaculture training workshops in African Regional Aquaculture Centre (ARAC) questions are frequently asked as to the suitability of municipal water supplies for aquaculture operations.
Municipal water supplies are typically contain residual chlorine at 1.0 mg/L to control bacteria. Chlorine levels as low as 0.02 mg/L can be stressful to fish. If such waters must be used to farm fish the residual chlorine must be removed by ageing, or aeration, with chemicals such as Sodium thiosulphate or filtration through activated carbon. Colorimetric kit can be used to measure chlorine.
BIOLOGICAL PARAMETERS
Bacteria:
Bacteria are present in pond/tank water. Some of these are harmful to fish while others are not. The bacteria in the pond water may be grouped into heterotrophic and autotrophic bacteria based on their feeding habit. The concentration of heterotrophs should not exceed l0,000 individuals/mL; while the total bacteria concentration is 1,000,000 individuals/mL. At this level water is fertile enough to culture fish.
Phytoplankton
Phytoplanktons are free floating microscopic plants present in the pond water. These add colour to the pond water and are the primary producers in a fish pond. Phytoplankton forms the food base for fish either directly or indirectly. The minimum required level of phytoplankton in non-aerated ponds is 20 mg/L. However, ponds that are regularly aerated, harvested and stocked with planktivorous fish can cope with higher levels up to 100 mg/L. This will ensure higher fish yields which means more value for your money (Ansa, 2006).
Some undesirable phytoplankton exists and includes some of the greens and the blue-greens. This can occur in a fish pond receiving heavy manure loading. Some blue-green algae (Cyanophyta) are weed species and form unpleasant growths leading to noxious water blooms e.g. Microcystis and Anabaena. The respiratory demands of the algae especially at night soon surpass their daylight oxygen depletion in the pond water. As a result, fish tend to come to the water surface early in the morning to gasp for air. Furthermore, some blue-greens produce dangerous toxins which results in massive fish kills when they are present in large numbers. Six species that are culprits are microcystis, Nodularia, Coclosphacrim, Glocotrichia, Anabaena, Aphanizomenon.
Zooplankton
Zooplanktons are the free floating microscopic animals present in the water body. A concentration of 5 mg/L of zooplankton is indicative that the water is fertile. Maximum level required is 15 mg/L. Above this level the phytoplankton biomass will be greatly reduced as zooplankton graze on phytoplankton. So for best results in fish growth a range of 5–8 mg/L of zooplankton is optimum.
Organic Detritus
Detritus consists of dead tissues of organic origin and other living organisms such as bacteria engaged in the decomposition of organics. Detritus can be defined as particulate that was part of a living organism. Detritus includes the following
a. Particles from recent dead plants and animals,
b. Finely disintegrated particles of dead animals and plants,
c. Faecal solids, and
d. Aggregates of colloidal particles, etc.
Aquatic Macrophytes
These are aquatic plants that are found in different ecological zones in the pond. Four ecological groups exist, namely:
- Floating weeds e.g. Eicchornia crassipes (water hyacinth)
- Rooted emergent weeds e.g. Nymphaea lotus (water lily)
- Submerged weeds e.g. Ulricularia (bladder wort)
-Marginal weeds e.g. Cyrtospermum (swamp arum), Dissotis (=Heterotis spp)
Aquatic macrophytes are not desirable in ponds because they reduce light penetration into the pond water thereby limiting photosynthetic activity of the phytoplankton which is the natural food organisms of fish. Therefore all aquatic macrophytes should be removed from the ponds before fertilization.
SUMMARY
The publication reviewed the major water quality parameters that are important in freshwater and brackish water aquaculture systems and methods to monitor and control them based on systems approach. Besides good nutrition, water quality determines the survival, well being and growth of fish in intensive aquaculture operations. The origin of water quality deterioration in fish culture systems such as nitrogen metabolism and respiration were explored. Water quality testing procedures, equipment and tools, and interpretation of results based on systems approach were discussed. It is hoped that it will provide deeper understanding of the subject matter especially among undergraduate students and fish farmers in Nigeria and beyond.
REFERENCES
Abowei,J.F.N. and Sikoki, F.D.(2005).Water pollution management and control. Doubletrust Publishing Company,Port Harcourt, p 59–60.
Ansa, E, J. (2006). Water quality management in fish pond/tank.Proceedings of train the trainers workshop on intensive aquaculture enterprise, ARAC. Port Harcourt 27–28 July, 2006. pp. 55–60.
Anyanwu, P.E (2006), Water recirculation system. Proceedings of train the trainers workshop on intensive aquaculture enterprise, ARAC. Port Harcourt 27–28 July, 2006.pp 23–36.
Boyd, C.E. (1990). Water quality in pond aquaculture, Birmingham Publishing Company, Birmingham, AL. 482 pp.
Boyd, C.E. and Lichtkoppler, F.(1979).Water quality management in pond fish culture Agric. Exp. Stn. Auburn Univ., Res. Dev. Series №22. 30 pp.
Uzukwu, P.U.(2013). Water quality management in warm water fish ponds: A systems approach. Cont. J. Biological Science 6 (1):16–25, 2013.
