In Aquaculture, fish perform all their bodily functions in water. Because fish are totally dependent upon water to breathe, feed and grow, excrete wastes, maintain a salt balance, and reproduce, understanding the physical and chemical qualities of water is critical to successful aquaculture. To a great extent, water determines the success or failure of an aquaculture operation.
1. Physical Characteristics of Water
Water can hold large amounts of heat with a relatively small change in temperature. This heat capacity has far reaching implications. It permits a body of water to act as a buffer against wide fluctuations in temperature. The larger the body of water, the slower the rate of
temperature change. Furthermore, aquatic organisms take on the temperature of their environment and cannot tolerate rapid changes in temperature.
Water has very unique density qualities. Most liquids become denser as they become cooler. Water, however, gets denser as it cools until it reaches a temperature of approximately 40C.
As it cools below this point, it becomes lighter until it freezes at 00C. As ice develops, water increases in volume by 11 percent. The increase in volume allows ice to float rather than sink, a characteristic that prevents pond from freezing solid.
2. Water Balance in Fish
The elimination of most nitrogen waste products in land animals is performed through the kidneys. In contrast, fish rely heavily on their gills for this function, excreting primarily ammonia. A fish’s gills are permeable to water and salts.
In the ocean the salinity of water is more concentrated than that of the fish’s body fluids. In this environment water is drawn out, but salts tend to diffuse inward. Hence marine fishes drink large amounts of sea water and excrete small amounts of highly salt-concentrated urine.
In fresh water fish, water regulation is the reverse of marine species. Salt is constantly being lost through the gills, and large amounts of water enter through the fish’s skin and gills.
This is because the salt concentration in a fish (approximately 0.5 percent) is higher than the salt concentration of the water in which it lives. Because the fish’s body is constantly struggling to prevent the “diffusion” of water into its body, large amounts of water are excreted by the kidneys.
As a result, the salt concentration of the urine is very low. By understanding the need to maintain a water balance in fresh waters fish, one can understand why using salt during transport is beneficial to fish.
3. Sources of Water
Water is always a limiting factor in commercial fish production. Many of the negative chemical and environmental factors associated with most operations have their origins in the source of water selected.
Final site selection has to be made based on both the quality and quantity of water available. The most common sources of water used for aquaculture are wells springs, rivers and lakes, groundwater, rainfall and municipal water. Of the sources mentioned, rainfall, wells and springs are considered to consistently be of high quality.
3.1. Water’s Physical Factors
(i) Temperature: After oxygen, water temperature may be the single most important factor affecting the welfare of fish. Fish are cold-blooded organisms and assume approximately the same temperature as their surroundings. The temperature of the water affects the activity, behaviour, feeding, growth, and reproduction of all fishes.
Metabolic rates in fish double for each -80C rise in temperature. Channel catfish and tilapia are examples of warm water species. Their temperature range for growth is between 24-320C. Temperature of 290C for catfish and 310C for tilapia is considered optimum.
Walleye and yellow perch are examples of cool water species. Ranges for optimum growth fall between 16 and 290C. Temperature in the upper end of this range is considered best for maximum growth for most cool water species. Coldwater species include all species of salmon and trout.
The most commonly cultured coldwater species in the Midwest is rainbow trout, whose optimal temperature range for growth is 9-180C. Ideally, species selection should be based in part on the temperature of the water supply. Any attempt to match a fish with less than ideal temperatures will involve energy expenditures for heating or cooling.
Temperature also determines the amount of dissolved gases (oxygen, carbon dioxide, nitrogen, e.t.c.) in the water. The cooler than water the more soluble the gas. Temperature plays a major role in the physical process called thermal stratification. As a result of the uniqueness of water in terms of densities and high heat-capacity, nutrients, dissolved gases, and fish wastes are evenly mixed throughout the pond.
As the days become warmer, the surface water becomes warmer and lighter while the cooler-denser water forms a layer underneath. Circulation of the colder bottom water is prevented because of the different densities between the two layers of water. Dissolved oxygen levels decreases in the bottom layer since photosynthesis and contact with the air is reduced.
The already low oxygen levels are further reduced through decomposition of waste products, which settles to the pond bottom. Localized dissolved oxygen depletion poses a very real problem to the fish farmer.
Read Also: Fishery Biodiversity Conservation
(ii) Suspended Solids: This is associated with plankton, fish wastes, uneaten fish feeds, or clay particles suspended in the water.
Suspended solids are large particles which usually settle out of standing water through time. Large clay particles are an exception while small clay particles are kept in suspension due to the negative electrical charges associated with them.
(iii) Plankton: Turbidity caused by phytoplankton (microscopic plants) and zooplankton (microscopic animals) is not directly harmful to fish. Phytoplankton (green algae) not only produces oxygen, but also provides a food source for zooplankton and filter feeding fish/shellfish.
Phytoplankton also uses ammonia produced by fish as a nutrient source. Zooplankton is a very important food source for fry and fingerlings such as hybrid striped bass and yellow perch. However, excessive amounts of algae can lead to increased rates of respiration during the night thereby consuming extra oxygen.
Excessive phytoplankton buildup or “blooms” which subsequently die will also consume extra oxygen. Any wide disparity between day and night oxygen levels can lead to dangerously low oxygen concentrations.
(iv) Fish Wastes: Suspended fish wastes are a serious concern for water recirculating culture systems. Large amounts of suspended and settle able solids are produced during fish production.
Therefore fish waste particles can be a major source of poor water quality since they may contain up to 70 percent of the nitrogen load in the system. These wastes not only irritate the fish’s gills, but can cause several problems to the biological filter.
The particulate waste can clog the biological filter, causing the vitrifying bacteria to die from lack of oxygen. Particulate waste can also promote the growth of bacteria that produces rather than consumes ammonia.
(v) Clay: Most clay turbidity problems are the result of exposed soil on the pond levee, exposed watershed, or feeding on bottom dwelling organisms. In natural bodies of water, turbidity values seldom exceed these critical levels. Even muddy looking ponds rarely have concentrations greater than 2,000 ppm.
Turbidity caused by clay or soil particles, however, can restrict light penetration and limit photosynthesis. Sedimentation of soil particles may also smother fish eggs and destroy beneficial communities of bottom organisms.
Removal of clay turbidity can be accomplished by adding materials that attach to the negative charges of the clay particles, forming particles heavy enough to settle to the bottom. Common remedies for clay turbidity are 7-10 square bales of hay per surface acre, or 300-500pounds of gypsum per surface acre. Gypsum application may be repeated at two week intervals if pond is still not clear.
3.2. Water’s Chemical Factors
(i) Photosynthesis: Photosynthesis is one of the important biological activities in standing pond aquaculture. Many water quality parameters such as dissolved oxygen, carbon dioxide, pH cycles, and nitrogenous wastes products are regulated by the photosynthetic reaction in phytoplankton.
It can be simply put as the process by which phytoplankton uses sunlight to convert carbon dioxide into food source and release oxygen as a by-product.
This process can be summarized as the equation below;
CH2O (food) + O2 ↔ CO2 + H2O + energy ↑
In addition to supplying oxygen in fish ponds, photosynthesis also removes several forms of nitrogenous wastes, such as ammonia, nitrates, and urea. The phytoplanktonic plant pigments involved in this chemical reaction is referred to as chlorophyll.
Because the photosynthetic process is driven by sunlight, greatest concentrations of oxygen occur when the sun is highest on then horizon while phytoplanktons primarily respire at night when photosynthesis ceases.
Respiration is the reverse of photosynthesis in that oxygen is used by phytoplankton to convert food to energy and carbon dioxide is released as a by-product. Phytoplankton respiration also occurs during the day but the fish framer is fortunate since there is usually a surplus of oxygen produced to compensate for the loss.
(ii) Dissolved Gases: These are gases which are in water solution.
An example of gas dissolved in water is soda water which has large quantities of dissolved carbon dioxide. Concentrations are measured in parts per million (ppm) or milligrams per liter (mg/l), both units of measure are the same. (One ppm or mg/l is the same as one pound added to 999,999 pounds to 1,000,000 pounds).
(iii) Oxygen: Dissolved oxygen (DO) is by far the most important chemical parameters in water for fish production.
Low DO levels are responsible for more fish kills, either directly or indirectly, than all other problems combined. Like human fish require oxygen for respiration and the amount of oxygen consumed by the fish is a function of its size, feeding rate, activity level and temperature. Small fish consumed more DO than do large fish because of their higher metabolic rate.
Fish farmer, in an attempt to maximise production, stock greater amounts of fish in a given body of water than found in nature especially when using surface pond or concrete tanks.
Therefore, it may be necessary to supply supplemental aeration to maintain adequate levels of dissolved oxygen. Whereas in Recirculatory aquaculture system (RAS) the farmer must supply 100 percent of the oxygen needed for the fish and beneficial nitrifying bacteria.
To obtain good growth, fish must be cultured at optimum levels of DO. A good rule of thumb is to maintain DO levels at saturation or at least 5ppm. DO less than 5ppm can place undue stress on the fish, and levels less than 2ppm will result in death. Some warm water fish species such as tilapia and carp are better adapted to withstand occasional low DO levels, while most coldwater species cannot.
Fish are not the only consumers of dissolved oxygen in aquaculture systems; bacteria, phytoplankton, and zooplankton consume large quantities of oxygen as well. Decomposition of organic materials (algae, bacteria, and fish wastes) is the single greatest consumer of oxygen in aquaculture system.
Problems encountered from water recirculating systems usually stem from excessive ammonia production in fish wastes. Consumption of oxygen by nitrifying bacteria that break down toxic ammonia to non-toxic forms depends on the amount of ammonia entering the system.
Oxygen enters the water primarily through direct diffusion at the air-water interface and through photosynthesis. Direct diffusion is relatively insignificant unless there is considerable wind and wave action. Several forms of mechanical aeration are available to the fish farmer.
Mechanical aeration can also increase DO levels. Because of the lack of photosynthesis in indoor water recirculating systems, mechanical means of aeration is the only alternative for supplying oxygen to aquatic animals cultured in the systems. Oxygen depletions can be calculated, but predictions can be misleading and should never be substituted for actual measurements. Categories of mechanical aerators include:
(a) Paddlewheels
(b) Agitators
(c) Vertical sprayers
(d) Impellers
(e) Airlift pumps
(f) Ventura pumps
(g) Liquid oxygen injection
(h) Air diffusers
(iv) Carbon Dioxide: Carbon dioxide (CO2) is commonly found in water from photosynthesis or from water sources originating from limestone bearing rock. Fish can tolerate concentrations of 10 ppm provided dissolved oxygen concentrations are high.
Water supporting good fish population normally contains less than 5ppm of free carbon dioxide. In water used for intensive pond fish culture, carbon dioxide levels may fluctuate from 0 ppm in the afternoon to 5-15 ppm at daybreak.
While in recirculating aquaculture systems carbon dioxide levels may regularly exceed 20 ppm. Excessively high levels of carbon dioxide (greater than 20 ppm) may interfere with the oxygen utilisation by the fish.
There are two common ways to remove free carbon dioxide. First, with well or spring water from limestone bearing rocks, aeration can blow off excess gas. The second option is to add some type of carbonate buffering material such as calcium carbonate (CaCO3) or sodium
bicarbonate (Na2CO3).
These additions require calculating the exact amount of the material needed for that purpose as it will initially remove all free carbon dioxide and store it in reserve as bicarbonate and carbonate buffers.
(v) Nitrogen: Dissolved gases, especially nitrogen, are usually measured in terms of “percent saturation”. Any value greater than the amount of gas the water normally holds at a given temperature constitute supersaturation. A gas supersaturation level above 110% is usually considered problematic.
Gas bubble disease which may vary in sign is a symptom of gas supersaturation. Bubbles may reach the heart or brain, and fish die without any visible external signs. Other symptoms may be bubbles just under the surface of the skin, in the eyes, or between the fin rays. Treatment of gas bubble disease involves sufficient aeration to decrease the gas concentration to saturation or below.
(vi) Ammonia: Fish excrete ammonia and lesser amounts of urea into the water as wastes. Two forms of ammonia occur in aquaculture systems, ionised and un-ionised. The un-ionized form of ammonia (NH3) is extremely toxic causing brown-blood disease in fish while the ionised form (NH4+) is not.
Both forms are grouped together as “total ammonia”. Through biological processes called nitrification, toxic ammonia can be degraded to harmless nitrates according to the following equation:
NH3 1½0 NO2 – 1½0 NO3
In natural waters, such as lakes, ammonia may never reach dangerous high levels because of the low densities of fish, but the fish farmer must maintain high densities of fish in his pond and, therefore, runs the risk of ammonia toxicity as it is mostly favoured by rise in temperature and high pH.
(vii) pH: The quantity of hydrogen ions (H+) in water will determine if it is acidic or basic. The scale for measuring the degree of acidity is called pH scale, which ranges from 1 to 14. A value of 7 is considered neutral, neither acidic nor basic; values below 7 are considered acidic while values above 7 are considered basic. The acceptable range for fish culture is between 6.5 and 9.0.
(viii) Alkalinity: Alkalinity is the capacity of water to neutralize acids without an increase in pH. This parameter is a measure of bases, bicarbonates (HCO3-), carbonates (CO3–) and, in rare instances, hydroxide (OH–). Total alkalinity is the sum of the carbonates and bicarbonates alkalinities.
The carbonate buffering system is important to the fish farmer regardless of the method of production as it helps to wide daily pH fluctuations.
In recirculating systems where photosynthesis is practically non-existing, a good buffering capacity can prevent excessive buildups of carbon dioxide and lethal decrease in pH. It is recommended that fish farmer maintain totally alkalinity values of al least 20 ppm for catfish production.
(ix) Hardness: Water hardness is similar to alkalinity but represents different measurements. Hardness is chiefly a measure of calcium and magnesium, but other ions such as aluminum, iron, manganese, strontium, zinc and hydrogen ions are also included.
When the hardness level is equal to the combined carbonate and bicarbonate alkalinity, it is refereed to as carbonate hardness. Hardness values greater than the sum of the carbonate and bicarbonate alkalinity are referred to as non-carbonated hardness.
Hardness values of at least 20 ppm should be maintained for optimum growth of fish and other aquatic organisms. Low-hardness values can be increased with the addition of ground agriculture lime.
(x) Other Metals and gases: Other metals such as iron and sodium, and gases, such as hydrogen sulphide, may sometimes present special problems to the fish farmer. Most complications arising from these can be prevented by properly pre-treating the water prior to adding it to ponds or tanks.
The range of treatments may be as simple as aeration, which removes hydrogen sulphide gas, to the expensive use of iron removal units. Normally iron will precipitate out of solution upon exposure to adequate concentrations of oxygen at a pH greater than 7.0.
Read Also: Diversity Pattern and Ecological Roles of Fresh Water Invertebrates
Read Also: How To Generate Money From Kitchen Wastes
