Fundamentals of Aquatic Veterinary Medicine. Группа авторов

concentrations of phosphorus may also be feasible. Alum is acidic and more suitable for use in waters of 500 mg/l total alkalinity and above. Gypsum is better for use in low alkalinity waters.

      1.3.3.4 Larger Aquatic Plants

      Larger aquatic plants or macrophytes include pondweed and milfoil. These plants are undesirable in aquasystems because they interfere with fish management (feeding and harvesting), compete with plankton for nutrients, provide shelter for undesirable fish, contribute to oxygen depletion and high ammonia levels when they decompose and contribute to water loss through transpiration. Drying and desilting of aquasystems every one or two years will keep the growth of aquatic plants in check.

      1.3.3.5 Pests and Predators

      Pests and predators are unwanted animals or plants that destroy fish or hinder the production of target fish species. Birds, fish, reptiles, amphibians, mammals and even certain invertebrates are known predators of cultured fish. Bird predation is the major source of fish loss at aquaculture facilities. Tadpoles feed on aquatic plants and animals, while frogs can eat fish fry. They lay eggs on the surface of ponds, which interferes with surface oxygenation, thereby restricting fish growth if not checked. Sea anemones can grow on boards, sluice gates and grooves, and on concrete dikes in high‐salinity, brackish water areas, and are poisonous to fish.

Fish predators can be used in controlling an overpopulation of fish in ponds. Wild fish are one of the potential sources of fish pathogenic organisms. Some of these wild species are predatory in nature and prey upon the young of cultivated species. They can rapidly multiply and compete for food with the cultivated fish. Control of nuisance fish can be done by poisoning or dewatering the pond. Piscicides of plant origin are preferred over common chemical insecticides used in agriculture due to the long‐lasting toxicity and residual effect of the latter. Moreover, fish killed by insecticides become unfit for human and animal consumption. Piscicides derived from plants such as mahua oil cake are most commonly used as their toxicity lasts for about two weeks and the fish killed are fit for human consumption. Mahua oil is applied at 200–250 ppm. However, the poisoning process kills only the unwanted fish species of the pond without affecting the pathogenic organisms they may carry. Disinfection of water is an effective means of disease control in fish culture by reducing the numbers of pathogens to minimum levels.

1.4 Monitoring and Regulation of Life‐Support Systems

Proper and timely management of soil and water by manipulating feeding, fertilization, liming, addition of water, aeration, bottom raking, and so on, eliminates most of the environmental stressors and provides better, healthy environments for the production of fish and other aquatic animals. Proper management also increases the immune response against pathogens. Eradication of predatory and nuisance fish, disinfecting the pond, selection of quality and healthy seed for stocking, maintaining proper species ratio and stocking density, water quality regulation, proper feeding and proper handling are the important steps of this management exercise. Water quality parameters that are commonly monitored in the aquaculture industry include temperature, dissolved oxygen, pH, alkalinity, hardness, ammonia, nitrites and nitrates. Depending on the culture system, carbon dioxide, chlorides, and salinity may also be monitored. Some parameters such as alkalinity and hardness are fairly stable, but others like dissolved oxygen and pH can fluctuate daily. Most of these parameters are measured with physical reagents or meters. However, there are several classical methods of water analysis which are still used today. Atomic absorption analysis provides the atomization of a sample in flame or electrothermal plasma. Liquid sample is turned into an atomic gas through desolvation, evaporation, and volatilization. Electrochemical methods are based on an analysis of the processes occurring at the electrodes and in the inter‐electrode space followed by the measurement of the potential and/or current in an electrochemical cell containing the analyte. Gravimetric analysis involves determining the amount of analyte through the measurement of mass (Table 1.1).

Relative concentration changes for dissolved oxygen, carbon dioxide and pH in ponds over 24 hours are shown in Table 1.2. Biological oxygen demand measurement requires taking two measurements. One is measured immediately for dissolved oxygen (initial), and the second is incubated in the laboratory for five days and then tested for dissolved oxygen remaining (final). This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period.

In ponds with moderate to high alkalinity (good buffering capacity) and similar hardness levels, pH is neutral or slightly basic (7.0–8.3) and does not fluctuate widely. Higher amounts of CO₂ (i.e. carbonic acid) or other acids are required to lower pH because more base is available to neutralize or buffer the acidity. The relationship of alkalinity, pH and CO₂ is shown in Table 1.3. The number (factor) found in the table which corresponds to the measured pH and water temperature is multiplied by the measured alkalinity value (mg/l as CaCO₃). The product of these numbers estimates CO₂ concentration (mg/l).

Table 1.1 Water quality factors, commonly used monitoring procedures, and preferred ranges for fish culture. Details for specific test procedures can be obtained from a commercial supplier or appropriate text (e.g. APHA 1989).

Water quality factor	Test procedure	Preferred ranges for fish culture
Temperature	Thermometer, telethermister	Species dependent
Dissolved oxygen	Titrimetric (modified Winkler) polarographic meter, calorimetric kits	> 4–5 ppm for most species
Total ammonia‐nitrogen (ionized and un‐ionized)	Calorimetric kits, (Nesslerization or salicylate), ion specific probes	NH < 0.02 ppm
Nitrite	Calorimetric kits (diazotization), ion specific probes	< 1 ppm; 0.1 ppm in soft water
pH	Electronic meter, calorimetric kits,	6–8 ppm
Alkalinity	Titrimetric with pH meter, titrimetric with chemical indicator	50–300 ppm calcium carbonate
Hardness	Titrimetric kit	> 50 ppm, preferably > 100 ppm calcium carbonate
Carbon dioxide	Titrimetric kit	< 10 ppm
Salinity	Conductivity meter	species dependent typically < 0.5–1.0 ppt for freshwater fish)
Hydrogen sulfide	Calorimetric kit	No detectable level
Clarity	Secchi disk, turbidimeter	Species dependent

Table 1.2 Relative concentration changes for dissolved oxygen, carbon dioxide, and pH in ponds over