Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 49

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 24 hours (Tucker 1984).

	Change
Time	Dissolved oxygen	Carbon dioxide	pH
Daylight	Increases	Decreases	Increases
Night	Decreases	Increases	Decreases

Since most test kits measure total ammonia nitrogen, it is important to determine what percentage of the total is toxic. In healthy ponds and tanks, ammonia levels should always be zero. Since the toxicity of UIA begins as low as 0.05 mg/l, a positive total ammonia nitrogen test needs to be followed by a test to find the actual concentration of UIA. Once the pH and temperature are known, the fraction of UIA present can be determined (Table 1.4), as presented in Table 1.5. Samples that cannot be submitted within a few hours should be filtered (0.45 mm) then frozen to reduce the loss of ammonia and changes in nitrate/nitrite concentrations.

Salinity is usually not measured directly, but is instead derived from the conductivity measurement (Wagner et al., 2006). This is known as practical salinity. These derivations compare the specific conductance of the sample to a salinity standard such as seawater. Salinity measurements based on conductivity values are unitless but are often followed by the notation of practical salinity units (Nelson and Siegel, 2014).

Table 1.3 Factors for calculating carbon dioxide concentrations in water with known pH, temperature and alkalinity measurements.^a

Source: Tucker, 1984.

pH	Temperatures (°C)
5	10	15	20	25	30	35
6.0	2.915	2.539	2.315	2.112	1.970	1.882	1.839
6.2	1.839	1.602	1.460	1.333	1.244	1.187	1.160
6.4	1.160	1.010	0.921	0.841	0.784	0.749	0.732
6.6	0.732	0.637	0.582	0.531	0.495	0.473	0.462
6.8	0.462	0.402	0.367	0.335	0.313	0.298	0.291
7.0	0.291	0.254	0.232	0.211	0.197	0.188	0.184
7.2	0.184	0.160	0.146	0.133	0.124	0.119	0.116
7.4	0.116	0.101	0.092	0.084	0.078	0.075	0.073
7.6	0.073	0.064	0.058	0.053	0.050	0.047	0.046
7.8	0.046	0.040	0.037	0.034	0.031	0.030	0.030
8.0	0.029	0.025	0.023	0.021	0.020	0.019	0.018
8.2	0.018	0.016	0.015	0.013	0.012	0.012	0.011
8.4	0.012	0.010	0.009	0.008	0.008	0.008	0.007

^a Factors should be multiplied by total alkalinity (mg/l) to get carbon dioxide (mg/l). For practical purposes, CO₂ concentrations are negligible above pH 8.4.

Total dissolved solids are reported in mg/l and can be measured by gravimetry or calculated by multiplying a conductivity value by an empirical factor; standard methods for the examination of water and wastewater accepts a total dissolved solids constant of 0.55–0.7 mg/l, although if the water source is known to be high in calcium or sulfate ions, a constant of 0.8 mg/l may be used (American Public Health Association et al., 2017). Depending on the ionic properties, excessive total dissolved solids can produce toxic effects on fish and fish eggs. Salmonids exposed to higher than average levels of CaSO₄ at various life stages experienced reduced survival and reproductive rates. When total dissolved solids ranged above 2200–3600 mg/l, salmonids, perch and pike all showed reduced hatching and egg survival rates (Scannel and Jacob, 2001).

It is important for the clinician to be familiar with the types of life‐support system monitoring reports used in modern aquatic facilities. Each facility will have its own template or digital form tailored to the specific monitoring procedures.