Fundamentals of Aquatic Veterinary Medicine. Группа авторов

species grow best at mid‐range from 21°C to 28°C

       Sudden temperature changes cause stress and even death.

      1.3.2.3 pH

The term “pH” is a mathematical transformation of the hydrogen ion (H⁺) concentration; it conveniently expresses the acidity or basicity of water. The lowercase letter “p” refers to “power” or exponent, and pH is defined as the negative logarithm of the hydrogen ion concentration. Each change of one pH unit represents a 10‐fold change in hydrogen ion concentration. The pH scale is usually represented as ranging from 0 to 14, where lower number reflects higher acidity and the higher number reflects higher alkalinity. Water with a pH of 4.5 or lower has no measurable alkalinity. Water with a pH of 8.3 or higher has no measurable acidity. At 25°C, a pH value of 7 is neutral; this describes the neutral point of water at which the concentrations of hydrogen and hydroxyl ions (OH^–) are equal (each at 10–7 moles/l). The pH of freshwater ecosystems can fluctuate considerably within daily and seasonal timeframes, and most freshwater animals have evolved to tolerate a relatively wide environmental pH range. Animals can, however, become stressed or die when exposed to pH extremes or when pH changes rapidly, even if the change occurs within a pH range that is normally tolerated. The pH of water in ponds often increases during the day and decreases at night. Fish and other vertebrates have an average blood pH of 7.4. Recommended pH range for cultured fish is 6.5–9.0. Acid death point is around 4; alkaline death point is around 11. The hydrogen ion concentration affects aqueous equilibria involving ammonia, hydrogen sulfide, chlorine and dissolved metals. Interactions of pH with these variables are often more important than the direct effects of pH on aquatic animals. For example, the toxicity of ammonia to fish increases with an increase in pH. Chemical interactions among carbon dioxide, hydrogen ions and the anions that produce alkalinity buffer the pH of most natural waters in a range of about 6–8.5.

      1.3.2.4 Dissolved Oxygen

      Dissolved oxygen monitoring is critical in aquaculture. Temperature, salinity and elevation affect dissolved oxygen. As these three factors increase, dissolved oxygen at saturation decreases. For example, the cold temperature of the Antarctic results in higher dissolved oxygen concentrations compared with warmer tropical waters. Freshwater at sea level holds 9.2 ppm at 20°C and 7.6 ppm at 30°C. Fish become more active and increase their metabolic oxygen needs as temperature increases. As temperature rises, fish also need more dissolved oxygen to grow muscle tissue. Minimum tolerable dissolved oxygen levels increase with a rise in temperature.

      In general, most fish species will grow and thrive within a dissolved oxygen range of 5–12 mg/l (parts per million). However, if levels drop below 4 mg/l they may stop feeding, become stressed and begin to die. Dissolved oxygen ranges for cultured fish are as follows:

       0–2 ppm – small fish may survive a short exposure, but lethal if exposure is prolonged. This range is lethal to larger fish.

       2–5 ppm – most fish survive, but growth is slower if prolonged; may be stressful; aeration devices are often used below 3 ppm.

       5 ppm to saturation – the desirable range for all.

       With rainbow trout, the minimum lethal limit is 1.6 ppm at lower temperatures and 2.5 ppm at higher temperatures.

      Oxygen and pH are best measured in situ (with probes) or as soon as possible after collection (preferably before leaving the site) as, in most situations, levels will change during storage and transport.

      Biological oxygen demand is a measure of the oxygen used by all organisms in an aquasystem.

      1.3.2.5 Carbon Dioxide

      Carbon dioxide (CO₂) is consumed during photosynthesis by plants and expired during respiration by animals, plants (at night) and bacteria in an aquasystem. When added to pond water by respiration or diffusion, it forms a weak acid (carbonic acid), which lowers the pH. Dissolved oxygen and pH cycles follow the same daily peaks and troughs. Carbon dioxide levels of below 10 mg/l are thought to be well tolerated by fish. While levels greater than 20 ppm often harm fish, especially if dissolved oxygen levels are low, sensitivity to CO₂ varies between species. The level of CO₂ in source water varies greatly, and is further affected by the respiratory and photosynthetic activity of animals and plants, and the level of decomposition of organic material in that water (a very significant contributor to CO₂ levels in some nutrient‐rich waters). CO₂ can build up to significantly high levels in systems with large numbers of animals and relatively slow water turnover.

The effect of increased CO₂ in water is to reduce the rate at which CO₂ from the animal's own metabolism can be released from the blood through the gills. As a result, CO₂ in the blood also increases (known as hypercapnia), resulting in a drop in the blood pH and acidosis. At the same time, the oxygen‐carrying ability of the hemoglobin in the blood is reduced. The animal can counteract the effect by balancing the acidosis with an exchange of ions, such as increasing the uptake of bicarbonate and losing hydrogen and phosphate ions and little harm is done. In the long term, this balancing act can have a more profound effect on the health of the animal. Carbon dioxide causes problems in recirculating aquaculture systems without aeration or degassing.

      1.3.2.6 Nitrogen

      The nitrogen (N₂) biogeochemistry of aquaculture ponds is dominated by biological transformations of nitrogen added to ponds in the form of inorganic or organic fertilizers and formulated feeds. Nitrogen application in excess of pond assimilatory capacity can lead to the deterioration of water quality through the accumulation of nitrogenous compounds (e.g., ammonia and nitrite) toxic to the fauna. Principal sources of nitrogen include animal excretion and sediment flux derived from the mineralization of organic matter and molecular diffusion from reduced sediment, although cyanobacterial nitrogen fixation and atmospheric deposition are occasionally important.

      1.3.2.7 Hydrogen Sulfide

      Hydrogen Sulfide (H₂S) is a poisonous gas with a “rotten egg” smell, produced by anaerobic decomposition of organics. Sulfur is an essential element for plants, animals and bacteria. H₂S is present in natural waters and in aquaculture systems, mainly as the sulfate ion. Sulfide can occur in water because it is a metabolite of Desulfovibrio species and certain other bacteria found in anaerobic zones, usually in sediment. These bacteria use oxygen from sulfate as an alternative to molecular oxygen in respiration. There are three forms of sulfide (H₂S, HS^– and S₂^–), and they exist in a pH and temperature‐dependent equilibrium. As pH increases, the proportion of H₂S declines, and that of HS^– rises until the two forms have roughly equal proportions at pH 7. At greater pH, HS^– is the dominant form, and there is no S₂^– until the pH is above 11. Hydrogen sulfide is toxic to aquatic animals because it interferes with reoxidation of cytochrome a3 in respiration. This effect is caused almost entirely by H₂S, while HS^– is essentially non‐toxic. Even if it is toxic, S₂^– is not an issue, because it does not occur at pH values found in aquaculture systems.

      1.3.2.8 Chlorine

      Chlorine is harmful/toxic to fish at values greater than 0.03 ppm. Tap water may range from 4.0 to 8.0 ppm. Sodium thiosulfate can be used to neutralize the chlorine. Chlorine may be used to disinfect equipment, tanks, countertops, and nets at 10 ppm for 24 hours or 200 ppm for 30–60 minutes. Effectiveness is reduced by organic material such as mud, slime and plant material. Sodium hypochlorite is available at concentrations of 15%, 50%, or 65% active.

      1.3.2.9 Alkalinity

      Alkalinity is the ability of the water to accept hydrogen ions and neutralize them and offers a buffering system to reduce pH swings. It is measured