Fundamentals of Aquatic Veterinary Medicine. Группа авторов

      1.3.2.16 Interaction of Chemical and Physical Properties of Water that Affect Aquatic Animal Health

      Some chemicophysical parameters of water have a direct influence upon fish health. Any abrupt or large fluctuations of these parameters often cause a state of stress in fish, sometimes resulting in widespread disease outbreaks. Dissolved oxygen content, pH, turbidity, temperature, introduction of some chemicals, detergents, pesticides and naturally produced toxic compounds like hydrogen sulfide, ammonia, and dinoflagellate toxins are potential stress‐related parameters. Carbon dioxide concentration up to 20–30 mg/l can be tolerated by fish provided that oxygen is near saturation. At lower levels of dissolved oxygen, the toxicity of carbon dioxide increases. The optimum pH range is between 6.7 and 8.6; liming agents may be applied to correct a low pH.

      Ammonia concentration above 1 mg/l can indicates organic pollution. Hydrogen sulfide toxicity increases with decreasing pH and it is harmful even at a concentration of 1 mg/l. Making the aquasystem environment more congenial and hygienic reduces stress and promotes fish health. For example, excessive application of inorganic fertilizers and accumulation of organic matter in older aquasystems may cause an over production of phytoplankton, and the appearance of algal and bacterial blooms, leading to dissolved oxygen depletion to lethal levels. For health and optimum growth, the dissolved oxygen level should not drop below 2–5 mg/l. Carbon dioxide concentration up to 20–30 mg/l can be tolerated by fish provided oxygen is near saturation. Nephrocalcinosis in salmonids has long been recognized as a pathological entity related to high dissolved CO₂, eventually leading to the formation of large mineralized deposits within the excretory tissue of the kidney and associated kidney pathology. The condition can result in poor condition and performance and occasional fish loss, particularly if other stressors are present.

At lower levels of dissolved oxygen, the toxicity of carbon dioxide increases. The optimum pH range is between 6.7 and 8.6; liming agents may be applied to correct low pH. Problems with high pH are common in fry nursery ponds and in ponds used to grow freshwater prawns (Macrobrachium rosenbergii). This is because fertilization practices used to prepare ponds for stocking are designed to promote fast‐growing phytoplankton blooms that rapidly take up carbon dioxide. Unfortunately, the early life stages of fish and crustaceans are particularly susceptible to pH toxicity and juveniles are less able than older animals to avoid areas of pH shift by moving to areas of stable pH in the pond (such as deeper waters).

      Ammonia concentration above 1 mg/l indicates organic pollution. Ammonia is very important in intensive systems; in small amounts, ammonia causes stress and gill damage. Fish exposed to low levels of ammonia over time are more susceptible to bacterial infections, have poor growth and will not tolerate routine handling. Deformities and significant behavioral changes associated with chronic exposure to nitrates have been documented in rainbow trout (Oncorhynchus mykiss) raised in recirculating aquaculture systems with nitrate concentrations at levels less than one‐tenth the recommended maximum nitrate nitrogen level of 1000 mg/l.

      Hydrogen sulfide toxicity increases with decreasing pH and it is harmful even at 1 mg/l concentration level. Proper and timely management of soil and water by manipulating feeding, fertilization, liming, addition of water and aeration eliminates most of the environmental stressors and provides better and healthier environments for the growth of fish. Hydrogen sulfide has been referred to as a silent killer of shrimp, causing tissue corrosiveness by irritating soft tissues in the gills, gut, stomach walls and hepatopancreas. H₂S stresses shrimp, lowering their resistance to infection. A safe level for H₂S in giant tiger shrimp (Penaeus monodon) ponds has been reported as 0.033 ppm while white shrimp (P. vannamei) post larvae has been said to tolerate up to 0.0087 ppm and juveniles up to 0.0185 ppm (Panakorn, 2016).

      1.3.3 Biotic Factors of Water

      1.3.3.1 Biological Oxygen Demand

      The biological oxygen demand is a measure of the oxygen used by all organisms in the aquasystem. Microbes (bacteria and fungi) use oxygen to decompose organics (may use 1–3 ppm dissolved oxygen in 24 hours). Phytoplankton respire at night to use oxygen (may use 5–15 ppm dissolved oxygen nightly). Fish respire day and night (may use 2‐6 ppm dissolved oxygen in 24 hours). Dissolved oxygen levels fall at night, since all organisms are respiring, and rise during the day, since plants photosynthesize to use carbon dioxide and eliminate oxygen (may gain 5–20 ppm dissolved oxygen daily). Diffusion and wave/wind action add oxygen (may add 1–5 ppm dissolved oxygen).

      The measurement of biological oxygen demand is a chemical procedure which determines the amount of dissolved oxygen needed by aerobic organisms in a body of water to break down organic material present in a given water sample at a certain temperature over a specific time period. It is not a precise quantitative test, although it is widely used as an indication of the organic quality of water. It is most commonly expressed in milligrams of oxygen consumed per liter of sample for five days of incubation at 20°C and is often used as a robust surrogate of the degree of organic pollution of water. Sources of biological oxygen demand include topsoil, leaves and woody debris; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food‐processing plants; failing septic systems; and urban rainwater runoff.

      1.3.3.2 Plants and Algae

The type and amount of vegetation in an aquatic environment can have a positive or negative effect on the health of the aquasystem. Vegetation can improve the growth of fish by allowing for shade, shelter, and removal of nitrates and phosphates from the water. At night, excessive vegetation can also decrease the oxygen available to the fish, since plants will use oxygen for respiration and may impede movement of the fish and decrease their ability to catch prey or avoid predation. Studies performed on bass, sunfish and brill have shown that different species of fish react differently to vegetation levels. In one study, the size and life stages of fish also affect the amount of vegetation that is most appropriate. In this study, bass were less tolerant of the plants filling the water column than brill or sunfish. Larger bass were negatively affected when 30% of the water column was filled with plants, while smaller bass were negatively affected when 50% of the water column was filled. These studies express the importance of knowing the species of plant and the species of fish for which the veterinarian is caring.

      Floating beds of vegetation are a good answer to nitrate and phosphate removal. Studies performed on rye grass in floating beds has shown that it effectively removes a significant amount of the NO₃ (Bartucca et al., 2016). By creating floating beds of vegetation, the nitrate and phosphate levels can be reduced without encumbering the water column. It is important to monitor the oxygen, nitrogen, and phosphorous levels to decide the best density of planting.

      1.3.3.3 Algal Bloom

      Overabundance of nutrients like phosphorus, particularly when the water is warm and the weather is calm, can lead to excessive growth of algae. According to the World Health Organization standard, when the population of algal cells exceeds 100 000 cells/ ml, the condition is termed an algal bloom. Excessive algal growth can lead to nocturnal hypoxic conditions where fish may start to gasp on the water surface, and mass mortality events may occur. Phytoplankton blooms can also cause large diurnal fluctuations in other water quality variables (e.g. very high pH due to excessive use of free CO₂ for photosynthesis) in mid‐afternoon. Such conditions are stressful to fish. Blue‐green algae can also produce toxic substances that are lethal to some fish (see also Harmful Algal Bloom below). They can also produce compounds that impart a strong off‐flavor to fish. In addition, dead algae can accumulate at the pond bottom and create toxic gases from decomposition.

      The best approach to controlling algae is to regulate nutrient inputs by moderate stocking and feeding rates. Phosphorus is one of the major factors responsible for increased levels of phytoplankton in an aquasystem. It is possible to precipitate phosphorus from pond water by applying sources of iron, aluminum, or calcium ions. These ions precipitate phosphorus as insoluble iron, aluminum or calcium phosphates. The use of alum (aluminum sulfate) 20–30 mg/l or gypsum (calcium sulfate) 100–200