Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 43

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).