Why do fish frequently get sick?

In aquaculture practice, there is often a phenomenon that varieties with strong disease resistance in the wild become susceptible to diseases because they live in the aquaculture environment for a long time.

The disease resistance of fish is mainly determined by its genetic factors, but in the end, the disease is determined by the environment. All kinds of stimuli of environment to fish are environmental stress factors. In aquaculture ecosystems, stresses on fish are often caused by environmental factors such as Nonionic ammonia, dissolved oxygen, pH and water temperature.

Generally speaking, ammonia nitrogen in aquaculture ecosystem comes from three aspects: one is the product of fish protein metabolism, the other is the product of bacterial decomposition of organic matter in water, and the last is the nitrogen fertilizer applied in water. Ammonia nitrogen in water exists in two forms: Nonionic ammonia and ammonium ion. The equilibrium relationship between them is directly regulated by pH value and water temperature. When pH value and water temperature increase, the proportion of Nonionic ammonia will increase. For example, when pH value rises from 8 to 9, Nonionic ammonia will increase 7 times. Nonionic ammonia is highly toxic to fish. When the concentration reaches 0.02 mg / L, it will cause fish chronic stress. When it reaches 0.05 mg / L, it will cause acute stress in fish, and when it reaches 0.4 mg / L, the fish has already begun to die. This is because Nonionic ammonia has a high fat solubility, can penetrate the cell membrane to poison cells, and eventually damage Gill, liver and other tissues.

Dissolved oxygen in water is not only the condition for fish to breathe and survive, but also plays a leading role in regulating the oxidation and decomposition of many substances in the water environment. The level of dissolved oxygen needed for healthy growth of fish is more than 6 mg / L, and the requirement of fish species is even higher, above 7 3 mg / L. The high content of dissolved oxygen in water can inhibit and reduce the toxic effect of Nonionic ammonia and other substances on fish. too low dissolved oxygen will cause floating head and even death of fish, and enhance the toxicity of ammonia. Due to the high culture density and the extensive use of formula feed, low dissolved oxygen has become a serious stress factor in the aquaculture environment, which greatly reduces the production performance of fish.

Due to the lack of dissolved oxygen, the oxidation process of excessive organic matter in aquaculture water is inhibited, and the residual bait, organic fertilizer and fish feces will gradually accumulate, resulting in the production and accumulation of various organic acids, resulting in a gradual decrease in pH value. The pH value of fish is generally between 7.4 and 7.8, and the pH value of water is too low, which will cause the disturbance of gas exchange and blood oxygen transport in fish gills, the imbalance of osmotic pressure regulation mechanism, the loss of serum acid ion regulation mechanism and the disorder of blood acid-base balance.

Because the fish is a variable temperature animal, its body temperature varies with the water temperature, so the water temperature is a very important environmental factor in the fish life. When the temperature approaches the limit that the fish can bear for a long time and the variation range of the water temperature is too large, will cause stress to the fish. The effect of this stress on fish in aquaculture water becomes very complex due to changes in dissolved oxygen and ammonia nitrogen.

With the development of intensive culture, people pay more and more attention to the impact of the above environmental factors on fish health. Aquaculture is developing in the direction of "green and healthy". Therefore, in addition to providing high-quality feed with balanced nutrition and advanced processing technology for farmed fish, it is also extremely important to regulate and improve water quality and maintain a water environment suitable for fish growth.