What are the reasons fish frequently get sick?
In aquaculture practice, a phenomenon is often observed: species with strong disease resistance in the wild become prone to disease after living in aquaculture environments for a long time.
The strength of a fish's disease resistance is mainly determined by its genetic factors, but whether it ultimately becomes ill is determined by the environment. Various stimuli from the environment to fish are called environmental stress factors. In aquaculture ecosystems, the stress that fish experience is often caused by environmental factors such as non-ionized ammonia, dissolved oxygen, pH, and water temperature.
Generally speaking, ammonia nitrogen in aquaculture ecosystems comes from three sources: first, it is a product of fish protein metabolism; second, it is a product of the bacterial decomposition of organic matter in the water; and finally, it comes from nitrogen fertilizer applied to the water. Ammonia nitrogen in water exists in two forms: non-ionized ammonia and ammonium ions. The equilibrium relationship between the two is directly regulated by pH and water temperature. When pH and water temperature rise, the proportion of non-ionized ammonia increases. For example, when pH rises from 8 to 9, non-ionized ammonia increases 7-fold. Non-ionized ammonia is highly toxic to fish. When the concentration reaches 0.02 mg/L, it causes chronic stress in fish; at 0.05 mg/L, it causes acute stress in fish; and at 0.4 mg/L, fish have already begun to die. This is because non-ionized ammonia has fairly high lipid solubility and can penetrate cell membranes to poison cells, ultimately damaging tissues such as the gills and liver.
Dissolved oxygen in water is not only a condition on which fish depend for respiration and survival, but also plays a leading role in regulating the oxidative decomposition of many substances in the aquatic environment. The dissolved oxygen level required for healthy fish growth is above 6 mg/L, and the requirement for fingerlings is even higher, above 7.3 mg/L. A high dissolved oxygen content in water can inhibit and reduce the toxic effects of substances such as non-ionized ammonia on fish. Too low dissolved oxygen causes fish to gasp at the surface or even die, and it also enhances the toxicity of ammonia. Due to excessively high stocking density and the extensive use of compound feed, low dissolved oxygen has become a serious stress factor in aquaculture environments. It greatly reduces the production performance of fish, and even with various aeration facilities, this stress always persists.
Due to insufficient dissolved oxygen, the oxidation process of excess organic matter in aquaculture water is inhibited. Residual bait, organic fertilizers, fish feces, and other materials gradually accumulate, resulting in the production and accumulation of various organic acids, which gradually lowers the pH. The pH inside a fish's body is generally between 7.4 and 7.8. If the pH of the water body is too low, it will cause obstacles to gas exchange in the gills and blood oxygen transport in fish, disrupt the osmotic pressure regulation mechanism, cause loss of the blood acid ion regulation mechanism, and disturb the acid-base balance of the blood.
Because fish are poikilothermic animals, their body temperature changes with water temperature. Therefore, water temperature is an extremely important environmental factor in the lives of fish. When the temperature approaches the limit that fish can tolerate for a long time, or when the range of water temperature fluctuation is too large, it will cause stress to the fish. In aquaculture water, the effects of this stress on fish become very complex because they involve changes in dissolved oxygen, ammonia nitrogen, and other factors.
The effects of the above environmental factors on fish health have attracted increasing attention with the development of intensive aquaculture. Aquaculture is developing in the direction of "green and healthy." Therefore, in addition to providing farmed fish with high-quality feed that is nutritionally balanced and made with advanced processing technology, regulating and improving water quality and maintaining a water environment suitable for fish growth are also extremely important.