Stress is a synthesis of the body's non-specific response to various external or internal stimuli, or stress is a non-specific response to any request made by the body to it, from the perspective of fish production, stress is a physiological or behavioral response made by fish to overcome the adverse effects of environmental and feeding management, so it is also called "systemic indication".
With the development of intensive aquaculture production, the farming density of various fish species is getting higher and higher, and the breeding environment and feeding conditions are getting worse and worse, these changes will affect the behavior and health status of farmed fish, causing fish to produce stress responses, thereby affecting the production performance of fish and the economic benefits of aquaculture. Therefore, how to take the necessary measures to minimize the impact of various stress factors on fish and achieve healthy fish farming should arouse our sufficient attention.
First, the factors causing fish stress can be divided into physical stress, chemical stress, biological stress, and nutritional stress according to different stress sources.
1． Physical stress
Caused by sudden changes in temperature during the breeding process, injection of new water, pulling nets, ponding and other physical factors or mechanical operations. The fish body is stressed due to the tolerance limit or acute changes of the above physical factors, which are manifested as jumping, wandering, turning, flipping, sinking or floating, trembling, spasms, coma, entanglement, fighting, body surface bleeding, etc., eliminating adverse factors, and the stress response can be alleviated. Fish colds, etc. belong to this category.
2． Chemical stress
Caused by factors such as low dissolved oxygen in water, excessive pH, salinity exceeding the adaptation range of fish, organic matter content, domestic sewage, industrial wastewater, pesticides, fish medicines, and pollution of the fishery itself. Acute stress is manifested by changes in body color, mucus secretion, rough and dull body surface, and imbalance, food stopping, and breathing disorders.
Long-term stress can cause a series of metabolic disorders such as scaly pine, black or white gill filaments, protruding eyes, body cavity ascites, liver enlargement and decay, liver color pale or hyperemia, gallbladder enlargement, light bile color, yellow bile, kidney swelling or decay, intestinal inedible, watery, congestion or blackness, refusal to eat or even death. Fish hepatobiliary syndrome in general is one of these categories.
3． Biological stress
Caused by excessive density of biological populations, unreasonable variety collocation, pathogenic infection, and invasion of predatory organisms. Farmed fish often exhibit sluggish movements, solitary swimming, ataxia, decreased appetite, growth retardation, emaciation, darkening of body color, and increased frequency of gill caps and tail fin movements. Tail-wagging disease caused by Chinese loach is one of these categories.
4． Nutritional stress
Nutritional stress includes sudden changes in feed variety, quality, quantity and odor, sudden changes in feed raw materials, feed formulas, long-term deficiency or excess of certain nutrients, changes in nutritional levels, etc. can also cause fish anorexia, swarm swimming, body color variation, congestion, moths, rotten skin and other stress responses. This is the case with horse racing, which is susceptible to occur at the fry stage.
Second, the harm of stress to fish The harm of stress is concentrated in the third stage of the stress response, and appropriate natural stress is the body's gradual adaptation to the environment to improve the production performance of fish.
However, if the stimulus intensity of the stress source exceeds the compensatory ability of the body's defense system, or the stimulation effect is continued, so that the fish are subjected to high-intensity stressors or long-term stimulation, it will have serious adverse effects, thus endangering the body.
The harm caused by stress to fish is both single and comprehensive, and its effects are multifaceted. To sum up, there are mainly the following aspects:
1． Excessive stress that leads to a decrease in the body's immunity and weakened resistance to disease will bring irreparable damage to the body, destroy the body's defense system, and cause the body's resistance to stress and disease to be lost.
A large number of experiments have shown that after animals are stimulated by stressors, due to the large secretion of glucocorticoids, the thymus, spleen and lymphoid tissues are atrophied, which hinders the production, differentiation and activity of eosinophils and T and B lymphocytes, weakens the blood phagocytic activity, and reduces the level of antibodies in the body, resulting in a decline in collective immunity and a weakening of disease resistance. It has been reported that the low pH water environment increases the plasma cortisol concentration of Chinese turtles, reduces serum lysozyme activity and phagocyte phagocytosis activity, reduces disease resistance, and triggers fish diseases.
2． Causing a decrease in fish fecundity, stress can increase or decrease the secretion of follicle-stimulating hormone, luteinizing hormone, prolactin, etc., thereby causing a decrease in the fecundity of fish. Acute stimulation can increase the blood of brown trout gonadotropins, while the blood of sub-mouth fish can reduce the amount of gonadotropins in the blood, and prolonging the time of stress stimulation can make rainbow trout gonadotropin levels below normal levels. Wang Jiqiao (2002) also reported that disturbance or mechanical manipulation can cause rainbow trout to predate or delay reproduction, and can also accelerate or completely inhibit the reproduction of Nile tilapia.
3． Leading to a decrease in production performance, when stressed, the body must mobilize a large amount of energy to deal with the stimulation of stressors, and the body's proteins, carbohydrates, fats and other catabolism is enhanced, and anabolic metabolism is reduced. During stress, a large amount of cortisol is produced, and cortisol can inhibit the extraction of amino acids by muscle tissue, increase the content of amino acids in the blood, and transfer to the liver, thereby promoting gluconeogenesis, at the same time, it can also accelerate the fat oxidation process, resulting in growth arrest and wasting.
4． Leading to metabolic disorders of the body, excessive stress, will cause the body's metabolism to be disordered, resulting in a large number of free radicals and some toxic metabolites, so that organ tissues due to the accumulation of free radicals and poisons caused by damage, affecting the healthy growth of the body, severe timing will be due to metabolic disorders and death. If the oxidized and moldy feed is fed, it may cause lesions such as body color variation of fish.
Third, the control of stress is beyond doubt, stress has seriously endangered a variety of farmed fish, to a certain extent hindered the rapid development of aquaculture industry.
This is a big contradiction with people's high-density farming in order to maximize economic benefits. In order to ensure high-yield and efficient aquaculture production and reduce or avoid losses caused by stress, some aquatic science and technology workers have carried out a lot of research work and proposed many effective control measures.
1． Improve the breeding environment, strengthen the breeding management In the current aquaculture, in order to maximize the breeding output and increase economic benefits, farmers often ignore the requirements of the environmental conditions of farmed animals in the feeding management, ignore the biological characteristics of the farmed animals in the natural state, and forcibly add many management ingredients that are not suitable for the healthy growth of farmed animals, which is a kind of stress.
Therefore, in order to alleviate the harm of stress to aquatic animals such as fish, the first thing is to create an environment suitable for the life and growth of farmed animals, and at the same time to formulate a set of humanized breeding programs and breeding management systems with animal welfare.
2． Feeding high-quality feed, improving the quality of the fish body is not only the material basis for promoting the growth of the fish body, but also the material basis for ensuring the health of the fish body, therefore, in the healthy breeding management of the fish, it is necessary to choose the freshest and most complete high-quality compound feed feeding, which not only helps to ensure the rapid growth of the fish body, but also helps to improve the physique and anti-stress ability of the fish body and achieve the healthy growth of the fish body.
3． Scientific drug use, avoiding drug abuse and using illegal drugs are currently one of the main reasons for the frequent occurrence of stress diseases in fish. Drugs are a double-edged sword, if the medication is symptomatic, the dosage is appropriate, and the method of use is correct, it can play a positive role in curing diseases and preventing diseases;
On the contrary, if the medication is not correct, the dosage is improper, and the method of use is wrong, it is constantly difficult to control the disease, and it can also produce drug harm, cause drug-borne diseases, damage the liver of the fish body, and reduce the anti-stress ability of the fish body. In addition, some drugs are inherently irritating, and when used, they will cause a stress response in fish. Therefore, in order to reduce or avoid the occurrence of stress, it is necessary to rationally choose fish drugs, use drugs scientifically, and avoid the occurrence of drug damage as much as possible.
4． Rational use of anti-stress additives, prevention of problems in advance In order to improve the anti-stress ability of the fish body and prevent the occurrence of stress, it is usually possible to add some anti-stress additives or drugs to the feed. Anti-stress feed additives have the effect of alleviating and preventing stress syndrome caused by stress sources, which can weaken the effect of stress on the body, improve the body's non-specific resistance, and thus improve the body's anti-stress effect.