In the past ten years, special aquaculture has gradually become the main mode of social aquatic product supply. The growth of special aquaculture species is particularly dependent on the development of high-protein aquatic feeds. In particular, fish meal has a high content of essential amino acids and fatty acids, low carbohydrate content, good palatability, low anti-nutritional factors and can be well digested by farmed animals. Absorption and other characteristics have always been an indispensable source of high-quality protein in aquafeeds. However, with the increase in the price of fish meal, in order to meet the nutritional needs of special aquatic products, plant protein and animal protein have become the main ways to replace fish meal, but at the same time they have also caused certain harm to the health of fish and the water environment.

1 Affect growth performance

At present, special aquaculture animals such as sea bass, yellow catfish, mandarin fish, vannamei prawns, etc. are mainly omnivorous or carnivorous fishes, and their growth is particularly dependent on fish meal in feed, but if vegetable protein is used, fish meal Substitution, due to changes in food taste and odor, the palatability of plant protein feeds deteriorates, and the rate of fish ingestion decreases, resulting in slower growth of fish and higher feed coefficients. At the same time, most plant proteins contain anti-nutritional factors, such as: trypsin inhibitor, hemagglutinin, phytic acid, gossypol, cyclopropene fatty acid, glucosinolate, erucic acid, aflatoxin and thiaminease, etc. Anti-nutritional factors cannot be well digested and absorbed in the intestines, resulting in decreased growth performance. Studies have shown that when more than 20%-30% vegetable protein soybean meal is added to the feed, the growth performance of aquaculture animals such as rainbow trout, Atlantic salmon, and flounder will decrease and the feed coefficient will increase. When cottonseed meal is added to the feed to replace fish meal by more than 30%, Due to the existence of mustard oil in rapeseed meal, the growth of turbot, rainbow trout, Atlantic salmon, channel catfish and other high-protein diet cultured species is inhibited. Compared with fish meal, the lack of restrictive amino acids such as lysine and methionine in plant protein results in imbalanced amino acid nutrition, resulting in a wooden barrel effect and affecting the growth of aquaculture animals.

2 Damage to the intestines

The intestine is an important basis for the nutrient absorption of aquaculture animals. Therefore, the normal structure and function of the intestinal mucosa are extremely important for the digestion and absorption of nutrients. However, because other proteins are used in the feed instead of fish meal, the feed process needs to be adjusted to ensure the hardness of the feed pellets. When the feed pellets are too hard, the fish cannot digest in time in the intestines after ingestion, but will cause mechanical scratches on the intestinal surface. Especially when the anti-nutritional factors in the feed cannot be degraded by heating, for example, the soy glycinin in soybeans can cause hypersensitivity to the intestinal tract of farmed animals, causing intestinal allergies and inflammatory reactions in the animals, leading to increased intestinal mucosal damage. The type and number of vacuoles increased, and the submucosal inflammation increased. There are a large number of microorganisms in the intestine. When the anti-nutritional factors such as oligosaccharides in high-protein diets cannot be digested and absorbed well, they will accumulate in the intestine and become mucous to form an intestine suitable for pathogenic bacteria. The environment causes the intestinal homeostasis to be broken, causing intestinal inflammation. In the early stage, amino acids are mainly used for the synthesis of fish growth protein. Fish, especially carnivorous fish, are very sensitive to restrictive amino acids. As the substitution level continues to increase, the intestines cannot absorb nutrients well, which is also caused The reason for the decline in growth performance.

3 Damage to the liver

The liver is the largest immune organ of aquaculture animals. Many harmful substances in the body need to be broken down by the liver to convert metabolites into urea and excreted. Once the liver is damaged or diseased, it will often cause fish nutrition metabolism disorders and immune system disorders. The disease resistance is reduced, and it is very easy to cause the outbreak of other secondary diseases, causing a large number of fish deaths. Unreasonable nutrient matching in the feed often leads to liver disease. For example, the presence of peanut meal in the feed may cause mildew in the feed. When fish eat this type of feed, aflatoxin will cause damage to the liver. When the protein content in the feed is too high, the digestive burden of the liver increases, and at the same time it induces fat accumulation, destroys the liver tissue morphology, and causes liver diseases. Studies have shown that when plant protein is used to replace fish meal, it will induce oxidative stress in the liver to prevent the body from being damaged by oxidative free radicals. As the replacement amount gradually increases, the antioxidant enzymes in the antioxidant protection mechanism are less active than In the fish meal group, aspartate aminotransferase (GOT) and alanine aminotransferase (GPT) in the body were elevated at the same time, indicating that liver cells were destroyed and liver function was impaired.

There is a very important two-way communication pathway between the intestine and the liver, called the "gut-liver axis". Simply put, it is the passage of substances back and forth between the intestine and the liver, which is a kind of "axis" that can influence each other. In other words, the liver can affect the intestines, which in turn can affect the liver. The intestine has an intestinal barrier function, which is the first "line of defense" for foreign harmful substances to invade the body. When the intestinal barrier function is impaired, the permeability of the intestinal mucosa increases, causing foreign harmful substances to enter the blood circulation and reach the liver. , Which leads to liver inflammation. When liver disease occurs, the hematopoietic function of the liver is destroyed, which leads to a decrease in intestinal blood flow and causes hemorrhagic necrosis of the intestine. Once liver injury occurs in aquaculture animals, its blood vessels are easy to rupture and blood is difficult to coagulate. Irritable stimulation will cause stress hemorrhage syndrome, the mortality rate is extremely high, and it will cause huge losses in production. At the same time, since the liver is responsible for digestion, metabolism, excretion, detoxification, and immunity at the same time, nutrients from external things need to be synthesized, decomposed, transformed, and stored in the liver. Therefore, when the liver is damaged, the digestion of substance.

4 Pollution of the water environment

Aquaculture animals live in water bodies, and changes in the water environment affect the health of aquaculture animals. When the protein content in the feed is too high, the liver converts the protein into urea to eliminate the body. At the same time, the addition of plant protein makes the feed poorly palatable, the amount of residual bait in the water increases, or the presence of anti-nutritional factors in the feed leads to the digestibility of the feed. Under the action of microorganisms in the water, the content of ammonia nitrogen and nitrite increases. When nitrite is absorbed by fish, it combines with hemoglobin to produce methemoglobin, which loses the ability to carry oxygen, causing breeding animals appetite is reduced, disease resistance is weakened, severe bleeding can be induced, and even a large number of deaths. When the ammonia nitrogen content in the water exceeds the standard, it will hinder the excretion of ammonia in aquatic animals, corrode the gill tissue of the fish body, and damage the deep tissues in severe cases, causing the fish to breathe difficultly, until the death of suffocation. At the same time, non-ionic ammonia can also damage the liver and kidney tissues of the fish body, making the "intestinal liver axis" function diminished, and the feed cannot be digested and absorbed well, even causing the farmed animals to not eat, which again leads to the increase of ammonia nitrogen and nitrite in the water body. High, forming a vicious circle, causing large-scale deaths of farmed animals and causing economic losses.