In the second half of the 20th century, fish and shellfish aquaculture developed rapidly. The FAO has formulated new plans for the next 40 years of development. There are about 24,000 kinds of fish species in the world. However, there are only a little more than 1,000 kinds of aquatic products cultivated or considered for breeding. There are still great potentials for breeding varieties. At present, aquaculture faces the same environmental challenges as livestock and poultry farming. Aquaculture regulations in some countries are even stricter than other terrestrial animals. Phosphorus and nitrogen (crude protein and amino acids) are essential nutrients for fish and shellfish, but excessive amounts of these two elements can lead to deterioration of the water environment. NPK is a basic nutrient of plants. Fertilizing in aquaculture water can promote the growth of algae and produce oxygen. Overfeeding will cause algae to multiply. When food or nutrients are depleted, algae die and break down, which is a process of aerobic. When the water body is eutrophic, oxygen-depleting organisms often cause oxygen depletion and death of fish and shellfish. Wastes from aquaculture are directly discharged into water and purification treatment is difficult.
First, the demand for phosphorus in fish and shellfish
Phosphorus is the primary mineral for high-quality aquafeeds. Phosphorus cannot be absorbed from water and must be provided by feed. Calcium is actively absorbed by freshwater surimi using specific ATPases, and uptake of radon can meet the requirement of 80% calcium. Marine fish drink large amounts of water to maintain osmotic pressure balance, calcium concentration in drinking water is higher, so marine fish can meet the needs of calcium by regulating osmotic pressure.
The concentration of phosphorus in brine is low, and it is apparent that marine fish need phosphorus. Researchers fed purified diets on fish and shellfish and supplemented most or all of the phosphorus with inorganic phosphate (sodium, potassium, or calcium). The phosphorus requirements were studied, usually expressed as available phosphorus. According to published data, phosphorus The range of requirements is narrow, ranging from 0.3 to 0.86% of the diet.
In the absence of phosphorus, weight gain was slow. In vivo, calcium and P, blood P, liver glycogen, bone ash, bone P, and bone Ca were decreased, while body fat concentration, gluconeogenesis activity, and serum alkaline phosphatase activity were increased. The ratio of calcium to phosphorus in aquatic animal diets is not as important as in terrestrial animal diets, but high calcium in some fishes is not conducive to the use of phosphorus. Most formulators keep Ca:P at a 1:1 level and over 2:1 are considered excessive. The exception is the truth, the diet Ca:P is about 1:2.
Second, the use of phosphorus
Phosphorus utilization varies with the type of fish and the source of phosphorus. In general, the higher the solubility of salt is, the higher the salt utilization rate is. The fish that can secrete stomach acid into the gastrointestinal tract can absorb more phosphorus. Gastro-intestinal fish such as carp absorb less phosphorus. The first batch of phosphorus utilization data was obtained and published 20 to 25 years ago using methods for terrestrial vertebrates. In recent years, the measurement methods have been improved and more accurate data have been obtained.
It is difficult to determine the digestibility or utilization of phosphorus in aquatic animals because they live in water, feed and feces are exposed to water, and key nutrients may be lost before sampling or before analysis. The determination of the utilization rate requires a chemical analysis to determine the part that disappears through the animal. If the loss before and after consumption is not taken into consideration, an accurate value will not be obtained.
Most fish eat feeds faster and lose less, while shellfish are slow to feed, and feed pellets are smaller before they are eaten. Therefore, many water-soluble nutrients are lost before they are eaten, making it difficult to quantify. At present, the only way is to put the feed in water, stir gently, collect dry matter after a period of time, and analyze the nutrients to be tested. Water can also be analyzed after a period of time in the water-dipping feed. Use this method to correct the concentration of nutrient intake. Treatment of feces may be more difficult.
The digestibility of crude protein and energy can be determined by collecting feces from water, but the utilization of phosphorus can be measured more accurately by collecting feces by necropsy without exposing the feces to water. This unmeasured endogenous phosphorus output results in apparent utilization. The true utilization rate of rainbow trout has been determined recently.
Third, the collection of endogenous phosphorus
Determination of true utilization must accurately measure endogenous phosphorus, and endogenous phosphorus is a typical decline function. For example, in order to determine the amount of endogenous phosphorus released, low phosphorus diets were fed for 10 days. Samples were collected during this period to measure the endogenous phosphorus in the feces. During the 10 days, the output continued to drop during the Phosphorus deficiency clinical symptoms (phosphorus concentration) appear. Therefore, the determination of the true utilization rate requires that both test diets and non-nutritional diets be fed at the same time, and feces samples be collected for measurement. Feces are collected before or after the designed collection time, and the results will show errors. Fish, like other non-ruminants, usually collect feces 4 to 6 days after the test diet is fed for testing.
At present, most of the values ​​of phosphorus utilization are from single feed materials, and diets usually contain only one source of phosphorus. The results of the study showed that the phosphorus utilization rate of plant feed was low, while the phosphorus utilization rate of fish meal was higher. The results of the determination of dietary availability with only one source of phosphorus are more accurate, but it does not represent the availability of multicomponent feed phosphorus, as the availability of phosphorus in the diet may not be a simple addition of the feed composition utilisation. Although we used the same group of fish to determine the phosphorus utilization rate of a single raw material in the same experimental system, we could not accurately predict the phosphorus utilization rate of multi-component feeds.
The interaction between phosphorus and other mineral elements in diets of aquatic animals has not been fully elucidated, but it seems to interact with some cations. If this interaction starts from the gastrointestinal tract, it may affect the absorption rate. Phytic acid will increase the amount of zinc needed for salmon and squid, and this interaction appears to occur in the stomach, directly affecting the utilization of the cation. It is generally believed that phytate phosphorus can not be used by aquatic animals, except for carp, and the utilization rate of phytic acid in carp is 8-38%, which may be the effect of microbial degradation. The interaction with its mineral elements remains to be studied.
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