4. The relationship between flow rate and oxygen consumption of organisms
The flow rate should be less than the speed required to exercise the fish. The water flow rate is 0.5-2.0 times the body length/second, which is the best speed to maintain the health, muscle tone and breathing of the fish. In a circular culture pond, the water flow rate gradually decreases with the distance from the pool wall, and the fish can choose a more comfortable flow area, while the speed is uniform in the raceway-type culture pond design. Fish tend to concentrate in the front third of the raceway, swimming toward the water inlet, but rarely occupy the rear two-thirds.
Contrary to what one might expect, forced swimming in fish seems to reduce oxygen consumption. This is due to physiological adaptations such as increased white muscle activity, improved cardiac output, and increased oxygen-carrying capacity of the blood. There are also savings in "breathing" costs. Fish that can maintain their position in turbulent water can breathe simply by opening their mouths and ventilating their gills. This is called impingement ventilation. Impingement ventilation saves energy in two ways: the fish does not need to open and close its gills to draw water, which reduces turbulence; and the water flows around a more streamlined body, reducing the need to constantly adjust its position. This hydrodynamic advantage results in a small but measurable reduction in oxygen consumption.
5. Production and elimination of nitrogen
Nitrogen is an essential nutrient for all living things and is a necessary element for the formation of proteins, nucleic acids, adenylic acid, pyridine nucleotides and pigments.
Fish produce and excrete various nitrogenous wastes through diffusion through the gills, cation exchange, urine and fecal excretion. In addition to ammonia, urea, uric acid and amino acids produced by fish, nitrogenous wastes are also produced and accumulated through organic debris from dead microorganisms, leftover food and nitrogen in the atmosphere.
NH4+ and NH3 are collectively called total ammonia nitrogen (TAN). Non-ionized ammonia nitrogen (free ammonia) is the most toxic form of ammonia and can pass through cell membranes. Therefore, the toxicity of total ammonia nitrogen mainly depends on the percentage of non-ionized ammonia nitrogen. Increases in pH, temperature and salinity will increase the proportion of non-ionized ammonia nitrogen.
Generally, warm-water fish are more susceptible to ammonia toxicity than cold-water fish, while freshwater fish are more tolerant than marine fish. For commercial production, the concentration of non-ionized ammonia nitrogen should be less than 0.05 mg/L, and the total ammonia nitrogen should be less than 1.0 mg/L. Nitrite can be quickly converted into nitrate by nitrifying bacteria in ozone or stable biofilters, but it will accumulate rapidly because the bacteria in the second-stage nitrification reaction are easily ineffective. The toxicity of nitrite is reflected in its impact on the oxygen-carrying capacity of hemoglobin. When it enters the blood, it oxidizes the iron on the hemoglobin molecule to change it from divalent to trivalent, producing methemoglobin, which is a very characteristic brown color, commonly known as "brown blood disease."
The amount of nitrite that enters the blood depends on the ratio of nitrite to chloride ions in the water. Increasing the chloride ion level can reduce the amount of nitrite absorbed by the blood. The chloride ion level can be increased by adding common salt (sodium chloride) or calcium chloride (this is why seawater is afraid of Vibrio and freshwater is afraid of salinity).
For channel catfish, tilapia and rainbow trout, the recommended chloride to nitrite ratio is at least 20:1. Nitrate is the end product of nitrification and the least toxic of the nitrogen compounds. In recirculating aquaculture systems, nitrate levels are controlled by daily water changes, while denitrification is particularly important in systems with low water exchange rates or high hydraulic retention times.
6. The combined effects of pH, alkalinity and carbon dioxide
pH is the negative logarithm of hydrogen ion concentration. The pH of most water bodies is buffered by the bicarbonate-carbonate system. Exposure to extreme pH can be stressful or lethal, but the indirect effects of pH and other variables are more important in aquaculture. pH controls a variety of dissolution and equilibrium reactions, the most important of which is the relationship between the ionized and bound forms of ammonia and nitrite. It also affects the toxicity of hydrogen sulfide, copper, cadmium, zinc and aluminum.
In broad terms, alkalinity is a measure of the pH buffering capacity or acid neutralizing ability of water, and is often expressed in terms of equivalent calcium carbonate concentrations in mg/L. The relationship between pH and alkalinity becomes critical, and alkalinity and carbon dioxide levels need to be carefully monitored and adjusted to maintain the optimum pH level for aquatic growth and biofilter operation. Alkalinity is easily adjusted by adding ammonium bicarbonate, which is safe, inexpensive and easy to use. Carbon dioxide is conventionally controlled by degassing systems, such as countercurrent degassing towers. Hardness is used to indicate the ability of water to precipitate soapy liquids, and the harder the water, the more soapy water needs to be added to obtain the same purification effect. In chemical terms, hardness is defined as the total concentration of calcium ions, magnesium ions, iron and manganese, expressed in mg/L equivalent of calcium carbonate.
In practice, hardness is determined by chemical titration. The total hardness of natural waters ranges from 5 mg/L to 10,000 mg/L. Water is traditionally classified as soft (0-75), medium hard (75-150), hard (150-300) and very hard (>300). Hardness is often confused with alkalinity because both are defined by mg/L calcium carbonate. In fact, if the alkalinity of water is derived from limestone, the two are very close. In contrast, many groundwaters in flat coastal areas have high alkalinity and low hardness. Aquifers in basalt and granite areas have low total hardness and alkalinity because the solubility rate of these minerals is quite low. For aquaculture, dissolved calcium is added to harden the water to promote the hatching of newly fertilized freshwater fish eggs and the calcification of the skeleton structure of fry. Calcium and magnesium also reduce the toxicity of dissolved metals. The recommended range for total hardness is 20-300 mg/L.
Exposure to high concentrations of carbon dioxide will reduce the respiratory efficiency of organisms and their ability to tolerate low oxygen. High levels of carbon dioxide in water will inhibit the excretion of carbon dioxide from the gills of fish, leading to increased concentrations in the blood and lower plasma pH, which in turn causes respiratory acidosis. In this case, even if the dissolved oxygen is high, the oxygen carrying capacity of hemoglobin will decrease and respiratory distress will occur. High concentrations (60-80) of carbon dioxide have an anesthetic effect on living animals and are used as temporary anesthetics as a management technique to reduce stress during operation and handling.
High concentrations can also be used to drive fish away. What really reflects the amount of dissolved carbon dioxide is carbonic acid + carbon dioxide. Adding alkali to increase pH and change the chemical balance of carbonic acid can control the concentration of carbon dioxide in the aquaculture system. Usually, adding alkali does not remove dissolved inorganic carbon in the solution, but only reduces the concentration of carbon dioxide. When the pH is increased, bicarbonate and carbonate ions are generated by changing the carbonic acid carbon balance. In aquaculture, two types of chemicals are used to control pH and reduce carbon dioxide concentration: ① Strong bases that do not contain carbon, such as sodium hydroxide ② Bases that contain carbon, such as sodium bicarbonate.
High levels of carbon dioxide may lead to nephrocalcinosis, the presence of white calcium deposits in the kidneys. The severity of this condition appears to vary significantly with dietary and environmental factors, particularly the type of drugs or substances used to increase alkalinity or pH. The use of agricultural grade limestone instead of sodium bicarbonate in high-density tilapia culture systems has been reported to result in an increased incidence of nephrocalcinosis.