"Deepwater raise fish," this argument makes sense under certain conditions. The shallow water in the pool is vulnerable to changes in the weather. If the temperature changes too much and exceeds the range of fish and Other animals, it will cause stagnation or even death. Especially in the north, fish need to spend a long period of winter. When the water is too shallow, it will freeze to the end or there will be too little water under the ice, and fish will be unable to survive. However, the pool of fish is not as deep as possible. The deep water not only fails to keep the fish well, but sometimes it also causes a “warm leap†in the water. What is the reason?
There is a "temperate layer" in the water
Water's heat mainly comes from the solar thermal energy, but solar energy is absorbed in most of the water within 20 cm of the pool. If the water does not move, the water itself conducts heat very slowly, and it can only heat the surface of the water. Since the salt-free water has the highest density at 4°C, if the pool water is relatively deep, the 4°C water is always at the bottom. When heated during the day, the upper water temperature rises and the bottom water does not rise to the upper layer. Only after the sun sets, the surface water becomes cold and it gradually sinks to the lower layers. If the water is quite deep and the temperature does not drop drastically, the surface water will not fall to the bottom and stop at the middle level. With the gradual warming of the weather, the depth of subsidence is getting shallower and shallower. After a day's accumulation, a thin layer of water with intense temperature changes is formed in the middle layer of water. In a short water layer, the temperature can differ by a few degrees. Degrees, in the pond, some thin 0.6 meters of water, the temperature can be a difference of 6 °C. Because this layer of water temperature changes violently, it is called "the thermocline." The thermocline looks like a layer of invisible walls, strictly separating the upper and lower layers of water. Because the upper water can receive sunlight, tiny plants in the water can carry out photosynthesis and produce abundant oxygen. As the upper water changes with the day and night of the water temperature, convection can occur and oxygen can be sent to the water layer above the thermocline. This water layer is called "upper water." Due to the convection of day and night, the upper water has a small difference in temperature and dissolved oxygen is also sufficient. The bottom layer of water below the thermocline has been kept at a low temperature since it cannot be in contact with the upper layer of water; fish ponds have many organic substances in the water and consume large amounts of oxygen. The original oxygen will soon be consumed up, and because of the lack of The addition of dissolved oxygen to the upper layer becomes stagnant stagnant water that is low-temperature, oxygen-free, and has no production capacity. Because of the lack of oxygen, the bottom layer of the thermocline is not able to inhabit because of lack of oxygen, and the most abundant nutrients, such as residual sediment, biological feces and metabolites, are the main material sources for the growth of pond organisms. However, because the upper water and the bottom water cannot be convective, the upper water does not get this nutrient, and the breeding of organisms is very slow. As a result, the entire body of water becomes less productive. So the pool is too deep and harmful.
Seasonal influence "The thermocline"
The thermocline began to form when the spring climate became warmer, and in the summer, it was very solid; when the autumn came, the temperature dropped significantly, while the warming was weak, and the thermocline gradually disappeared from top to bottom. When the surface water temperature falls below the bottom water, the surface water will settle to the bottom layer, the bottom water rises, and convection occurs. Finally, the temperature, dissolved oxygen, and other chemical components of the entire pool water tend to be consistent. At this time, the pool water is called “ The same period of time, also known as the "turnover period." At this point, as long as a small wind or temperature drop will stir up the bottom, the long-term accumulation of residue leftovers will turn over to the entire water layer so that the dissolved oxygen is consumed, thus resulting in the risk of oxygen-dead fish. When the entire pool water temperature drops below 4°C, the water at 4°C will float in the upper layer, and the vertical temperature distribution opposite to the warm season will occur.
The low dissolved oxygen under the thermocline is the actual water temperature distribution measured by an aquaculture expert and is the highest water temperature from September 7th to 8th. It can be seen from the figure that the water temperature changes from the water surface to the water depth of 1 meter is very small and can be considered as the surface layer of the pool water; from the water depth of 1 to 3 meters, the water temperature changes drastically and can be seen as a thermocline; 3 to 6 meters The water temperature basically does not change, it is the bottom layer of water. Because of the large number of fish in the pond, at 15 o'clock on the same day, dissolved oxygen was only 4.90 mg/l at a depth of 1 meter, 1.23 mg/l at 2 meters, and 1.05 mg/l at 3 meters. Visible, the depth of 3 meters, the fish can no longer survive.
Storm waves can damage the thermocline. The wind waves can mix water layers with little difference in water temperature. They can also increase the contact between air and water surface and make more oxygen in the air dissolve into water. However, the role of storms is related to the area of ​​the water body. The small waves formed by small ponds are very small, and the upper water cannot reach the bottom. If the pond is large and the waves are large, the thermocline can be destroyed and the upper water and the deep water can be mixed. The change of production from small ponds to big ponds can increase production, mainly to increase the role of wind waves. In larger ponds, because it is not easy to form a stable thermocline, ponds can be dug deeper to increase the range of activity of fish and dissolved oxygen in water.
The depth of the fish pond is suitable for this depending on the area of ​​the pond and the local climate characteristics, including whether it is the windy and windy area or the temperature difference between days. In areas with windy and diurnal temperatures, such as Xinjiang, the pool water can be deeper; in regions with relatively small temperature difference between Jiangsu and Zhejiang, and with little wind, the pool water should not be too deep. Generally speaking, about 10 hectares of ponds, water depth 2.5--3 meters is more appropriate.
(Shi Weiliang, Dalian Fisheries University, Liaoning Province, China 116023)
There is a "temperate layer" in the water
Water's heat mainly comes from the solar thermal energy, but solar energy is absorbed in most of the water within 20 cm of the pool. If the water does not move, the water itself conducts heat very slowly, and it can only heat the surface of the water. Since the salt-free water has the highest density at 4°C, if the pool water is relatively deep, the 4°C water is always at the bottom. When heated during the day, the upper water temperature rises and the bottom water does not rise to the upper layer. Only after the sun sets, the surface water becomes cold and it gradually sinks to the lower layers. If the water is quite deep and the temperature does not drop drastically, the surface water will not fall to the bottom and stop at the middle level. With the gradual warming of the weather, the depth of subsidence is getting shallower and shallower. After a day's accumulation, a thin layer of water with intense temperature changes is formed in the middle layer of water. In a short water layer, the temperature can differ by a few degrees. Degrees, in the pond, some thin 0.6 meters of water, the temperature can be a difference of 6 °C. Because this layer of water temperature changes violently, it is called "the thermocline." The thermocline looks like a layer of invisible walls, strictly separating the upper and lower layers of water. Because the upper water can receive sunlight, tiny plants in the water can carry out photosynthesis and produce abundant oxygen. As the upper water changes with the day and night of the water temperature, convection can occur and oxygen can be sent to the water layer above the thermocline. This water layer is called "upper water." Due to the convection of day and night, the upper water has a small difference in temperature and dissolved oxygen is also sufficient. The bottom layer of water below the thermocline has been kept at a low temperature since it cannot be in contact with the upper layer of water; fish ponds have many organic substances in the water and consume large amounts of oxygen. The original oxygen will soon be consumed up, and because of the lack of The addition of dissolved oxygen to the upper layer becomes stagnant stagnant water that is low-temperature, oxygen-free, and has no production capacity. Because of the lack of oxygen, the bottom layer of the thermocline is not able to inhabit because of lack of oxygen, and the most abundant nutrients, such as residual sediment, biological feces and metabolites, are the main material sources for the growth of pond organisms. However, because the upper water and the bottom water cannot be convective, the upper water does not get this nutrient, and the breeding of organisms is very slow. As a result, the entire body of water becomes less productive. So the pool is too deep and harmful.
Seasonal influence "The thermocline"
The thermocline began to form when the spring climate became warmer, and in the summer, it was very solid; when the autumn came, the temperature dropped significantly, while the warming was weak, and the thermocline gradually disappeared from top to bottom. When the surface water temperature falls below the bottom water, the surface water will settle to the bottom layer, the bottom water rises, and convection occurs. Finally, the temperature, dissolved oxygen, and other chemical components of the entire pool water tend to be consistent. At this time, the pool water is called “ The same period of time, also known as the "turnover period." At this point, as long as a small wind or temperature drop will stir up the bottom, the long-term accumulation of residue leftovers will turn over to the entire water layer so that the dissolved oxygen is consumed, thus resulting in the risk of oxygen-dead fish. When the entire pool water temperature drops below 4°C, the water at 4°C will float in the upper layer, and the vertical temperature distribution opposite to the warm season will occur.
The low dissolved oxygen under the thermocline is the actual water temperature distribution measured by an aquaculture expert and is the highest water temperature from September 7th to 8th. It can be seen from the figure that the water temperature changes from the water surface to the water depth of 1 meter is very small and can be considered as the surface layer of the pool water; from the water depth of 1 to 3 meters, the water temperature changes drastically and can be seen as a thermocline; 3 to 6 meters The water temperature basically does not change, it is the bottom layer of water. Because of the large number of fish in the pond, at 15 o'clock on the same day, dissolved oxygen was only 4.90 mg/l at a depth of 1 meter, 1.23 mg/l at 2 meters, and 1.05 mg/l at 3 meters. Visible, the depth of 3 meters, the fish can no longer survive.
Storm waves can damage the thermocline. The wind waves can mix water layers with little difference in water temperature. They can also increase the contact between air and water surface and make more oxygen in the air dissolve into water. However, the role of storms is related to the area of ​​the water body. The small waves formed by small ponds are very small, and the upper water cannot reach the bottom. If the pond is large and the waves are large, the thermocline can be destroyed and the upper water and the deep water can be mixed. The change of production from small ponds to big ponds can increase production, mainly to increase the role of wind waves. In larger ponds, because it is not easy to form a stable thermocline, ponds can be dug deeper to increase the range of activity of fish and dissolved oxygen in water.
The depth of the fish pond is suitable for this depending on the area of ​​the pond and the local climate characteristics, including whether it is the windy and windy area or the temperature difference between days. In areas with windy and diurnal temperatures, such as Xinjiang, the pool water can be deeper; in regions with relatively small temperature difference between Jiangsu and Zhejiang, and with little wind, the pool water should not be too deep. Generally speaking, about 10 hectares of ponds, water depth 2.5--3 meters is more appropriate.
(Shi Weiliang, Dalian Fisheries University, Liaoning Province, China 116023)
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