To improve the survival rate of fry stocking, it is essential to carefully plan and execute several key steps before and during the process. First, proper preparation of the fish pond is crucial. This includes thorough disinfection to eliminate harmful organisms and create a safe environment for the fry.
Disinfection can be done using two main methods: the dry method and the wet method. In the dry method, the pond is drained and exposed to sunlight. Afterward, quicklime (about 75 grams per square meter) is evenly spread across the bottom. The pond is then refilled with fresh water, left for 4–5 days, and then changed again. After 2–3 more days, the water becomes suitable for stocking. In the wet method, 220 grams of quicklime per cubic meter of water is used, mixed thoroughly and distributed evenly. The toxicity of the lime should be checked before introducing the fry. This can be done by measuring the pH level—if it drops below 7, the toxicity has passed. Alternatively, if no water remains in the test container, the toxicity is gone.
After disinfection, the next step is to prepare the water quality. One week before stocking, the pond should be filled to a depth of 50–60 cm. Then, 250–300 kg of livestock manure water per mu is added to enrich the water with natural food sources for the fry. Additionally, pulling nets before stocking is important. A few empty nets are placed in the pond to catch any remaining wild fish, frog eggs, or aquatic insects that could prey on the fry.
When it comes to stocking the fry themselves, careful selection and handling are necessary. The fry should be healthy, disease-free, and at least 3 cm in size. They should also be of similar size to prevent cannibalism. Before placing them in the pond, they should be disinfected by soaking in a salt solution (1 kg of salt in 30 liters of water) for about 5 minutes.
The timing of stocking is also critical. It should occur during the peak of rotifer breeding, typically 5–7 days after fertilization. Ideal conditions include sunny days when the water temperature is between 20°C and 25°C. Stocking density varies depending on the species—usually around 100,000 to 200,000 fry per mu. Grass carp generally require lower densities, while other species may allow higher numbers. It’s best to stock one species at a time and avoid mixing different sizes or species.
Before stocking, it's important to check for any residual drug toxicity. This can be done by placing a small number of fry in a cage within the pond. If they survive for at least a day, the toxicity has dissipated. Another way is to monitor the pH level; if it drops below 9, the quicklime toxicity has disappeared.
The timing of fry introduction into the pond should also be considered. Fry should be introduced when they are strong enough to swim freely, usually 4–5 days after hatching. Introducing them too early can lead to poor survival due to weak swimming ability, while delaying can reduce their chances of adapting.
It’s also recommended to stock only one species in the same pond, ensuring they are from the same batch. Polyculture or mixing different sizes should be avoided. Stocking should be done on sunny days, as the water is warmer and has higher dissolved oxygen levels, which helps the fry adjust better. On cloudy or rainy days, the water temperature and oxygen levels drop, making it harder for the fry to survive.
Applying manure to the pond is another key step. It helps increase the population of zooplankton, such as rotifers and nauplii, which serve as natural food for the fry. The ideal density is 5,000–10,000 per liter. Base fertilizer should be applied five days before stocking, with 200–500 kg per acre. New ponds may also benefit from a small amount of chemical fertilizer.
Before stocking, it’s important to check the presence of food organisms. If large zooplankton like copepods or red worms are present, they should be removed using an appropriate solution. A mesh net with a size of 200–300 mesh can be used, but all nets must be clean and free of disease.
Finally, before releasing the fry, pull a dense net through the pond to remove any remaining predators. Also, ensure the temperature difference between the fry’s holding container and the pond is no more than 3°C. If there is a significant difference, gradually acclimate the fry to the new temperature. When placing the fry in the pond, do so gently in a sheltered area, avoiding sudden dumping, especially on windy days. Place the fry upwind to prevent them from being blown toward the edges of the pond.
Bone Screw
Bone screws, also known as fracture fixation screws, are commonly used in clinical practice to fix orthopedic implants.
Bone screws are usually used to fix internal fractures or dislocations by directly screing into two different bone blocks or fixing an internal implant such as a bone plate to achieve fracture fixation, position the bone and promote its healing. Bone screws are used in a wide range of areas, including the shoulder, elbow, hip, knee, spine, etc. For example, pedicle screw systems are used for spinal fusion, and compression bone screws are commonly used for foot and ankle surgery or fixation of other fractures under pressure. Similar to traditional mechanical screws, the main structures of bone screws also include nail cap, nail body, and nail tip. Screws can be used to fix bone plates or bone fragments. When used for the former, they are called plate screws, and when used for the latter (to prevent the collapse of bone fragments), they are called positional screws. The latter can be inserted into the plate holes, and can be placed on the bone alone (also known as compression screws). Compression screws can be used to increase interfragment pressure.
(1) The screw cap
The screw cap has three main functions: the first is to optimize the force. The protruding cap makes the contact area between the screw and bone larger, increases the load area, optimizes the local force at the screw insertion site, and reduces the risk of bone rupture caused by excessive stress. The second is the positioning effect. The prominent cap makes the bone nail can only be screwed into a certain depth to prevent the whole bone nail from being screwed into the bone completely. The third function is to provide the position of force application by rotating the force groove at the cap to move the bone nail forward and drive it into the bone. Now, the force groove is mostly inner hexagonal, which does not require axial force to maintain the actuator in the center position and is suitable for a wider range of fractures.
(2) Screw the body
The size of the nail body determines the strength and fatigue resistance of the bone nail. The larger the diameter of the nail body, the stronger the strength will be, and the corresponding fatigue resistance will be better. In addition to the diameter of the nail body, the pitch and tooth depth of the screw body thread are also the key parameters of the nail body design. Different thread design has an important effect on the pressure and occlusal performance of the screw.
(3) Screw tip
Tapping is the process of phalangeal nail cutting thread in the bone. According to the shape of the nail tip, the bone nail can be divided into self-tapping nail and non-self-tapping nail. The nail tip of the self-tapping nail is sharper and can be directly screwed into the bone without pre-drilling. Usually, self-tapping screws are used for Cancellous bone, and the bone is compressed when the screw is inserted, so as to increase the bone density of the occlusal part locally and enhance the occlusal effect. However, when inserting screws in Cortical bone, the screw channel is generally pre-punched, and then the bone screw is screwed. Usually, the self-tapping screw is not directly used to prevent the bone screw from being stuck or damaged because the cortical bone is too hard.
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