Breeding management of Brassica in winter

The wintering period in Brazil varies in duration depending on the region. In southern Brazil, it is relatively short, while in the north, it lasts longer. In eastern China, the wintering season typically runs from November to April of the following year, lasting about six months. Throughout this entire period, the survival and health of Brazilian fish during winter do not directly impact their reproductive performance in the next breeding season. Instead, the success of the overwintering process is closely tied to the quality of daily management. If husbandry practices are neglected, a large number of fish can die, making consistent care essential. Therefore, someone must be assigned to monitor feeding and overall management during the winter. Winter management for Brazilian fish involves several key aspects: **(I) Transport and Acclimation to Wintering Pools** At the start of winter, fish are moved from rearing ponds to wintering pools. This process can cause stress and injuries due to handling and transportation. To minimize this risk, before entering the pool, fish should be bathed in 1–2 mg/l methylene blue for 30 minutes to disinfect them. It's also recommended to gradually raise the water temperature to around 25°C, allowing the fish to adapt to the new environment. Once they begin eating normally, the temperature can be adjusted to match the wintering conditions. For fingerlings, maintain the temperature at 20–23°C, while broodstock can be kept at 22–26°C depending on the situation. Within the first week after transfer, mortality is usually highest. Dead fish should be removed promptly to prevent water pollution and potential disease outbreaks. Water changes of about 30% per day are recommended, but care must be taken to avoid large temperature fluctuations. **(II) Daily Management** 1. **Feeding Practices** Brazilian fish are omnivorous, so their feed requirements are not overly strict. In addition to specialized feed, general squid or shrimp-based feeds can also be used. The amount of food varies with water temperature and growth stage. For overwintering juveniles, maintaining a water temperature of 20–23°C is ideal, with daily feeding at about 3% of their body weight, divided into three meals at 8:00, 12:00, and 16:00. For adult fish, the focus is on survival, with a water temperature of 18–21°C and 1–2% of body weight fed twice daily at 9:00 and 15:00. Broodstock require more attention, as their gonadal development during winter is crucial for successful reproduction in the next season. They should be kept at 22–26°C, with 3–5% of their body weight fed once daily, with 1/3 of the feed given in the morning and 2/3 in the afternoon. Feeding should follow the "four timing" principle—fixed time, fixed location, fixed quantity, and fixed quality. A feeding platform should be set up, and daily feed intake should be monitored. Adjustments should be made based on water quality and temperature changes. 2. **Water Quality Management** Maintaining good water quality is critical for the health and growth of fish during winter. Since frequent water changes are avoided for heat retention, waste and leftover food accumulate, depleting oxygen and releasing harmful substances. Regular monitoring of water quality is necessary, along with periodic removal of pollutants. When changing water, sweep debris toward the drain. If there’s a lot of leftover food, use a siphon to remove it. Although Brazilian fish have low oxygen requirements, high stocking density often necessitates the use of aerators or oxygenation systems. These should be operated for 30 minutes every four hours to ensure adequate dissolved oxygen levels. 3. **Ongoing Monitoring and Care** Throughout the winter, someone should be responsible for daily oversight. This includes observing feeding behavior, measuring and adjusting water temperature, removing uneaten food and feces, and checking water quality. If fish are seen swimming near the surface or showing reduced appetite, it could signal a problem. Immediate checks should be conducted to assess water quality, temperature, and possible pathogens, followed by appropriate treatment measures. 4. **Disease Prevention** Due to high stocking density and fluctuating water conditions, disease outbreaks are common during winter. To prevent this, it is advisable to treat the water every other week with 1 mg/l bleach or 0.2 mg/l methylene blue. Proactive measures can significantly reduce the risk of illness and improve overall survival rates.

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