In the middle and later stages of shrimp farming, when shrimp reach a size of 7-8 cm, several factors contribute to significant changes in pond water quality and bottom conditions. As feeding amounts increase, shrimp waste accumulates, algae die off, and leftover feed and organic matter decompose, leading to shifts in water chemistry, microbial populations, and overall pond health.
The water environment typically shows signs such as rising pH levels, changing color from greenish to brown or tea-green, reduced transparency, and increased turbidity. The pond bottom becomes dark and malodorous, while harmful microorganisms proliferate. Shrimp may display symptoms like red tails and antennae, tail damage, slow growth, weak physique, poor response, and even ciliate infestations. Some ponds may see issues like enteritis, jaundice, floating heads, soft shells, red legs, eye rot, and carapace ulcers. Based on years of experience, key management strategies have been developed for this critical phase.
To stabilize water color and reduce turbidity, it's important to address the lack of magnesium ions, which are essential for green algae growth. Applying 20–25 ppm of dolomite once every 7–10 days helps maintain stable water color. To reduce turbidity, photosynthetic bacteria at 10–15 ppm can be used once weekly. Regular disinfection is also crucial—disinfecting the entire pond every 10–15 days with dibutyl hydantoin at therapeutic concentration in the morning, followed by organic iodine and quaternary ammonium salt (0.2 ppm) the next day. Adjustments can be made if needed.
Balancing nutrients in the water is another key step. Adding dicalcium phosphate (2–3 ppm) every 10–15 days and dolomite powder (20 ppm) once during that period ensures sufficient calcium, magnesium, and phosphorus levels.
Improving the sediment environment and managing pH involves applying beneficial microbes at 0.3 ppm every 10–15 days or using a zeolite-based bottom improver at 10–15 ppm every five days. If heavy rain causes a drop in pH below 7.5, adding 5–8 kg of lime per 667 square meters can help restore balance.
Water exchange should be done gradually: starting with 10 cm on the first day, increasing to 20 cm, 30 cm, and up to 50 cm over the following days. This promotes better water quality and shrimp growth. Maintaining proper water levels and investing in oxygenation equipment is essential for large-scale operations.
Feeding practices must be optimized. Feed shrimp four times daily, with the first meal given 1–2 hours after sunrise, ensuring dissolved oxygen levels exceed 4 mg/L. The second feeding occurs after six hours, and the amount should be enough for shrimp to consume within 45 minutes to an hour.
Boosting shrimp immunity and stress resistance involves adding immune polysaccharides (0.1%), biological enzyme additives (0.2%), vitamin C (0.1–0.2%), and vitamin B (0.05%) to their feed twice a week. Garlicin (0.05–0.1%) can also be added to enhance disease resistance.
Regular health checks are vital. If more than 20% of the shrimp show signs of illness, such as reduced appetite or floating heads, immediate action is required. Sampling 200–300 shrimp every 3–5 days using a fine-mesh net helps monitor health. Sudden changes in feed consumption, weather fluctuations, or post-water exchange events call for prompt sampling using a rotary screen to detect early signs of disease.
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