In response to the low survival rate of bigeye larvae in river crabs, years of research and practical experience have significantly improved the quality and survival rate of crab seedlings. Below are the key techniques for early-stage healthy crab breeding.
1. **Temporary Rearing and Warming of Broodstock Crabs**
Due to market demand, river crabs are often stocked early in spring, which is also related to their limited cold tolerance. As a result, breeding has been advanced, with crabs typically introduced into rearing rooms before mid-March. However, the temperature rise during this period is slower than usual. This is because the broodstock may be in poor physical condition, and the outdoor temperature is still low when they are moved indoors, leading to underdeveloped gonads. If the temperature increases too quickly, the larvae will be weak, resulting in lower yields, poor-quality seedlings, or even breeding failure. Therefore, the warming process should be gradual: from the day the crabs are introduced until hatching, it should take 30–35 days. When water temperature is below 10°C, increase by 1°C every two days; once it reaches 10°C, increase by 1°C every three days.
2. **Salinity and Temperature Control During Breeding**
The salinity during crab breeding is usually around 26‰. When all metamorphosis reaches the zoea V stage, the salinity should be reduced to 22‰ to enhance the metamorphosis rate of large larvae. After the larvae become megalopae, the salinity should gradually be lowered to below 5‰ before they emerge. The incubation temperature varies depending on the developmental stage: 18–20°C for early stages, 22°C for juvenile larvae, 23°C for iliac zoea, 24°C for III–IV stage juveniles, and 25°C for juvenile juveniles. Bigeye larvae require a temperature of 26°C.
3. **Density Management in Breeding Pools**
The density of crabs in cloth pools should be carefully adjusted based on the planned output and overall nursery conditions. It is generally recommended that the density should not exceed 300,000 per cubic meter.
4. **Water Quality Management**
4.1 Water exchange should be adjusted according to larval density and feeding amounts. In the early stages, such as juvenile larvae, about 1/5 of the water should be changed daily. In the II and III zoea stages, change approximately 1/2 of the water each day. For IV-stage variegated larvae, change about 2/3 of the water daily, and for V-stage and megalopa, replace 100% of the water each day.
4.2 Before water changes in the late zoea stages (IV and V), apply 110 ppm potassium permanganate to purify the water.
4.3 Regularly add photosynthetic bacteria or other biological agents to promote beneficial bacterial growth, reduce ammonia nitrogen, and inhibit harmful bacteria.
5. **Feeding Principles and Bait Types**
Feeding should follow the principles of freshness, palatability, and appropriate quantity without overfeeding. In the early stages, crabs may not tolerate fresh live bait well, so substitute baits are used. The types of substitute baits vary depending on the developmental stage.
During the juvenile period, feed egg yolk, algae powder, shrimp slices, yeast, mini bait, and a small amount of rotifers (fresh or frozen). After the anaphase change, Artemia nauplii are suitable for zoea stages. In the IV and V stages, feed cakes, mini baits, and a mix of rotifers and Artemia nauplii. Once the bigeye larvae mature, feed processed shrimp or adult Artemia (live or frozen).
6. **Disease Prevention and Treatment**
Common diseases include polycondensation (or single worms) and filamentous bacteria. To prevent these, implement the following measures:
6.1 Strictly sterilize the nursery ponds and disinfect the crabs before hatching.
6.2 Maintain good water quality by controlling feeding amounts and water exchange.
6.3 Regularly apply photosynthetic bacteria or other biological agents to promote beneficial bacteria and suppress harmful ones.
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