In response to the low survival rate of bigeye larvae in river crabs, years of practical experience and research have significantly improved both the quality and survival rate of crab seedlings. Below are the early-stage health breeding techniques for crabs.
1. **Temporary Rearing and Warming of Broodstock Crabs**
Due to market demands, river crabs are often stocked early in spring, which is also related to their limited tolerance for low temperatures. As a result, breeding times have been advanced in recent years. Typically, crabs are introduced into rearing rooms before mid-March. However, the temperature increase for broodstock is slower than usual. This is because the physical condition of the broodstock is poor, and the outdoor temperature is still low before they are moved indoors, causing the gonads to be underdeveloped. If the water temperature is increased too quickly, the resulting larvae may be weak, leading to low nursery yields, poor seedling quality, or even breeding failure. Therefore, the temperature should be gradually raised: from the day the crabs are introduced until hatching, it should take 30–35 days. When the water temperature is below 10°C, increase it by 1°C every two days; once it reaches 10°C, increase it by 1°C every three days.
2. **Salinity and Temperature in Crab Breeding**
The salinity during crab breeding is typically 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 be gradually adjusted to less than 5‰ before emergence. The incubation temperature should be maintained at 18–20°C, with 22°C for juvenile larvae, 23°C for iliac zoea, 24°C for III–IV stage juveniles, and 25°C for juvenile stages. Bigeye larvae require a temperature of 26°C.
3. **Density Management in Cloth Pools**
The density of cloth pools should be arranged reasonably based on the planned output and overall conditions of the nursery. 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 carefully managed according to larval density and feeding levels. In the early juvenile stage, about 1/5 of the water should be changed daily. During the II stage of zoea, change approximately 1/5 of the water each day, while in the III stage, change about 1/2 daily. In the IV stage of variegated larvae, change about 2/3 of the water daily, and for V-stage megalopae, replace 100% of the water each day.
4.2 In the late zoea stages (IV and V), apply 110 ppm potassium permanganate before water changes to purify the water and improve the quality of the exchange.
4.3 Regularly add photosynthetic bacteria or other biological agents to the nursery ponds to promote beneficial bacteria, degrade ammonia nitrogen, and inhibit harmful bacteria growth.
5. **Feeding Principles for Crabs**
Feeding should follow the principles of freshness, palatability, and appropriate quantity without overfeeding. In the early stages of healthy seedlings, live bait is not always suitable, so substitute baits are used. Careful attention should be paid to the conversion and amount of different types of bait. Here are the recommended substitutes for different stages:
- During the juvenile period (II stage), feed an appropriate amount of egg yolk, algae powder, shrimp slices, yeast, mini bait, and a small amount of rotifers (fresh or frozen).
- After the anaphase change, feed Artemia nauplii in appropriate amounts.
- For zoea IV and V stages, feed mainly with cakes and mini baits, along with rotifers and a certain amount of Artemia nauplii depending on the number of larvae.
- Once the bigeye larvae mature, feed processed shrimp or adult Artemia (live or frozen).
6. **Disease Prevention and Treatment**
Common diseases in crab breeding include polycondensation (or single worms) and filamentous bacteria. To prevent these, the following measures are recommended:
6.1 Strictly sterilize the nursery ponds and disinfect the crabs before hatching.
6.2 Maintain good water quality by managing feed and water exchange appropriately.
6.3 Regularly apply photosynthetic bacteria or other biological agents to the ponds to establish beneficial bacterial populations and suppress harmful bacteria.
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