Experiences in polyculture farming of mud eels and freshwater crabs.

Raising mud eels and freshwater crabs together in rice paddies is a promising new approach that has attracted much attention from local people due to its low investment cost, ease of breeding, minimal disease risk, and readily available natural food sources.

Rice paddies for raising mud eels and freshwater crabs.

1. Biological characteristics of freshwater eels and freshwater crabs

- Mud eels have a habit of living burrowed in the mud.

- Freshwater crabs crawl along the water's surface and dig burrows to live in, or hide in aquatic plants and roots. Crabs are capable of crawling on land and traveling long distances; each time they molt, their weight increases by 20-50% of their potential weight.

- Both species share the common characteristic of being nocturnal, disliking strong light, so during the day they stay in burrows or hide in the mud, rarely venturing out until dark to forage. Their main food sources are shrimp, prawns, small fish, worms, snails, and grains. They can thrive in water temperatures ranging from 20-30°C, ideally 25-27°C, with a pH of 6.5-8. During the rainy season (from March to August each year), mud eels and freshwater crabs grow, develop, and reproduce abundantly.

2. Prepare the pond for aquaculture.

To ensure that mud eels and freshwater crabs grow and develop well and to minimize losses, the breeding pond must be properly prepared before stocking.

- Selecting the right pond for aquaculture: The pond should be flat, retain water well, have an abundant water source, convenient water supply and drainage, unpolluted water, and loamy soil.

- Pond area for farming: Depending on conditions, a suitable pond area ranges from 3,000 to 5,000 m2. After harvesting the winter-spring rice crop, the straw should be cut and left long as stubble to be used as food for eels and crabs. Then, flood the pond with water to a height of 0.5 - 0.7 meters.

- Dig surrounding ditches and a central ditch. The surrounding ditch at the base of the embankment should be 0.8-1.0m deep and 3-5m wide. If the field is large, dig an additional ditch in the middle of the field in a "+" or "#" shape, 1-1.5m wide and 0.5-0.8m deep. The total area of ​​all three types of ditches should occupy approximately 15-20% of the field area.

The purpose of digging trenches in rice fields is to create shelters for mud eels and crabs during periods of hot or cold weather, or when the environment changes unexpectedly.

- The field embankments must be high and wide. Pay attention to compacting the soil tightly to prevent water leakage. The inlet and outlet gates must be blocked with suitable bamboo fences or netting, and the culvert foundations must be firmly compacted.

- Place a net barrier around the rice field. If using thin nylon netting, stakes must be driven 40-50cm above the embankment, and the stakes connected with steel wire. Fold the nylon sheet in half over the wire so that it hangs down in two layers, then bury it 15-20cm deep in the embankment soil. If using plastic sheets or fiber cement, bury them only 15-20cm deep, with the top end at least 40cm above the ground and the four corners curved. This is to prevent flooding during rain, which could allow eels and crabs to escape.

- Plant aquatic grasses to cover the bottom of the ditch and release floating plants such as duckweed, water hyacinth, and water spinach... covering about 1/3 of the water surface. This provides shelter for eels and crabs during hot or cold weather.

- If conditions permit, build additional embankments for mud eels and crabs to dig burrows for shelter. These embankments should be about 1 meter wide and 30-40 cm higher than the water level at its highest point. Sprinkle water hyacinth seeds on the embankment to protect the crabs and mud eels from sun and cold when the outside temperature changes. Therefore, the wider and more sturdy the embankment, the less likely it is to be blown over by wind when the water hyacinth grows well and develops a root system. At the same time, mud eels and crabs will have less impact on the main embankments surrounding the pond. Depending on the shape and size of the pond, the embankments can be built in a cross or parallel shape.

- One to two weeks before raising the crabs, drain the water to eliminate all predators of the crabs and kill pathogens by applying lime at a rate of 7-10 kg/100 m2, drying in the sun for 3-5 days, then refill the field with water but do not allow it to overflow. Only when the rice is about to flower should water be added to the field so that the crabs can come out to find food.

- Induce water coloration using manure or chemical fertilizers to create a source of plankton that will serve as food for newly released crab juveniles.

3. Select the variety

Currently, there are no facilities on the market specializing in the artificial breeding of mud eels and freshwater crabs. To obtain fry for stocking, farmers can purchase them in bulk from traders. When buying, pay attention to: Choosing to buy from reliable sources that guarantee quality according to requirements.

+ Freshwater eels: healthy, uniformly sized (150-200 eels/kg), shiny, not slimy, active swimmers, and free from scratches. Avoid buying them at the market as you might accidentally buy eels that have been electrocuted, have damaged spines, or are deformed, which will not grow well and will reduce economic efficiency.

+ Freshwater crabs: healthy, uniform size (150-160 crabs/kg), no broken claws or gills, shiny shells, and no algae growth.

4. Release the fry.

- The appropriate stocking season is March-April each year. Stocking should be done in the early morning or late afternoon.

- The appropriate stocking density for mixed-species mud eels and freshwater crabs is 20 individuals/m2, of which...

+ Mud eels: 16 eels/m2

+ Freshwater crabs: 4 crabs/m2
Once stocked, no further stocking is needed because the mud eels and freshwater crabs will reproduce naturally in the ponds where they are raised.

5. Care

Mud eels and freshwater crabs are highly resistant to environmental conditions and are almost disease-free. Their natural food sources are readily available, such as golden apple snails, which are a good option due to their high protein content. Other suitable feed sources include shrimp, prawns, ground fish, fishmeal, rice bran, corn bran, rice stubble, and cassava flour.

We can prepare our own mixed pellet feed consisting of: ground rice + fish meal + golden apple snails. Then, we can press it into pellets, dry them, and feed them to freshwater eels and crabs gradually.

- Feeding begins 1-2 days after stocking. Initially, feed is spread evenly throughout the field, then the feeding area is gradually reduced, and finally, feeding is done at a few fixed points in the field so that the mud eels and crabs become accustomed to eating, facilitating the harvesting process later.

Feeding method: Protein and concentrated feed should be given in combination, once every two days at a fixed time in the evening. The amount of feed should be approximately 5% of the crab's or eel's body weight.

Additionally, 25-30 kg of organic fertilizer per 100 m2 of rice field per month can be applied to promote the growth of plankton in the rice paddy, which serves as food for eels and crabs.

Note: In the feeding areas, place fine mesh feeding trays at the bottom of the pond, with an area of ​​about 1.0-1.5 m2, then put the feed into the trays. The next morning, check the amount of leftover feed to adjust the feeding schedule accordingly. Since mud eels and freshwater crabs are mainly nocturnal, feed them in the late afternoon, in the area where they were caught. Based on the rate of water evaporation, regularly add water to the pond to maintain a clean water environment, consistently maintaining a water level of 0.5-0.7m. Absolutely no pesticides should be used in the pond.

6. Harvesting

After about 4-5 months of rearing, selective harvesting can be carried out. After 5-6 months, mud eels reach 20-25 grams/each, and freshwater crabs reach 25-30 grams/each. Harvesting can be done selectively or all at once.

Stop feeding for 1-2 days before harvesting.

- Selective harvesting: Place a trap containing bait such as groundnuts or shelled golden apple snails in the feeding area. Place the trap in the evening before, and remove it early the next morning to collect the larger snails, releasing the smaller ones back into the pond for further rearing.

- Complete harvesting of mud eels: Gradually drain the water so the mud eels swim along with the current, and place nets or traps at the drain to collect them.

Farmers should only harvest a portion of the fish, selecting the larger, healthier ones to continue raising them in the field so they can reproduce and provide seeds for the next farming season.

According to the National Agricultural Extension Center