Commercial crab farming techniques
Sea crabs are widely distributed in our country's waters, of which the green crab with the scientific name Scylla serrata (Forskal) is a relatively large species with high economic value...
1. Biological characteristics of sea crabs:
Sea crabs are distributed widely in our country's sea, including green crabs with the scientific nameScylla serrata(Forskal) is a relatively large species with high economic value.
Eating:
Sea crabs are omnivorous animals. The larval stage feeds on zooplankton (rotifers, moina, artemia…). The stage from baby crabs to adults feeds on fresh fish, snails, and shrimp.
Growth:
From larva to adult, crabs must go through many molts and each molt changes the size and actual structural morphology of the crab.
In the larval and powdery crab stages, the time between molts is usually short: 2-3 days or 3-5 days. In the adult stage, crabs often molt during the tidal cycle (early water).
Living conditions:
pH:Crabs live in brackish water with a pH range of 7.5 - 9.5, most suitable is 7.5 - 8.2. However, crabs can tolerate water with a pH lower than 6.5.
Salinity:Crabs have a high adaptability to changes in water salinity. Crabs can live in almost fresh water up to 33% salinity.0.
Water temperature:Sea crabs are widely distributed and in high latitudes, crabs can tolerate low water temperatures well. In the southern sea of our country, sea crabs adapt to water temperatures from 25 to 29 degrees Celsius.0C. High temperatures often adversely affect the physiological activities of crabs, and are one of the causes of death.
Place of residence: Crabs like to live in places with lots of aquatic plants, semi-submerged areas, and banks to dig burrows and find shelter, especially during molting. Many crabs live in coastal estuaries and mangrove forests.
2. Breeding techniques:
Pond construction:
Ponds for raising baby crabs into commercial crabs usually have an area of 500m2.2up to 5000m2This is an intensive farming method: stocking, active feeding, and close care and management.
Location: The pond should be built in an area where water can be easily changed by tides to reduce costs. In areas with clean water sources, not polluted by industry, agriculture, daily life, with few strong winds and large currents. The pond bank is made of soil (can also be built with bricks if possible), the bank needs to be compacted to prevent water from seeping, leaking and eroding. The pond bank is 3-4m wide, 1.5 to 2m high, at least 0.5m higher than the highest tide level. Each pond should have two culverts at two opposite ends, if the pond is rectangular, the two culverts will be placed on the two banks of the width. The drain is placed close to the bottom and connected to the canal in the pond. Inside the pond, 2-3m from the bank, dig a 3-4m wide canal surrounding the pond. In the middle of the pond, build a mound 0.2-0.3m higher than the pond water level. In the canal, add more rocks for the crabs to hide. Build a barrier around the bank to prevent crabs from going out. You can use a sparse mosquito net, bamboo net, etc. The barrier must be tilted towards the inside of the pond at a 60 degree angle.0, the post must be 0.8-1m high.
Depending on the acidity of the pond bottom, there is a suitable treatment method. If the soil pH is below 6, sprinkle lime powder (CaCO3) for pond bottom: drain water, spread lime evenly on the pond bottom, both the canal bed and the pond edge. The amount of lime to spread is 7-10 kg/100m2pond. Dry the pond bottom for 2-3 days, then let water in and out 3-4 times to rinse off the alum water.
Water salinity: baby crabs in the growing stage live in brackish water with salinity of 15-25%0However, crabs can withstand large changes in salinity. They live and grow well at 5% salinity.0 up to 30%o.
Seed release:
The source of crab seeds for crab farming mainly depends on natural sources. The source of crab seeds is collected from the bottom, by dredging boats at river mouths, and caught in mudflats in mangrove areas.
Crabs come in the following sizes:
Small type 60-120 pieces/kg
Medium size 25-50 pieces/kg
Large size 10-15 pieces/kg
It is best to buy crab seeds from the surrounding area and after catching, quickly transport them to the stocking site. Calculate the quantity needed to stock and order for several consecutive days to stock the pond in a relatively short time.
In each pond, crabs should be stocked with the same size. You can also choose the desired size of crabs at the place where the crab breed is supplied, or select them before stocking.
Stocking density:
Small crabs 3-5/m2, medium size 2-3 fish/m2, large size 0.5-1 fish/m2. Release the seeds at different points in the pond. Cut the ropes, slowly let the crabs on the edge of the bank so that they can go down to the water by themselves. This is a way to check the health of the crabs: the healthy ones quickly run down to the water, the weak ones usually stay in place or crawl slowly. Collect such crabs and put them in a stage to monitor, if they recover, release them back into the pond. Each pond should release the seeds for one or two days to have enough quantity.
Management and care:
Feeding: Crabs raised in ponds mainly rely on daily food supply, the amount of natural food in the pond is not much. The main food is fresh food: scrap fish, crabs, crabs, fish heads... The daily amount of food is about 4 - 6% of the crab's weight, crabs are often active in the evening. Feed crabs once a day from 5 - 7 pm. Food is spread evenly around the pond so that crabs do not compete. You can use a feeding tray to check the crab's appetite. Periodically collect samples to calculate the crab yield in the pond and adjust the amount of food accordingly. After 2-3 hours of feeding, check the feeding tray. If the crabs eat all the food in the tray, you can increase the amount of food. If there is still food left, reduce the amount of food.
Crabs must be fed daily, do not let them go hungry. Large crabs that are hungry will kill small crabs for meat. Therefore, crab farming must have food reserves. On days when there is no fresh food, feed the crabs with dry food: small fish, shrimp, dried crabs. Before spreading the dried fish in the pond for the crabs to eat, soak the dried fish in water for a few dozen minutes to soften the fish.
Ensuring a clean water environment is very important for crabs, especially when raising them at high density and feeding them live food. In places where the tides rise and fall daily, water needs to be changed regularly. Change 20-30% of the water in the pond every day. Change all the water in the pond once a week. When changing water, take water from the lower and middle layers, avoid taking water from the surface layer which is often polluted. New, clean water stimulates crabs to be active, eat a lot, and molt well.
Regularly check the condition of banks, culverts and fences to avoid crab loss.
During the rearing period, about every 2 weeks, catch and weigh the crabs to see their growth, examine their condition: are they agile, do not have parasites on the shell, and check if there are parasites in the gill cavity. If there are signs of infection, find the cause and treatment.
At the end of the farming season, the weight of crabs in the pond increases, and a lot of food is given, so the environment is easily contaminated. Therefore, changing the water regularly to check the environment is very important. In some cases, the bottom of the pond accumulates a lot of leftover food, rotting, it may be necessary to drain, decant the crabs and clean the bottom of the pond: scrape off the mud on the surface and leftover rotting food.
Harvest:
Test the crab to check the quality. Regular crabs must weigh 250g or more. The crabs must be firm or have roe (female crabs). When the crabs meet the standards and have a good price, harvest them for sale.
Crabs that have not reached the right size, weight, or are not yet full of roe, if they are still healthy, can be raised in small ponds, actively raised and after a period of time when they reach the harvest standards, they will be sold for a higher price.
Raising commercial crabs from baby crabs, for 3-8 months, usually has a relatively large loss rate (40-60%) but the crab weight increases 3-4 times (from 60-80g/crab to 250-350g/crab). The total weight of commercial crabs increases 1.5-2 times the total weight of the seed crabs.
Some points to note for successful crab farming:
- Crabs must be of the same size and released at the same time;
- There must be enough clean water to change regularly;
- There must be enough fresh food sources;
- There must be a barrier on the pond bank;
- In the pond there must be mounds as shelter for crabs.
According to khuyennongkiengiang - LY