Techniques for commercially farming sea crabs
Sea crabs are widely distributed in Vietnam's waters, among which the green crab, scientifically known as Scylla serrata (Forskal), is a relatively large species with high economic value...
1. Biological characteristics of sea crabs:
Sea crabs are widely distributed in the waters of our country, including the green crab, whose scientific name isScylla serrataForskal is a relatively large species with high economic value.
Eating habits:
Crabs are omnivorous, primarily feeding on animals. During the larval stage, their diet consists of plankton (rotifers, moina, artemia, etc.). From juvenile to adult, their diet includes live fish, snails, and shrimp.
Growth:
From larva to adult, crabs undergo several molts, and each molt brings about changes in size and the crab's true morphological structure.
In the larval and juvenile stages, the time between molts is usually short, ranging from 2-3 days to 3-5 days. Adult crabs typically molt during the tidal cycle (the beginning of the tide).
Living environmental conditions:
pHCrabs live in brackish water with a pH range of 7.5 – 9.5, ideally 7.5 – 8.2. However, crabs can tolerate water with a pH lower than 6.5.
SalinityCrabs are highly adaptable to changes in water salinity. They can live in water ranging from near-freshwater to salinity levels of 33%.0.
Water temperatureSea crabs have a very wide distribution, and in high latitude regions, they tolerate low water temperatures well. In the southern seas of Vietnam, sea crabs adapt to water temperatures from 25 to 29 degrees Celsius.0C. High temperatures often negatively affect the physiological functions of crabs, and are one of the causes of death.
Place of residenceCrabs prefer to live in areas with abundant aquatic plants, semi-submerged areas, and banks where they can dig burrows and find shelter, especially during molting. Coastal mangrove forests near river mouths are home to many crabs.
2. Rearing techniques:
Constructing fish ponds:
Ponds used to raise juvenile crabs into marketable crabs typically have an area of 500 square meters.2up to 5000m2This is a form of intensive farming: stocking, intensive feeding, and strict care and management.
Location: The pond should be built in an area where water can be easily changed by tides to reduce costs. It should be in an area with clean water, free from industrial, agricultural, or domestic pollution, with minimal strong waves and currents. The pond banks should be made of earth (brick construction is also possible if investment is feasible), and the banks need to be compacted to prevent erosion, leakage, and landslides. The base of the pond bank should be 3-4m wide and 1.5-2m high, at least 0.5m above the highest tide level. Each pond should have two drains at opposite ends; if the pond is rectangular, the two drains should be placed on the two banks along the width. The drains should be placed near the bottom and connected to the internal pond channel. Inside the pond, 2-3m from the bank, dig a 3-4m wide channel surrounding the pond. In the middle of the pond, build a raised mound 0.2-0.3m higher than the pond water surface. Add brushwood in the channel for the crabs to hide in. Construct a barrier around the pond to prevent crabs from escaping. This can be done using fine mesh netting, bamboo fences, etc. The barrier should be angled inwards towards the pond at 60 degrees.0The sign must be between 0.8 and 1 meter high.
Depending on the acidity of the pond bottom, appropriate treatment methods should be used. If the soil pH is below 6, apply lime powder (CaCO₃).3For the pond bottom: Drain the water and spread lime evenly on the pond bottom, including the canal bed and pond banks. The amount of lime to spread is 7-10 kg/100m².2The pond should be left to dry for 2-3 days, then filled and drained 3-4 times to remove the acidic water.
Water salinity: juvenile crabs in their growth stage live in brackish water with a salinity of 15-25%.0However, crabs are highly sensitive to salinity variations; they thrive and grow best at a salinity of 5%.0 up to 30%o.
Releasing the seeds:
The primary source of crab larvae for crab farming is natural broodstock. These larvae are obtained from bottom trawls, trawling boats in river estuaries, and from fishing in mangrove swamps.
Crabs are available in the following sizes:
Small size: 60-120 pieces/kg
Medium size, 25-50 pieces/kg
Large size: 10-15 pieces/kg
It is best to purchase juvenile crabs from nearby areas and, after catching them, transport them quickly to the breeding site. Calculate the number needed for stocking and order them over several consecutive days so that the crabs can be released into the breeding pond within a relatively short period.
Crabs of the same size should be stocked in each pond. Alternatively, you can choose the desired size of crabs directly from the supplier or select them before stocking.
Stocking density:
Small crabs: 3-5 crabs/m²2, medium size 2-3 fish/m2, large size 0.5-1 fish/m2Release the crabs at different points in the pond. Cut the ties and slowly lower the crabs along the edge of the bank so they can naturally enter the water. This is a way to check the crabs' health: healthy crabs will quickly swim into the water, while weak ones will remain still or crawl slowly. Collect these crabs and place them in a cage for observation; if they recover, release them back into the pond. Each pond should be stocked with crabs for one or two days to ensure sufficient numbers.
Management and care:
Feeding: Crabs raised in ponds rely primarily on daily food supplies; the amount of natural food in the pond is limited. The main food source is live food: fish scraps, small crabs, mud crabs, fish heads, etc. The daily food intake is approximately 4-6% of the crab's weight. Crabs are usually active foraging in the evening. Feed the crabs once a day between 5 PM and 7 PM. Food should be evenly distributed around the pond to prevent competition. Feeding trays can be used to monitor the crabs' appetite. Regularly collect samples to calculate the crab yield and adjust the amount of food accordingly. After 2-3 hours of feeding, check the feeding trays; if the crabs have consumed all the food, increase the amount; if there is still food remaining, reduce the amount.
Crabs must be fed daily; they must not be allowed to go hungry. Large, hungry crabs will kill and eat smaller ones. Therefore, raising crabs requires a food reserve. On days when fresh food is unavailable, feed them dry food: dried fish scraps, shrimp, and krill. Before scattering the dried fish in the pond, soak it in water for a few dozen minutes to soften it.
Maintaining clean water is crucial for crabs, especially in high-density farming with live feed. In areas with daily tidal fluctuations, frequent water changes are necessary. Replace 20-30% of the pond water daily. Replace all the water once a week. When changing water, take water from the lower and middle layers, avoiding the surface layer which is often polluted. Fresh, clean water stimulates crab activity, encourages increased feeding, and promotes molting.
Regularly inspect the condition of embankments, drains, and fences to prevent crab escape.
During the rearing period, the crabs are weighed and measured approximately every two weeks to monitor their growth and condition: checking for activity, absence of external parasites, and presence of parasites in the gill cavities. If any signs of disease are detected, the cause must be identified and appropriate treatment implemented.
Towards the end of the farming season, the weight of crabs in the pond increases, and increased feeding leads to environmental pollution. Therefore, regular water changes and environmental monitoring are very important. In some cases, the bottom of the pond accumulates a lot of leftover, rotting food, requiring draining, removing the crabs, and cleaning the pond bottom: scraping away the surface mud and rotting leftover food.
Harvest:
Test the crabs to check their quality. Good quality crabs should weigh 250g or more per crab. They should be meaty or full of roe (female crabs). Once the crabs meet the standards and fetch a good price, harvest them for sale.
Crabs that haven't reached the required size and weight, or those that are thin or not fully roe-filled, if they are still healthy, can be raised in small ponds and intensively nurtured. After a period of time, they will reach the harvesting standard and fetch a higher price.
Raising commercial crabs from juvenile crabs takes 3-8 months, and while the mortality rate is relatively high (40-60%), the crabs' weight increases 3-4 times (from 60-80g/crab to 250-350g/crab). The total weight of marketable crabs increases 1.5-2 times the total weight of the juvenile crabs.
Some points to note for successful crab farming.:
- The crablings must be of the same size and released at the same time;
- There must be sufficient clean water for regular replacement;
- There must be an adequate supply of fresh food;
- There must be a barrier or fence on the edge of the pond;
- The pond must have mounds of brushwood to provide shelter for the crabs.
According to khuyennongkiengiang - LY
