THE ERADICATION OF CRABS IN MACROBRACHIUM PONDS
Chacheongsao Fisheries Station
Freshwater Fisheries Division, Department of Fisheries
Ministry of Agriculture and Cooperatives
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THE ERADICATION OF CRABS IN MACROBRACHIUM PONDS1
Land crabs are predators of giant freshwater prawns (Macrobrachium rosenbergii) both directly. By ‘directly’, I mean that crab will catch prawns for food and by ‘indirectly’ that crab will complete for the food of the prawns and moreover, occupy space in the ponds. The disadvantage of having the holes that land crabs make in the rearing ponds, apart from causing loss of water, is that it endangers prawns since crab's holes can trap prawns when they use them as hiding dwelling places. It has been observed that when a prawn wants to get into a crab's hole it will do so by moving backwards. The owner of the hole will wait quietly and patiently until the intruder comes in and the former will squeeze the prey with its pincers and retain it for consumption later.
From past experience, it has been found that in some location, although the rearing pond was well prepared, a large number of land crabs was still living in the pond after the juvenile prawns had been introduced. In particular, during the rainy season, the number of land crabs multiplies rapidly due to the richness of food. This is one of the crucial problems in giant freshwater prawn farming. An additional problem is that there are few measures which will eradicate land crabs without endangering other aquatic animals. For this reason, chemical treatment cannot be used. Some inefficient methods are being used, for example using tools, such as shovels, for digging land crabs out of their holes, or using hooks to drag them out. The holes are later destroyed. However, in doing this, the person involved is often attacked by the crab's pincers or bitten by a snake. Additionally, the task has to be carried out during the night, the operator being guided by a flashlight and equipped only with a spear as a weapon to stab the land crabs. Moreover, the turbidity of the pond, its depth, and the weediness of the area along the edges of the ponds often results in a poor rate of success.
1 Translated from an article in the Thai Fisheries Gazette, 1979, 32: 403–406
2 Fisheries Biologist, Chacheongsao Fisheries Station, Department of Fisheries, Thailand.
If one really takes a good look at a land crab, one can see it has thoracic legs designed only for walking, which is different from the blue crab (Scylla serrata) and the horse crab (Portanus sp.) whose fifth pair of thoracic legs are wide and flat similar to the shape of a paddle. These paddle shaped legs are used for swimming. The ends of the land crab's legs are sharp which is the reason why they cannot be used for swimming. However, they are good for walking on the bottom of the pond and for climbing on aquatic plants or tree branches and other materials which are below the water surface. Land crabs cannot climb over an object with a hard and smooth surface which is at all tall or perpendicular. Using this fact, rice farmers in the olden days used to eradicate land crabs, which destroyed paddy sprouts ready for transplanting in the paddy-field, by burying earthen jars under the surface which acted as traps for which they could not escape. These earthen jars contained food (bait) which attached land crabs by its strong smell. When the land crabs fell into the earth jars, the farmers could get rid of them easily (but today's farmers are rarely aware of this method of eradication of land crab). Experiments were therefore carried out to see if this technique could be used in freshwater prawn culture to eradicate this predater. The following notes contain the results of this work.
The earthen jar should have a diameter of more than 25 cm and height of 40 cm. The mouth of the jar should be approximately 12 cm wide. The reason for using such a big jar is so that there is enough space for several prawns, crabs and fish, which will probably love to live in there. If the jar is too small and too shallow, crabs will be able to climb out and, at the same time, prawns and fish will not be able to swim in and out at their convenience thus facilitating the ability of the crabs in catching them for food. The vitally important characteristic of the earthen jar is its surface. It must be smooth in order to prevent the land crab from climbing up easily. In burying the earthen jar, the ideal place chosen should be located along the edges of pond. The mouth of the jar should be located at about 20–30 cm under the level of the water surface and approximately 2–3 cm above ground level to prevent earth from falling into it.
The bait used for trapping land crabs should consist of any kind of food with a strong smell, for example shrimp paste, baked rice bran, baked coconut flesh, etc., to spread the odour of food to the greatest distance so that the crabs will come to the trap quickly. This means that the eradication of crab from the pond will be undertaken rapidly. Before introducing the bait into the jar, the food should be mixed with mud and formed it into a ball with a diameter of about 4 cm. There are two reasons for mixing food with mud:
to prevent food from dissolving in water too quickly and to help to maintain the smell of the food for a longer time;
to prevent prawns, crabs and fish from taking food out of the jar.
The freshwater prawn rearing pond used for this experiment a surface area of two rais (0.32 ha) and was located at Tambon Nhong Bua, Amphur Ban Poh, Chacheongsao Province. Fifteen thousand juvenile prawns were stocked in the rearing pond.
In the evening, after burying the jars in the shallow sides of the pond, leaving their mouths at 2–3 cm above the ground as mentioned above, they were filled with prepared bait. In the following evening, a large number of land crabs was found in the jars with high mortality rate. After the land crabs were caught the leftover particles of food and other debris was removed by hand without disturbing the jars. The jars were then baited again. (If the number of land crabs is large, the catching of crabs should be undertaken twice; in the morning and again in the evening, re-baiting on each occasion). The task of catching land crabs and changing food in our pond had to be done everyday until it was found out that there were no more crabs left; then there was no need to bait them anymore. What is still necessary is to clean the jars every 7–15 days. It is recommended that the jars should be left in site till the end of the rearing period. The surviving land crabs or newcomers to the pond will be trapped. Crabs which die in the jars and not removed will float to the surface when they go rotten. After harvest when the water in our pond was pumped out it was found that no land crabs had survived.
From this experiment it has been found that once the jars had been buried they not only eradicated existing land crabs but prevented further predation from this source once and for all. This was achieved if, during the preparation of the rearing pond (before or after introducing juvenile prawns), jars were buried along the edge of the pond in the water inlet and out areas or in other areas at intervals of 30–50 m. If done at this stage no bait seems necessary. Cleaning of the jars by hand shifting out water to get rid of earth and sediments which fell in was however necessary once or twice a month.
The following recommendations are based on our experience in this and subsequent work:-
In order to get rid of land crabs efficiently, baited earthen jars should be buried in prawn rearing ponds. From the experiment conducted, a two rai pond was cleared of land crabs all through the rearing period.
If the earthen jars are buried after the preparation of the rearing pond or soon after juveniles are stocked, crabs can be eradicated completely even if there is no bait to attract them.
Apart from burying earthen jars in the ponds, other containers having high sides and with mouths which are smaller or equal in diameter to the sides and having a smooth and hard interior surface can also be placed in position. For example, plastic pails, kerosene cans, metal cans, etc., can be utilised.
This technique for eradication of land crabs can be applied to other types of aquaculture and for eradicating land crabs in paddy fields without chemical treatment.
This method of eradication for land crabs wherever it is used, does no harm to either plants nor animals in the nearby area and does not pollute the environment in any way.
The burying of earthen jars or other containers to get rid of land crabs can be applied to catch other kinds of crabs whose legs are used for walking rather than swimming. However, the technique must be adapted to make it appropriate to the habits of each category of crabs. For example, to trap ghost crabs one has to be aware that this habit is to emerge and search for food on the beach during the night. If the fisherman wants to catch them he should be equipped with kerosene or metal cans such as are normally used for holding water. He should then bury them in the sand on the beach at night and apply shrimp paste to their inside walls. Two to three hours later he will come back to find a large quantity of ghost crab filling the cans.