Shellfish Breeding and Hatchery Management (2+1)

22.3.3 Larval rearing & collection of juveniles

Rearing of the post larvae
Preparation of the rearing tanks
All rearing tanks used in breeding, especially new tanks must be scrubbed clean and filled with seawater for 20 days. During this period the water in the tanks is repeatedly changed. Before the tanks are used, they are then scrubbed and filled with seawater containing 40 ppm bleaching powder and then washed clean with filtered seawater.
Rearing density
Strict control over the rearing density of the larvae (i.e. the number of larvae per ml of water) is maintained. At present there are two methods used to rear the larvae: still water rearing and flowing water rearing. Auricularia larvae during their early and middle stages concentrate at the surface of the water. If the density of the larvae is high, they will form agglomerations and sink to the bottom of the tank resulting in their death. Rearing density, therefore, should be controlled to ensure better survival rates. The desirable density of auricularia is 300-700 per litre. In a one tonne tank filled with 750 litres of water, 0.3 million auriculariae can be reared.
Selecting and counting of larvae
After the embryos are transferred to rearing tanks, they develop into auricularia larvae in about 30 hours. Healthy larvae occupy the surface layer of the water while deformed larvae and dead embryos are found in the lower layer of the water column or on the bottom of the tank. A sample can be removed for counting the larvae. Samples can be taken separately from the two ends and the middle of the tank using 250 ml beakers. The sample is stirred and a 1 ml aliquot is taken with a pipette and placed on a plankton counting chamber to assess the number of larvae. Two more samples are taken and the average of three counts is taken as an indication of the density of the larvae. When the auricularia larvae are in the early stage of development, they should be reared at a density of about 500 larvae per litre. The development of the auricularia can be divided into three stages: early, middle and late.
Water management
During their development, the larvae eject faeces and consume dissolved oxygen constantly. Some of the larvae will die. These, together with excess food, will produce harmful substances such as hydrogen sulphide and ammonia. In addition, bacteria reproduce rapidly with the rise of temperature. Poor water quality directly affects the normal development of the larvae. Therefore proper water management and sanitation is essential, including regular cleaning of the tanks and frequent changing of water. Sediment and deformed larvae at the bottom of the tank have to be siphoned out preferably on a daily basis.
During water changes a sieve (80 µm mesh size) is used to prevent loss of eggs and/or larvae. While the water is being changed, it is advisable to constantly stir the water lightly around the tank. This will prevent damage to the larvae during the water change as without stirring the larvae would be forced into the sieve causing mechanical injury.
Larval feeding and feeding rates
High quality microalgae and proper feeding schedules are key factors in the successful rearing of sea cucumber larvae. Early auriculariae possess a well formed alimentary tract and must be fed. Ingestion by these larvae consists of conveying the suspended particles of food into the alimentary canal through the mouth parts by the swaying of peristomial cilia. The effectiveness of Isochrysis galbana, Dunaliella salina, Dicrateria spp. and mixed diets consisting of all the above mentioned microalgae was tested. The best growth rates and lower mortality were observed when larvae were first fed I. galbana and supplemented with mixed cultures, chiefly consisting of Chaetoceros spp., four or five days later. Unicellular algae were given twice a day but the amount given depended on the particular stage of the larvae. In general, a concentration of 20 000 to 30 000 cells per ml was maintained in the rearing tank. The amount of food given should be increased or decreased depending on the abundance of food observed in the stomach of the larvae. This can be visually assessed everyday before feeding.