Propagation of crabs in Japan
Seed stock of the Japanese blue crab Neptunuspelagicus is regularly produced in hatcheries forstocking open waters. As with other species of crabs, cannibalism has been a major problem in the grow-out of this species to adult size. The provision of various types of shelters on the pond bottom has been tried with some success in improving survival rates, but commercial culture has not yet become successful. The larval rearing of this species, as well as of Portunus trituberculatus, is carried out in hatcheries. Juveniles have been experimentally raised to adult size in eight to ten months using fresh fish and shellfish as food. But the main interest has been in growing them just to the juvenile stage for stocking the open sea. Berried females collected from the wild are kept in tanks filled with natural sea water of about 33–34 ppt salinity. The hatched early larvae are fed on cultured marine Chlorella, but later larval stages are fed on Artemia nauplii. In about 25 days after hatching the metamorphosis is complete. The juveniles are fed on fresh fish, such as anchovies, and after a further three weeks of rearing are stocked in coastal areas.
The king crab, Paralithodes camtschatica, has also been successfully spawned and the larvae reared in the laboratory. As in the case of other crabs, egg-bearing females are used for propa gation. The eggs hatch out in about a week at temperatures below 10°C. The zoeae are reared in nylon net cages in running water and fed on Artemia nauplii supplemented with shrimp juice, clams and the brown seaweed Laminaria. Net cages are comparatively efficient for rearing post-larvae, but the survival rate has been very low at this stage, probably due to cannibalism and the unsuitability of the food used.