Induced spawning of the Japanese eel

Catadromous female eels were administered once a week with commercial gonadotropins, DES-Na and Vitamin E for a total of six occasions, and then with the acetone-dried pituitary glands of fishes several times over a suitably short period. The administration of LH-RH analogue was also tried at the later stage of maturation. Catadromous male eels were injected with the commercial gonadotropins and V.E on more than 5 occasions. The matured female and male eels were then kept in the same tank (1.6×3.5×0.9m, water depth 0.6m) and the above-mentioned hormone treatments were continued. When they were kept at a water temperature of 18-20°C, spawning did not occur at all and even when fertilized eggs were obtained without any observable spawning behaviour the eggs did not hatch. But when the water temperature was raised to 21-22°C, spawning behaviour was observed at night and the spawning took place in the early morning. The fertilized eggs from this spawning hatched normally. The abdomen of the spawned eels swelled up once again over a considerably short duration in almost all cases, so it can be presumed that spawning can take place at least twice. It is presumed that the normal body weight of female eels ranged between 110-130% (about 120%) of that at the start of the experiment, just before spawning. Mature female eels with a body weight of more than 130% always gave rise to denatured eggs.