Volume 64 (1998) Issue 1 Pages 77-82
The effects of rearing period in seawater on induced maturation by hormonal treatments were examined in the cultured female Japanese eels Anguilla japonica. The oocytes of female eel (30 month old) kept in seawater for 3 months completed vitellogeneisis by significantly smaller number of injection of salmon pituitary extracts than those of female kept in seawater for one week. Fertility and hatching rates were not significantly different between the oocytes of the two experimental groups. Similar results were obtained in the experiment which was performed using 6-month older females. These results indicate that long term rearing in seawater increased responsiveness to salmon pituitary extract in ovarian maturation. Histological observation revealed that oocytes of females kept in seawater for 3 months became large and attained at the primary yolk globule stage, whereas oocytes of the females kept in seawater for one week were at the oil drop stage. Electron microscopic observation showed that gonadotrophs became active in the pituitary of females kept in seawater for 3 months. These results are the first findings showing that vitellogenesis is triggered under artificial rearing condition without any hormone treatments in cultured immature eels. These morphological results indicate that long term rearing in seawater induces vitellogneisis in the females, resulting in the shortened period for compeletion of vitellogeneisis by the injection of salmon pituitary extract.