Synchronous changes of morphology and gonadal development of silvering Japanese eel Anguilla japonica

Abstract

The gonadal development of Japanese eel Anguilla japonica during silvering was examined via gonadal histology and morphometric characteristics. Specimens were collected from the Kaoping River of Taiwan between November 1998 and November 2001. Female eels predominated, constituting 87.6% of the sex-determined eels. The degree of gonadal development was assessed by skin coloration, with female eels divided into yellow, pre-silver, and silver phases. Males were divided into yellow and silver phases because of small sample size. Silver phase eels predominated in the winter. Mean (±S.E.) total length of silver-phase eels was significantly larger in females (642.2±10.4) than males (564.8±14.6) (p<0.001). The mean (±S.E.) gonadosomatic index (GSI) of females increased significantly from 0.27±0.01 for yellow phase, to 0.55±0.03 for pre-silver and 1.32±0.07 for silver phase. Oocyte development progressed from the chromatin nucleolus stage in the yellow phase eel, through the peri-nucleolus stage in the pre-silver phase eel and to the oil-drop stage in the silver phase eel. Spermatogenesis was active for silver phase males which had a larger mean GSI (0.15±0.01) than yellow phase males (0.07±0.02), but the gonadal development of males was slower than that of females. GSI, ocular index (OI), and fin-index (FI) were positively correlated, and gut-index (GI) was negatively correlated with total length in both sexes. Residual analysis of the regression of the indices GSI, OI, FI, and GI on TL indicated that the variation of the indices with growth was greatly influenced by the developmental phase of the eel. On the other hand, OI and FI were positively correlated and GI was negatively correlated with GSI, irrespective of eel phases. These results suggested that the skin coloration, OI, and FI were reliable criteria in selecting mature eel brooders for artificial propagation purposes.