ビワマスの形態ならびに生理・生態に関する研究 Ｉ ビワマスのパー・スモルト変態

Abstract

Parr-smolt transformation, growth rate, maturity and appearance or disappearance of red spots on the body were observed in juvenile biwa salmon Oncorhynchus rhodurus indigenous to Lake Biwa, reared in pond for eighteen months, and compared with those of amago salmon reared under the same conditions. Biwa salmon transformed from juvenile parr to silvery smolt in underyearling June to July (standard length 7.5-8.0cm), and 60-80% remained as smolt till yearling autumn, Smolts of amago salmon appeared mainly in underyearling September to yearling March, and all of them transformed back to parr or silvery parr by yearling May. Most of the smolt of biwa salmon, unlike those of amago and masu salmon, did not show complete silvering of body color or intense blackening at outer extremities of dosal fin. Growth after smolt transformation in biwa salmon was slower than that of amago salmon, though there was no significant difference between the juveniles of biwa and amago salmon in underyearling April to May. Almost all of the amago salmon matured in yearling autumn, wherease only a part of the biwa salmon males matured at the same time. Red spots appeared on body of almost all of the amago salmon, and the number and density increased with growth. On the contrary, those of biwa salmon decreased during smolt transformation, though a few red spots existed at parr stage. Thus biwa salmon differed from amago salmon in several characteristics during smoltification.