Morphological, physiological and ecological studies on biwa salmon-IV. Seasonal changes hypoosmoregulatory ability of biwa salmon Oncorhynchus rhodurus and amago salmon O. rhodurus.

Abstract

Seasonal changes in hypoosmoregulatory ability of hatchery-reared biwa salmon Oncorhynchus rhodurus and amago salmon O. rhodurus were examined by following changes in plasma Na+ concentration after transfer to 70% seawater. The landlocked biwa salmon lose parr marks and become silvery bodied (smolt) underyearling during early summer. They showed the maximum hypoosmoregulatory ability from underyearling May to July, when they transformed from parr to smolt and migrated to the lake. However, the smolt did not show increased seawater adaptability as compared with parr in underyearling May, and the ability decreased markedly in autumn and winter. There was no essential difference in seawater adaptability between smolt and dark parr or yearling parr. On the other hand, amago salmon, which smoltify and go down stream to the sea in late autumn and early winter, showed better seawater adaptability than biwa salmon throughout all developmental stages examined. Especially, the smolts during the period from November to January showed good osmoregulatory ability. The poor hypoosmoregulatory ability of biwa salmon as compared with amago salmon may be related to their lacustrine nature and a long period passed since this species was landlocked in Lake Biwa.