Occurrence of summer upstream migration in Biwa salmon (Oncorhynchus masou subsp.)

Abstract

Salmon (Oncorhynchus masou) of uncertain subspecific identity migrating upstream in inflowing rivers around Lake Biwa much earlier than the main autumn migration of Biwa salmon (O. m. subsp.) raised the question of whether or not summer migrating Biwa salmon individuals might be mixed with the very similar redspotted masu salmon (O. m. ishikawae). Accordingly, 17 individuals of summer migrating salmon were captured from inflowing streams of Lake Biwa from May to July in 1998 and 1999, and their morphological characters and mitochondrial DNA analyzed. Twelve individuals were found to be Biwa salmon and three to be redspotted masu salmon, based on both morphological and DNA characters. The remaining two individuals could not be identified owing to discordant the morphological and molecular characters. The study confirmed the occurrence of summer upstream migration in Biwa salmon. Although the majority of Biwa salmon migrate upstream just before the breeding season in autumn, the possibility exists that summer migrating individuals might contribute to the genetic diversity of the Biwa salmon population.