Sneaking behavior of mature male parr directed at pair of Lake-run form Biwa salmon

Abstract

Sneaking behavior directed towards a pair of Lake-run form Biwa salmon (ca. 50 cm TL) by a mature male parr (ca. 10cm TL), observed in the Oura River, a small inlet stream of Lake Biwa, central Japan on 25 October 2020, resulted in successful sperm release by the latter. If the parr individual was a Biwa salmon, the observation is important for improved understanding of spawning in the latter, as such sneaking has not been previously recorded with any certainty. On the other hand, if the parr was a stocked Amago salmon individual, it likely represented an unregulated stock, the release of such possibly promoting genetic introgression into the Lake-run form. A third possibility, that the sneaking parr was a native Amago salmon, would support the hypothesis that outbreeding occurs between native Amago salmon and Biwa salmon in the Lake Biwa water system.