Nearly year-round reproduction among individuals of a Japanese freshwater threespine stickleback population

The freshwater threespine stickleback (Gasterosteus aculeatus) shows tremendous interpopulation variation in a variety of phenotypic traits, including reproductive characteristics. We investigated individual-level reproductive patterns in the freshwater Ono population in Japan, which occurs nearly at the southern global range limit for this species. Although G. aculeatus populations generally only reproduce in spring and/or summer, we directly observed tagged individuals in semi-natural enclosures and found that both males and females reproduced almost the year long. We observed two peaks in reproductive activity, spring to early summer and autumn to early winter, at both the population and individual levels. Thus, we found unique features in the timing and duration of reproduction of this freshwater population, which inhabits spring-fed areas with constant year-round water temperatures.