Hierarchical organization of life‐history variation in a salmonid fish across riverscape: Relevance of seasonal growth opportunity and maturation decision window

Understanding life-history variation maintained across landscapes is crucial for revealing the spatial scale at which adaptation and population stability manifest. Previous studies have often focused on population mean life history and its differences between habitats, overlooking variation within local habitats and its relative contribution to life-history variation at the landscape scale. By integrating a year-round survey of spatial heterogeneity in stream temperature and prey resource dynamics with a life-history survey for a masu salmon Oncorhynchus masou masou inhabiting a large watershed, we examined here how relative phenologies of seasonal growth and the maturation decision window have potentially shaped within-habitat and between-habitat variation in life history across the watershed. We found that fast-life individuals tended to dominate downstream reaches, characterized by warmer temperature, abundant aquatic invertebrate prey, and fewer terrestrial invertebrate inputs. On the other hand, slow-life individuals were more prevalent in upstream reaches, characterized by colder temperature, fewer aquatic invertebrate prey, but relatively abundant terrestrial invertebrate subsidies. However, there was considerable variation in the age of mature fish in all reaches except the most downstream and upstream reaches. The variation partitioning for the age of mature fish across the watershed revealed that the proportional contribution of within-habitat variation was unexpectedly higher (84% in females, 88% in males) than that of between-habitat variation in both sexes (16% in females, 12% in males). These results highlight the need to acknowledge the hierarchical structure of life-history variation, especially within-habitat variation, to fully understand the spatial scales at which the life-history variation is maintained and its functions, such as population stabilization and adaptive evolution, are operated.