Ecological features of upriver migration in Kitakami River chum salmon and their connection to aerobic thermal performance.

The physiological performance of ectotherms is influenced by temperature, raising concerns about the impact of global warming on ectotherms. Understanding the relationship between ecologically relevant temperatures and the physiological performance of ectotherms provides a basis for assessing their resilience to changing environments. Absolute aerobic scope (AAS) is a functional metric of the thermal performance of aquatic ectotherms. The thermal profile of chum salmon (Oncorhynchus keta) returning to the Kitakami River, Japan, in early October has already been explored in a previous study; however, the ecological aspects of their upriver migration (e.g. spawning site, migratory duration and experienced temperature) and their connection to AAS thermal profiles are not fully understood. To address this gap, we released 53 marked chum salmon throughout the spawning season (October-November), of which 48 were tracked using radio telemetry. Over 3 years, 18 were successfully tracked to their spawning sites, and 13 were tracked partway. The longest track was 93 km. The spawning sites of Kitakami River chum salmon depended on migration timing, with earlier run salmon tending towards upriver sites. Chum salmon returning in October spawned in the middle basin, typically requiring >5 days to reach the spawning sites, whereas those returning in November spawned in the lower sections in 1-3 days. Comparing the estimated thermal occupancy of migrating salmon with the published AAS profile, we found that Kitakami River chum salmon in early October spent almost all of their time within the optimal temperature window for AAS and tended to be below the peak temperature of AAS. Our findings provide a basis for the ecological features of migrating chum salmon in rivers and shed light on their aerobic thermal performance in the natural environment.