Acute Salinity Challenges in <i>Oncorhynchus masou</i> Under Warm Temperature: Differential Responses of Osmolality, Plasma Ions, Hormone Levels, and Immune Parameters

Being a key ecological and economic fish species, migratory salmon typically hatch in freshwater rivers, migrate to the ocean for maturation, and return to their natal streams to spawn. This life cycle necessitates physiological adjustments to manage fluctuating salinity levels, particularly in estuaries—critical transitional zones for smoltification. Concurrently, climate warming may directly impact the metabolic efficiency and growth rates of migrating salmon. Understanding the adaptive demands of salmon is essential, as the ability of anadromous fish to cope with salinity and thermal variability determines their resilience in a rapidly changing environment. This study employed masu salmon ( Oncorhynchus masou ), a nationally protected endangered salmonid species with anadromous and resident phenotypes during their life history, as an object of study to evaluate the combined effects of acute salinity stress (0, 15, and 30 ppt) and elevated temperature (10°C vs. 14°C) on osmoregulation, ion concentration, hormone levels, and immune regulations. The results revealed that exposure to seawater adversely affected the physiological and immune parameters of masu salmon, which was exacerbated under high‐temperature conditions. Furthermore, anadromous individuals exhibited greater sensitivity to rapid changes in salinity compared to resident individuals. Our findings highlight the impact of climate warming on water salinity during masu salmon migration and provide evidence of the distinct physiological and immune strategies employed by anadromous and resident fish in response to the combined influences of temperature and salinity.