Straightforward upriver migration to spawning sites by chum salmon Oncorhynchus keta homing to coastal short rivers in the Sanriku region

Abstract

In chum salmon (Oncorhynchus keta) homed to the Sanriku region, Japan, most of the fish are matured in bays and spawn near river mouths in coastal short rivers; therefore, their upriver migration is extremely short, but their behavioural characteristics have remained unknown. Upriver migration in the Otsuchi River, a typical coastal river, was evaluated from behavioural and physiological aspects. Homing salmon tracked in Otsuchi Bay held in the inner bay for less than 1 day to more than 10 days before river entry. The varied holding duration was negatively correlated with plasma 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) concentration, an indicator of maturation. After river entry, however, most fish were captured in weirs near the river mouths within 2 days regardless of the DHP concentration. Of the 34 fish released in the river, on the contrary, eighteen and five fish were seen next day in the main spawning sites located at c. 1.5 km upstream and in the branch creek, respectively, and 85% of the fish held position there until their death. The mean survival time of released fish was 5.8 days. Plasma DHP level suggested that preparations for spawning were already completed at the timing of the release. Taken together, homing salmon completed spawning preparation in the bay, and then they moved to their spawning sites immediately after river entry and spawned there during their short remaining life. This upriver migration contrasts with those of other populations, such as early migrants and long river migrants, whose maturation is completed during upriver migration.

Behavioral patterns of Chum salmon (Oncorhynchus keta) during spawning migration across the coastal water-river continuum in Northeastern Korea

“…The biological rhythms of Chum can vary depending on the timing of their arrival on the coast, potentially in uencing the timing and speed of their migration from the coast to rivers 44,45 . Studies in Japan, Alaska, and other areas have investigated changes in salmon migration patterns between coastal and river environments based on variations in biological rhythms using methods such as EMG telemetry and blood samples 46,47 . It has been reported that migration patterns vary depending on hormone levels and maturity 47,48 .…”

“…Studies in Japan, Alaska, and other areas have investigated changes in salmon migration patterns between coastal and river environments based on variations in biological rhythms using methods such as EMG telemetry and blood samples 46,47 . It has been reported that migration patterns vary depending on hormone levels and maturity 47,48 . During the spawning migration period, salmon with immature gonads may stay in relatively warm coastal areas for an extended period to regulate the timing of spawning before slowly migrating to rivers, while well-matured Chum demonstrate a strategy of quickly moving to colder rivers as spawning approaches 47 .…”

“…It has been reported that migration patterns vary depending on hormone levels and maturity 47,48 . During the spawning migration period, salmon with immature gonads may stay in relatively warm coastal areas for an extended period to regulate the timing of spawning before slowly migrating to rivers, while well-matured Chum demonstrate a strategy of quickly moving to colder rivers as spawning approaches 47 . However, since such studies were not conducted in this research, there are limitations in understanding the in uence of seasonal factors such as biological rhythms on Chum migration patterns along the NECWs.…”