Relationship between in-river return rate of hatchery-origin chum salmon Oncorhynchus keta and coastal water temperature and body weight at release on the Japan Sea side of Honshu, Japan

In Japan, chum salmon Oncorhynchus keta resources sustained by hatchery programs have been experiencing a decline in return rates since the 2000s, prompting the need to understand the underlying cause. On the Japan Sea side of Honshu, this study estimated the return rates of chum salmon using mass mark-recapture surveys. It examined the factors affecting return rates using statistical models. Groups of chum fry from 2011-2016 brood years tagged with otolith thermal marks were released at different release times (late January to late March) and sizes in two rivers. Their return rates into natal rivers as age 3-5 in 2014-2020 were estimated at 0-0.45%. The best model showed that the return rates increased with lower coastal water temperature at release (9.7-12.1 degrees C). The recent increase in coastal water temperature during spring, associated with global warming, may have diminished the return rate. Notably, the model indicated a positive correlation between return rates and larger release size (0.34-1.16 g), although these effects were weak in the river where relatively large fry were released. This suggests a threshold size of approximately 1 g that may improve return rates. Based on these results, release strategies to enhance the return rates of the two rivers are discussed.