Pacific salmon (Oncorhynchus spp.) are exposed to environmental and anthropogenic stresses due to their wide geographic distribution, long migrations, and complex life histories. Understanding how these stressors interact to affect population abundance is essential for conservation and sustainable management of salmon. We investigate the causal effects on population dynamics of hatchery-enhanced Japanese chum salmon (O. keta), focusing on the period of sharp decline since the early 2000s and the rebound in 2022. We used 50 years of fishery and hatchery release data and 40 years of high-resolution SST datasets in 30 coastal areas in Japan, as well as monthly mean SST data along the Japanese chum salmon migration route in the high seas. We provide initial evidence that SST at release and that in the migration route, size at release, northward expansion of a piscivorous fish, and intraspecific competition influence the abundance of Japanese chum salmon. The sudden increase in 2022 was driven by an increase in Age-4 fish. The age structure shift towards younger age at maturity was observed in all areas except the Honshu Pacific and was particularly pronounced in the Hokkaido Sea of Japan and the Sea of Okhotsk and, where winter SST in the North Pacific and the Gulf of Alaska had a positive effect on return rates. Size at release and Russian chum salmon also had a positive effect. On the other hand, SST at release and yellowtail (Seriola quinqueradiata), as well as summer SST in the Sea of Okhotsk had a negative effect on return rate. Our regression model predicted the return rate well, while explaining 43-75% of the variation in regional populations. Our results highlight winter SST as a driver of younger age at maturity but the trade-off between returned fish and body size.