• Energy Research

Pacific salmon (Oncorhynchus spp., Salmonidae) are exposed to environmental and anthropogenic stresses due to their wide geographic distribution and complex life history. We investigated the causal effects on return rates of Japanese chum salmon (O. keta, Salmonidae), focusing on the period of sharp decline since the early 2000s, using more than 40 years of fishery and hatchery release data and high-resolution sea surface temperature (SST) datasets along the Japanese coasts and SST along the Japanese chum salmon migration route. We examined the geographic distribution of chum salmon allele frequencies using published isoenzyme datasets and a single nucleotide polymorphism (SNP) dataset collected from the range, focusing on lactate dehydrogenase (LDH), growth hormone, gonadotropin-releasing hormone and mitochondrial DNA loci. The effect of SST increases or decreases varied depending on the life history stage of the chum salmon. Winter SST in overwintering areas had a positive effect on return rates, whereas summer SST in coastal areas had a negative effect. The spike in 2022 was driven by an increase in age 4 fish, and the shift in age structure towards younger age at maturity occurred in all areas, being consistent with recent studies throughout Alaska. Allele frequencies varied with geographical patterns, with those in Japan significantly different from the overall trend, and those involved in metabolism and growth were close to Alaskan populations. We hypothesise that relaxed selection in hatcheries has altered allele frequencies, resulting in better growth in the first and second winters, thereby promoting a younger age at maturity.

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.