Although chum salmon (Oncorhynchus keta, Salmonidae) biomass in the North Pacific is at a historical maximum, the number of individuals returning to Japan, the location of the world’s largest chum salmon hatchery program, has declined substantially over two decades. To search for potential causes of this decline in the context of evolutionary history, we synthesized catch/release and sea surface temperature (SST) statistics, and published genetic data, namely, 53 single nucleotide polymorphisms (SNPs) from 114 locations, lactate dehydrogenase (LDH) and tripeptide aminopeptidase (PEPB) isozymes from 81 locations, and mitochondrial DNA (mtDNA) D-loop haplotypes from 48 locations, in the distribution range (n = 27,170). SST in the summer, when juveniles inhabit Japanese coasts, was found to be negatively correlated with adult return rates 4 years later (r = −0.62). According to SNP-based population-specific FST values, the species may have originated in western Alaska and expanded its distribution southward. At the southern edge of the Asian distribution, the population structure of chum salmon has been rearranged by hatchery operations. Key functions of nine genes characterizing the distinct nature of the Japanese populations are reproduction, regulation of circadian rhythm, bacteria/parasite defense, DNA damage repair, growth and energy metabolism. Thermally adapted, derived alleles of LDH-A1*100, predominantly expressed in skeletal muscle, have often been replaced by ancestral alleles, while the ancestral mtDNA-B3 haplotype is significantly rarer. This genetic replacement might result in lower metabolic efficiencies in skeletal muscle and mitochondria at higher temperatures, thereby leading to the survival decline of juveniles in a warming climate.