Long-term decline in egg size of Japanese hatchery chum salmon links to declines in return rate and abundance

Variation in life-history traits contributes to fitness and population productivity in salmonids. To investigate long-term changes in egg size and body size of Japanese hatchery chum salmon and their links to declines in return rate and abundance, I analyzed egg size and female fork length (FL) of approximately 21,000 age-4 and age-5 fish from 13 hatchery-enhanced rivers in Japan during 1999-2019. River-specific generalized additive models (GAMs) identified synchronous declines in egg size across most rivers. Hierarchical GAMs adjusting for female FL, age, and river effects revealed a nationwide decline in egg size, whereas adjusted FL showed pronounced interannual variation without a long-term decline. Mean age at return also declined after approximately 2010 because of shifts in age composition. Standardized exponential decay rates were remarkably similar for egg size, return rate, and chum salmon abundance, suggesting common underlying environmental and evolutionary processes. These findings suggest that long-term declines in maternal investment may have contributed to declining return rates and abundance, underscoring the importance of life-history traits in evaluating the long-term consequences of hatchery enhancement programs.