Body size and egg size trends in Japanese hatchery chum salmon: Links to declines in marine survival and abundance

Japanese chum salmon (Oncorhynchus keta) populations supported by one of the world largest hatchery programs have experienced severe declines in marine survival and abundance. To evaluate long-term changes in reproductive traits, we analyzed temporal and geographic variation in female fork length (FL) and egg size from 13 hatchery-enhanced rivers in Japan during 1999-2019. Females from northern rivers tended to have smaller FL but produced larger eggs, indicating persistent latitudinal differentiation in reproductive traits. Despite this geographic structure, both FL and egg size declined synchronously among rivers over time. Using nationwide annual means, exponential decay analyses showed that egg size declined by approximately 2% per generation, whereas marine survival and population abundance declined by approximately 22% per generation. Mean egg size was positively correlated with subsequent marine survival, explaining 45% of the variation in return rates. Mean female FL explained 64% of the variation in mean egg size. Exponential decay rates estimated from standardized variables were nearly identical among egg size, marine survival, and chum salmon abundance, suggesting common underlying drivers of long-term decline.