Long-term variation in body size of the Japanese flying squid (Todarodes pacificus): Seasonal, spatial, and thermal relationships

Body size is a fundamental ecological trait that integrates growth condition, size structure, and population response to environmental change. Using fishery-independent surveys conducted in the Yellow Sea and adjacent waters during 2000–2025, we quantified seasonal, interannual, and decadal variation in mantle length and body weight of the Japanese flying squid ( Todarodes pacificus ), and evaluated the roles of sea surface temperature (SST) and latitude. Mantle-length distributions differed significantly among seasons and among decadal groups (Holm-adjusted pairwise Kolmogorov–Smirnov tests), with a shift in the observed mantle-length distribution toward smaller individuals in the most recent period, particularly in summer and autumn. A linear model based on mean mantle length indicated a positive SST correlation and a significant SST × decade interaction, implying decade-specific SST–size relationships. Bayesian hierarchical models revealed the allometric exponent ( b ) was highest in the earliest decade and lower thereafter, indicating temporal variation in weight-at-length. Model comparison using leave-one-out cross-validation indicated that SST contributed to predictive performance of body-weight variation, while spatial predictions highlighted geographic heterogeneity, with higher predicted body weight concentrated in the southeastern part of the study area. Together, these results demonstrate that size metrics from long-term surveys provide sensitive indicators of changes in body-size and weight-at-length patterns of short-lived cephalopods, and underscore the importance of accounting for non-stationary environment-size relationships.