Latitudinal Differences in Age at First Maturity of the Largehead Hairtail ( <scp> <i>Trichiurus japonicus</i> </scp> ) Under Environmental Variability

ABSTRACT The largehead hairtail ( Trichiurus japonicus ), a key commercial species in China Seas, has undergone life‐history modifications including earlier maturation and decreased body size, attributed to prolonged overharvesting and environmental changes. However, spatial patterns of Age at first maturity (AFM) across its distribution range remain systematically uncharacterized. This study compared AFM between the Bohai Sea (temperate) and the East China Sea (subtropical) populations through otolith microstructure analysis and probabilistic maturation reaction norms (PMRNs). Daily age distributions showed no regional structural differences (independent t ‐test, p &gt; 0.05), enabling construction of a unified PMRN model. The model identified a baseline maturation threshold at 178 days and showed the presence of population‐specific deviations, with 50% maturity occurred at 176 days in Bohai specimens versus 181 days in East China Sea. Gradient forest analysis revealed sea bottom temperature (SBT) as the primary environmental factor influencing AFM difference. Comparative quantification demonstrated that specimens from warmer habitats exhibited systematically earlier maturation than those in cooler regions, with SBT variations accounting for the observed 5‐day interpopulation difference. The methodological integration of PMRNs with environmental covariates provides a replicable framework for predicting climate‐mediated shifts in fishery species phenology. Results advocate for regionally adaptive management protocols that account for temperature‐dependent maturation plasticity to stabilize recruitment in spatially heterogeneous marine environments.