Temporal Variation in Growth and Mortality of Japanese Sardine based on Chinese survey in the Northwest Pacific

Japanese sardine is an important small pelagic species in the Northwest Pacific and a key resource within the NPFC Convention Area. This study estimated growth and natural mortality parameters using survey data collected by RV Songhang from 2021 to 2025. Fork length distributions showed clear interannual variation, with fish sampled in 2024-2025 generally smaller than those collected in previous years. Length-weight relationships also varied among years, while the mixed-effects model indicated a common allometric exponent close to isometric growth (b = 2.94). Growth parameters estimated from length-frequency data exhibited noticeable temporal variability, with pooled estimates of the von Bertalanffy growth function of L_∞ = 25.31 cm, k = 0.31 y⁻¹, and t₀ = −0.94 y. An age-length key (ALK) constructed from otolith readings identified three age groups (0+, 1+, and 2+), with age‑0 and age‑1 fish comprising the vast majority (50.9% and 43.9%, respectively) of the aged individuals. Natural mortality (M) estimated using eight empirical methods ranged from 0.23 to 1.41, and the annual geometric mean varied from 0.46 to 0.77, with pooled estimates at 0.63. These results highlight pronounced temporal variability in Japanese sardine life history, and we recommend that future stock assessments adopt our time‑varying, high‑seas survey‑based estimates for the stock, as they better reflect actual conditions in the Convention Area than fixed parameters.