Adaptability of Chub mackerel (Scomber japonicus) under El Niño and La Niña conditions: Body condition, feeding, and contaminant analysis

This study analyzes the effects of the El Niño and La Niña phenomena on the physiological ecology and mercury contamination levels of Chub mackerel, using samples collected from the Northwest Pacific region in 2021 (a La Niña year) and 2023 (an El Niño year). We analyzed body condition (Fulton's K index), trophic dynamics (δ¹³C, δ¹⁵N isotopes), and mercury levels in individuals across size classes (100-300 mm). The results indicate that, smaller mackerel (100-200 mm) exhibited significantly higher K values during El Niño (p < 0.05), whereas larger conspecifics (>250 mm) showed reduced condition. Sexual dimorphism in K (females > males) observed in 2021 disappeared under El Niño, suggesting climate-mediated shifts in energy allocation. Trophic niche breadth expanded during El Niño (6.88 vs. 4.12 SEAc), reflecting dietary diversification toward lower-trophic prey, as evidenced by depleted δ¹³C and δ¹⁵N values. Mercury concentrations surged in 2023, particularly in medium-to-large fish (151-300 mm, p < 0.01). This pattern likely reflects climate-driven intensification of mercury production under colder, oxygen-depleted conditions combined with trophic shifts toward larger, higher-trophic prey that biomagnify mercury more efficiently, collectively indicating that ENSO variability modulates both pollution bioavailability and contaminant transfer dynamics in pelagic food webs. These findings underscore the dual threats of climate-driven ecological disruption and pollution amplification in pelagic ecosystems. By elucidating size- and sex-specific vulnerabilities, this work provides actionable insights for adaptive fisheries management and highlights the urgent need to integrate climate and contaminant monitoring in marine conservation strategies.