Quantitative Environmental DNA Reveals Temperature‐Driven Spawning Phenology in a Genetically Distinct Peripheral Population of Pacific Cod ( Gadus macrocephalus )

Understanding spawning phenology in peripheral populations is essential for effective conservation under climate change. Here, quantitative environmental DNA (eDNA) monitoring was conducted to characterize Pacific cod ( Gadus macrocephalus ) spawning migration in Jinhaeman Bay, Korea—the southernmost spawning ground of this species—over 3 years (2019–2022). Species‐specific qPCR assays revealed that eDNA concentrations were completely absent in November, sharply increased in December, and peaked in December–January, with the estimated median spawning date consistently occurring in the December–January period across all 3 years. Linear mixed‐effects model analysis confirmed temperature as the primary environmental driver of spawning migration ( β = −0.330, p = 0.004), with segmented regression identifying 10.3°C as the critical thermal threshold for spawning initiation. Spatial analysis revealed that deep‐water stations (15–24 m) accounted for 95%–97% of bay‐wide eDNA, indicating concentrated spawning activity at these depths. Commercial fishery catches peaked approximately 1 month before peak eDNA detection within the bay, demonstrating temporal mismatch between peak spawning activity (December–January) and fishing effort (November–December). Because this genetically distinct peripheral population occurs at its thermal range limit, projected winter warming of 1.5°C–2.5°C by mid‐century could elevate temperatures above the 10.3°C threshold, potentially disrupting spawning phenology. These findings support extending seasonal closures from November through February and prioritizing deep‐water habitat protection. Overall, this study demonstrates that temperature‐responsive eDNA monitoring provides a cost‐effective early‐warning framework for detecting climate‐driven phenological shifts in coastal fish populations occupying marginal thermal habitats.