Spawning strategy drives divergent stock trends of walleye pollock and Pacific cod under ocean warming in the...

Walleye pollock and Pacific cod are cold-water semi-demersal species with similar life histories; however, their long-term catch trends have diverged since the late 1980s along their southern distribution limits in Korea and Japan. This study examined how warming-driven water-column changes differentially affect these species with contrasting spawning strategies (pelagic eggs: pollock; demersal eggs: cod). Using data from 1982 to 2023, spawning-season water-column structure, early life-stage thermal habitat suitability index (HSI), and habitat suitable depth (HSD; HSI ≥ 0.6) were compared with catches across three regional groups (A: Korean East/Japan Sea coast; B: Japanese East/Japan Sea coast; C: Japanese Pacific coast), and lead-lag relationships were assessed by cross-correlation function (CCF) analysis of detrended time series. Surface temperatures increased in all groups under Kuroshio-origin warm currents (A: East Korea Warm Current; B: Tsushima Warm Current; C: Kuroshio Extension), whereas bottom thermal conditions differed depending on regional cold-water masses. Pollock catches declined sharply in all groups after the late 1980s without recovery, even where surface HSI remained suitable. In Group A, pollock HSI declined or remained low in sub-regions, whereas suitable cod habitat persisted and catches increased. In parts of Groups B and C, cod catches declined after the early 2010s; HSD deepened in Group B and was negatively associated with catch five years later in B-1 (r = -0.72, p < 0.001), whereas HSD-catch relationships varied among Group C sub-regions. Contrasting spawning strategies therefore expose early life stages to different water-column layers, whereas catch responses depend on regional oceanographic conditions.