Abstract
The East Sea is warming at two-to-four times the global ocean rate over 2000–2025, and the once-dominant fishery for the ommastrephid squid Todarodes pacificus around Korea has contracted by more than 90%. Whether warming contributed to that decline cannot be settled from the catch record: catch and catch per unit effort (CPUE) confound abundance with fishing effort, and the strong raw correlation between sea-surface temperature and CPUE rests on a shared time trend that vanishes under detrending, so the fishery record motivates the question without being able to answer it. We therefore move observation to the recruitment stage, combining a 26-year reconstruction of suitable thermal spawning habitat (2000–2025) with a fishery-independent paralarval survey at the spawning ground (2018–2026). Two findings are robust. First, sea-surface temperature rose by 0.32–0.52 °C decade − ¹ across the spawning domain, with warming through the full 0- to 100-m water column, and the suitable thermal habitat shifted poleward (autumn centroid ≈ 27 km decade − ¹; 17–27 km decade − ¹ across the definitions reported here) while its area was maintained—a redistribution of suitable spawning temperature rather than a net loss. Second, paralarvae occupy a warm 50-m thermal niche that matches the published embryo-survival range and remains significant after controlling for latitude, so larvae track suitable-temperature water irrespective of latitude, including at the poleward stations; yet, larval occupancy, after a 2020–2021 peak, fell to a series low in 2025–2026 that persists within the suitable-temperature niche itself. Together, these describe a decoupling in time—a maintained thermal template that was not converted into realized occupancy—but on seven to eight survey years, we advance this as a hypothesis to be tested rather than a demonstrated mechanism: the occupancy decline is a fall to a series low, not a significant trend, and the habitat–occupancy association does not survive multiple-testing correction. The contribution is a fishery-independent recruitment-stage baseline for this boom–bust squid—a defined thermal niche, a reconstructed habitat geometry, and an occupancy series—at which a climate–recruitment relationship can be tested as survey years accumulate.