Abstract

The East Sea (Sea of Japan) is one of the world’s most rapidly warming marginal seas, a sensitive setting in which to examine how cetacean communities are structured by, and respond to, ocean temperature. Using visual sighting records from line-transect surveys off the Korean east coast (2015–2024) and analyses designed to be robust to heterogeneous survey effort rather than to estimate abundance, we matched 177 sightings of six species to satellite sea-surface temperature (SST) and tested whether the species form distinct thermal guilds. Cold-water species (Dall’s porpoise and Pacific white-sided dolphin) occurred in water averaging 11.6 °C and warm-water species (common, Risso’s, and bottlenose dolphins and false killer whale) in water averaging 17.8 °C—a 6.2 °C separation that was highly significant, very large (Cohen’s d = 1.84), and independent of location, defining a temperature-structured community. Over the same period, spring regional mean SST rose about 2 °C (0.22 °C yr−1). Strikingly, the cold-water guild was absent during the spring surveys in 2022 and 2024: it was absent in the two warmest-spring years despite the highest survey effort and full spatial coverage, its encounter rate fell with spring SST (ρ = −0.78), and—unlike the warm-water guild, which persisted unchanged—its loss was guild-specific. National bycatch statistics and local knowledge independently corroborate this decline. A sharply temperature-structured community, rapid warming, and the guild-specific loss of cold-water species together indicate that climate-driven reorganization of this assemblage is already underway, underscoring the need for sustained, all-season monitoring.

Keywords