Phenological shifts in plankton dynamics reshape pressures on anchovy recruitment in the southern coastal waters of the Korean peninsula

Abstract

The East Asian marginal seas support one of the world's highest levels of fish production and consumption, yet their coastal ecosystems are increasingly threatened by rapid warming and degradation driven by cumulative anthropogenic pressures, including habitat modification and overexploitation. Using a comprehensive dataset (2010-2020) from a major fishing ground in the southern coastal region of the Korean Peninsula, we investigate phenological changes in plankton communities across three statistically supported thermal regimes identified during the 2010s. Phytoplankton exhibited a progressive decline in the magnitude of the annual bloom, while the peak of the Nemopilema nomurai jellyfish bloom shifted earlier by similar to 1.5 months (from day of year [DOY] 268 to DOY 225), increasing temporal overlap with the anchovy (Engraulis japonicus) spawning season after 2014. During the 2010s, annual production of anchovy declined markedly from 113.7 & times; 10(3) metric tons in 2010-2013 to 85.01 & times; 10(3) metric tons in 2017-2020. Structural equation modeling revealed that these declines were associated with the combined effects of weakened phytoplankton productivity and earlier jellyfish blooms, suggesting interacting bottom-up (phytoplankton-zooplankton-anchovy) and top-down processes affecting anchovy early life stages. These patterns are consistent with a phenology-driven mechanism linking environmental variability to recruitment conditions, rather than a direct causal attribution. Our findings highlight the importance of phenological shifts in shaping forage fish productivity and underscore the need to incorporate jellyfish dynamics into ecosystem-based fisheries management in the region.