The rising demand of ecosystem services, due to the increasing human population in coastal areas, and the subsequent need to secure healthy and sustainable seas constitute a major challenge for marine ecosystems management. In addition, global anthropogenic changes have transformed the marine realm, thereby challenging ecosystem health and the services necessary for human welfare. These changes have opened ecological space for opportunistic organisms, such as jellyfish, resulting in ecosystem-wide and economic implications that threaten marine ecosystem services. Here, we used a comprehensive dataset of jellyfish hazards over the period 1960-2019 to track their dynamics and implications for human welfare. Our results revealed that their large-scale patterns have been mainly enhanced in human-perturbed Large Marine Ecosystems, although the contribution of jellyfish Class to hazard type changed across ocean regions. The long-term variability of these events suggests that their temporal patterns mirror the pace of ocean warming and ocean health degradation nurtured by global anthropogenic changes in recent decades. These results warn of the wide socioecological risks of jellyfish hazards, and their implications advocate for transboundary, regional cooperation to develop effective ecosystem-based management actions. Failure to integrate jellyfish into ocean surveys will compromise coastal ecosystem services governance. Classification: Social Sciences/Sustainability Science, Biological Sciences/Ecology.

The East Asian marginal seas are among the most productive fisheries grounds. However, in recent decades they experienced massive proliferations of jellyfish that pose vast challenges for the management of harvested fish stocks. In the Korean Peninsula, the common bloom-formers Scyphozoan species Aurelia coerulea and Nemopilema nomurai are of major concern due to their detrimental effects on coastal socio-ecological systems. Here, we used pluriannual field observations spanning over 14 years to test the extent of climate influence on the interannual variability and bloom dynamics of A. coerulea and N. nomurai. To depict climate-jellyfish interactions we assessed partitioning effects, direct/indirect links, and the relative importance of hydroclimate forces on the variability of these species. We show that jellyfish interannual patterns and bloom dynamics are shaped by forces playing out at disparate scales. While abundance changes and earlier blooms of A. coerulea were driven by local environmental conditions, N. nomurai interannual patterns and bloom dynamics were linked with regional climate processes. Our results provide a synoptic picture of cascading effects from large scale climate to jellyfish dynamics in the Korean Peninsula that may affect fisheries sustainability due to the prominent detrimental impact these species have in the region.

Temperate fisheries grounds are exposed to compound effects of jellyfish proliferations and fishing pressure, which affect local fisheries, cause economic losses, and threaten seafood supply. Here, we quantify the interlink between climate variability and jellyfish blooms and their impact on the Japanese anchovy (Engraulis japonicus), in the Korean coastal waters. We used a bioclimate dataset (2010–2019) that includes quantitative information of two major bloom-former species, Aurelia coerulea and Nemopilema nomurai, in the Korean Peninsula. We show that climate phenomena governing East Asia regions explain circa half of jellyfish variability. In turn, jellyfish blooms have a significant negative effect on anchovy interannual changes (r = -0.47, P < 0.01), which varies along with the bloom magnitude. Our results indicate that the intensity of jellyfish blooms, more than their duration, has a predominant effect on anchovy and coastal fisheries production. We also suggest the possibility of using climate signals for assessing and eventually predicting, interannual abundance changes of jellyfish in the Korean Peninsula. These results stress the challenge posed by jellyfish blooms to the provisioning of ecosystem services via their influence on marine harvested fish and further highlight the need for their integration into ecosystem-based management.

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 ∼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 × 10³ metric tons in 2010-2013 to 85.01 × 10³ 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.