Abstract: Chlorophyll a (Chl a) concentration, a key indicator of phytoplankton biomass, has become a variable in global marine ecosystem research. However, in the context of global climate change, the responses of Chl a concentration to changes in the physical environment across different fishing grounds remain unclear. In this study, the variations in sea surface Chl a concentrations over major global oceanic fishing grounds from 1998 to 2018 were systematically analyzed, aiming to quantify the response pattern of Chl a concentration to sea surface temperature (SST), and climate indices. Eight important oceanic fishing grounds in the Pacific, Atlantic, and Indian Oceans were selected as the study regions. Using a multi-sensor fused, high spatiotemporal continuity monthly satellite remote sensing dataset of global sea surface Chl a concentration, combined with the Seasonal-Trend decomposition procedure based on LOESS (STL), quantification of trends and time-series characteristics, and a mediation effect model, the associations between Chl a concentration and SST, as well as climate indices were quantified. Results show that significant regional differences in the fluctuation range, standard deviation, trend slope, seasonal strength, trend strength, dominant periods, and coefficient of variation of Chl a concentration exist among those oceanic fishing grounds. From the perspective of latitudinal distribution, high-latitude oceanic fishing grounds (e.g., fishing ground in the northwestern Pacific and the southwestern Atlantic Oceans) generally present higher and more variable Chl a concentrations, while mid- to low-latitude (near-equatorial) fishing grounds show lower Chl a concentrations and higher significant correlations with climate indices, which may be associated with climate variability dominated by ENSO events in equatorial regions. From the perspective of the hemispheric distribution, Chl a concentrations in most northern hemisphere fishing grounds (e.g., the northwestern Pacific, western Pacific, eastern Pacific, and central Atlantic Oceans) tend to decrease, while in some southern hemisphere regions (e.g., the southwestern Atlantic and western Indian Oceans), it tends to increase. Specially in the southeastern Pacific Ocean Chl a concentration shows a positive but extremely weak correlation with no significant, and in the eastern Indian Ocean, it represents a decreasing trend. Furthermore, mediation analysis indicates that 4 climate indices, namely the Ocean Nino Index (ONI), the Indian Ocean Dipole (IOD), the North Atlantic Oscillation (NAO), and the Pacific Interdecadal Oscillation (PDO), had significant effects on Chl a concentrations through SST in fishing grounds located in the northwestern and western Pacific, the central Atlantic and the eastern Indian Oceans. Both correlation analysis and mediation analysis suggest that marine ecosystems (as indicated by Chl a concentration) respond to climate change in a complex manner with pronounced regional heterogeneity. Consequently, future climate change will affect global oceanic fishing grounds differently, necessitating tailored adaptive management strategies for each region.