Mapping Aquaculture Ponds for the Coastal Zone of Asia with Sentinel-1 and Sentinel-2 Time Series

Aquaculture in Asia has undergone significant expansion, driven by rising fish demand and overfishing. Coastal pond systems, as crucial protein sources, are prevalent in low-lying coastal areas of South, Southeast, and East Asia and vital to regional food security and economies. Employing Earth observation time series data, this study monitored pond aquaculture development in Asia and reveals dramatic changes in the spatiotemporal distribution of these systems. Covering 22 countries, we focused on a 300,000 km coastline, applying a 200-km inland and seaward buffer, resulting in 261 coastal parcels for efficient satellite data processing. Using dense optical satellite imagery, the evolution of coastal pond aquaculture across Asia was analyzed at continental scale from the 1980s to 2019. An automatic multi-sensor approach combining Sentinel-1 Ground Range Detected (GRD) and Sentinel-2 Surface Reflectance (SR) images was used to generate a pond reference dataset of 3.6 million pond objects with an accuracy of 91.9%. Based on this reference pond object dataset, mean annual water masks were derived from the entire Landsat archive (1984–2019), providing multispectral data with 30-meter resolution. These water masks enabled long-term monitoring of the reference pond objects to determine the yearly aquaculture status (active/inactive) for each reference pond identified in 2019. The use of water indices (NDWI, MNDWI, AWEI, and WIFI) and receiver operating characteristic (ROC) curve analysis across eight test sites ensured precise water detection, with the WIFI indices performing best. Results indicate a dramatic expansion of pond aquaculture, with active pond areas increasing from 6,500 km² in 1988 to over 19,000 km² in 2019 - a more than threefold increase. China leads with 40% of the total pond area, followed by Indonesia (13%), India (11%), Vietnam (7.7%), and Thailand (7.2%). Comparisons with FAO aquaculture production data demonstrate correlations between reference pond area expansion and official reported aquaculture production volumes. However, deviations suggest that additional factors, such as changes environmental conditions, market dynamics, diseases, policy shifts, and economic factors, influence production trends beyond spatial expansion alone. These findings reveal the rapid transformation of Asia’s coastal aquaculture and emphasize the need for ongoing monitoring to ensure sustainable development. By integrating geospatial data and long-term satellite observations, this research provides essential insights for policymakers and resource managers, supporting strategies to balance economic growth with environmental conservation and reduce the ecological footprint of expanding pond aquaculture in the coastal zone.