Interannual changes of coastal aquaculture ponds in China at 10-m spatial resolution during 2016–2021

The rapid growth of coastal aquaculture has strongly supported global food security and economic development of coastal areas in the past few decades but has also caused remarkable impacts on coastal ecosystems. Accurate delineation of coastal aquaculture extent and changes is thus significant for improving coastal zone management toward sustainability. Here, by means of Sentinel-2 images, we propose an effective and rapid approach that combines simple non-iterative clustering (SNIC) with hierarchical decision trees (HDT), so-called SNIC-HDT, to generate fine-resolution coastal aquaculture pond data at the national scale of China. The SNIC-HDT fully considers the spatial heterogeneity of coastal aquaculture ponds and performs accurate segmentation by determining the optimal segmentation scale for coastal aquaculture ponds with different sizes and shapes. In addition, the SNIC-HDT incorporates temporal features into the establishment of HDT and thus effectively distinguishes coastal aquaculture ponds from other water bodies. In support of the Google Earth Engine platform, we employed 85,501 scenes of Sentinel-2 images and generated the first annual 10-m spatial resolution national coastal aquaculture pond dataset of China between 2016 and 2021 (China_CAP), with an overall classification accuracy of more than 90%. This dataset reveals that total area of China's coastal aquaculture ponds experienced a substantial loss of 13.21% from 9769 km2 in 2016 to 8629 km2 in 2021. The most remarkable areal reduction occurred in Zhejiang Province, with a decrease rate of 38.24%, followed by Guangdong (27.93%), and these reductions were mostly related to the policy of retuning aquaculture ponds to natural wetlands. Coastal aquaculture ponds in Fujian and Tianjin provinces experienced slight areal gains (7.24% and 2.13%). The results generated in this study could provide a basis for the conservation of habitats of migratory water birds on the East Asia-Australia Flyway and are of great scientific and practical importance to support the evaluation of Sustainable Development Goals.