Tracking changes in aquaculture ponds on the China coast using 30 years of Landsat images

Although coastal aquaculture ponds provide high-quality fish protein for billions of people, they are threatened by urbanization, pollution, and climate change. Moreover, colossal aquaculture ponds pose damages such as natural wetland shrinkage and water quality deterioration. However, understanding the trends and drivers of coastal aquaculture ponds on a national scale remains a challenge. In this study, a decision-tree classifier was used to quantify the spatiotemporal distribution of aquaculture ponds over last 30 years in 12 provinces located on the coast of China. Moreover, we analyzed their trends and drivers, including geographical conditions, socioeconomic factors, and development policies. The key results of the study include the following: (1) from 1990 to 2020, the cumulative area and holding area of aquaculture ponds reached 21997.90 km2 and 9613.66 km2, representing 3.7-fold and 1.6-fold increase, respectively, than the 1990 values. Based on geographical conditions, most aquaculture ponds tend to be located in coastal plains and bays following the distribution of low-lying land. (2) Influenced by different levels of socioeconomic development and changed policies, the holding area of aquaculture ponds experienced a “rapidly increasing period” from 1990 to 2011, growing by 246 km2 per year; “stable period” between 2011 and 2017; and “sharply shrinking period” after 2017 declining by 417 km2/year. (3) Coastal land reclamation played a critical role in the expansion of aquaculture ponds by cumulatively contributing approximately 22% of the land resource in the past 30 years. In the future, as a result of increasing land competition, the extent of aquaculture ponds on the China coast tends to decrease continually.