*2.1. Study Area*

The Guangdong–Hong Kong–Macao Greater Bay Area (GBA) is located in southcentral China, composed of the "9 + 2 urban agglomeration" which is composed of two

special administrative regions of Hong Kong and Macau and 9 cities from central Guangdong Province (namely, Guangzhou, Shenzhen, Zhuhai, Foshan, Huizhou, Dongguan, Zhongshan, Jiangmen and Zhaoqing). It ranges between 24.4◦~21.5◦ N, 111.4◦~115.4◦ E, with a total area of about 56,000 km<sup>2</sup> (Figure 1). The population of GBA reached approximately 70 million in 2017, with USD 1.51 trillion GDP and urbanization rate of 85.20% excluding Hong Kong and Macau. In some of the cities such as Shenzhen and Foshan, the urbanization even exceeded 90% [12]. The GBA is currently one of the strongest economic vitality regions in China. Not only that, but it is also one of the most typical areas of fish pond adoption in China. However, with the fast industrialization and urbanization in the GBA, fish ponds have been seriously degraded in past decades [13–15]. In recent years, with the further acceleration of the transition from rural population to urban population, the built-up area in the GBA has grown at an annual rate of 3.35% from 2000 to 2015. All high-density populated areas (>10 inhabitants/900 m2) are located in built-up areas, and the average population density in rural areas has also decreased at a rate of 1% per year [16]. Accompanied by this phenomenon, decreasing farmers in the GBA would be a potential threat for fish pond management. Thus, a comprehensive analysis of the long-term dynamics of fish ponds and the impacts of urbanization on their changes is indispensable in this context.

**Figure 1.** Administrative divisions of the Guangdong–Hong Kong–Macao Greater Bay Area (GBA). The study area covered the entire GBA. Because there are only a few fish ponds in Macau SAR, they were excluded in following investigations.

### *2.2. Data Collection and Image Analysis*
