**5. Conclusions**

In this study, we analyzed spatiotemporal changes in mulberry-dyke-fish ponds in the GBA using Landsat satellite images obtained from 1986 to 2019. combined the measurements of standard deviation, coefficient of variation, Theil coefficient, water body change index as well as expansion coefficient, the spatiotemporal changes were quantified.

From 1986 to 2019, the fish ponds in the GBA showed an overall increasing trend in the first stage and a sharp decreasing trend at the second stage. The year of 2013 was a milestone. A total 25 study units in the two periods before and after 2013 were studied. It was found that 18 units were transformed into shrinking trend. Additionally, the causes for the fish pond changes were analyzed, and the future development of fish ponds was also predicted. The results proved that human activities have continuously influenced the spatial distribution and size of fish ponds in the past 40 years. The fish ponds had transformed from a near-natural ponds with different sizes and a near-natural random distribution in the early stage into an artificial distribution and an artificial shape.

The increased social demands were the major causes for the steady growth in fish ponds in the GBA from 1986 to the beginning of the 21st century. The market price of aquatic products directly affected farmers' willingness to develop and maintain fish ponds. The policies reflected the local governments' attitudes toward the development of fish ponds. Urbanization was the main cause of shrinkage in fish ponds. The shrinkage of Shenzhen and Hong Kong before 2013 was due to the encroachment of urban expansion, while Foshan was due to the introduction of a large number of enterprises to promote industrial upgradation. After 2013, the policy for the development of metropolis in the GBA was given priority, and the development tended to be economic development and industrial upgradation. the outputs of aquacultural ponds are relatively low, thus, the shrinking of fish ponds will remain in future. However, increasing the economic outputs of fish ponds through deep excavation of the cultural value and construction of some new aquacultural demonstration areas can slow down the shrinking trend and enhance the social and cultural values of fish ponds.

**Author Contributions:** Investigation, Z.C., X.Y., W.Z. and J.Q. data preparation, Z.C. and W.Z. writing—original draft preparation, W.Z. and J.Q. writing—review and editing, X.Y., E.P., L.R. and X.X. supervision, X.Y. project administration, X.Y., L.R. and X.X. Funding acquisition, X.Y. and X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** The National Natural Science Foundation of China (Grant No.: 41871017 and 41476152) funded this research.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank the anonymous reviewers for their careful reading of the manuscript and their many insightful comments and suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.
