Tracking functional shifts in fishpond wetlands of Hong Kong's Deep Bay and their implications for conservation planning

Fishponds are important components of coastal wetland landscapes, supporting aquaculture, biodiversity conservation, and multiple ecosystem services. A key challenge in wetland management is to identify long-term ecological trajectories indicating functional transitions in managed fishpond landscapes. These transitions often evolve gradually through interacting shifts in hydrological condition, vegetation dynamics, and land use practices. This challenge is especially evident in Hong Kong's Deep Bay Area (DBA), where fishponds are subject to both changing management policies and increasing development pressure. To characterize these transitions, this study developed an integrated framework for deriving and assessing long-term ecological indicators of fishpond functional changes from 1986 to 2020. For this, we used annual Landsat composites and time-series analysis of the Modified Normalized Difference Water Index (MNDWI) and Normalized Difference Vegetation Index (NDVI). A multi-threshold classification framework was used to characterize hydrological and vegetation conditions, and temporal breakpoints were identified using the Breaks for Additive Season and Trend (BFAST) approach to distinguish stable, transitional, and functionally shifted fishponds. Mapping accuracy was evaluated using high-resolution WorldView-3 imagery and field-based reference data. The results showed that nearly half (51.4%) of the fishponds experienced functional shift, abandonment, or conversion, whereas only 48.6% retained relatively stable hydrological and ecological characteristics. These changes were accompanied by increasing vegetation cover and localized wetland succession including mangrove expansion along parts of the coastal fringe. The results demonstrate that long-term water and vegetation trajectories can serve as effective ecological indicators of fishpond functional change and wetland transition. This provides evidence to support adaptive conservation and land-use strategies for multifunctional wetland landscapes in Deep Bay and comparable coastal environments.