Daniel Sousa, Christopher Small

Recent decades have seen a considerable increase in shrimp aquaculture in tropical and subtropical developing countries. This practice is controversial due to its potential for serious economic, food security, and environmental impacts. Of particular concern is the widespread practice of intensive aquaculture in and near mangrove ecosystems where many shrimp species spawn. However, while considerable effort has been directed toward understanding the effects of aquaculture, consistent and objective estimates of its past and present spatial extent are lacking. This is in part because of the challenge of distinguishing aquaculture ponds (ghers) from other types of standing water using satellite imagery. In this work, we use visible and infrared satellite imagery to map past and present extent of standing water areas on the lower Ganges-Brahmaputra Delta (GBD), one of the largest and most expansive areas of recent aquaculture growth on Earth. Spatial extent and expansion of aquaculture on the lower GBD are of particular interest because of the immediate proximity to the Sundarbans mangrove forest. Using MODIS 16-day EVI composites, we characterize the dominant spatiotemporal patterns in vegetation phenology of the area and identify consistent seasonal differences in visible and infrared reflectance between flooded ghers and other land uses. Using this information, we then quantify the expansion of standing water bodies on the GBD over the 45 year span of the Landsat image archive, 1972 – 2017. Considerable non-uniqueness exists in the spectral signature of ghers on the GBD, resulting in uncertainty in estimates of spatial extent. We use a spectral mixture model to estimate extent of standing water bodies with three progressive decision boundaries to explicitly show this uncertainty and provide liberal, moderate, and conservative estimates of flooded gher extent on three different spatial scales. Using this approach of multiple extents and multiple thresholds, we quantify the size distribution of contiguous regions of flooded gher extent at ten year intervals. Applying a moderate threshold, we find the area of standing water bodies within the administrative boundaries of the Bangladeshi polders to have expanded considerably from less than 300 km2 in 1990 to over 1400 km2 in 2015. At all three scales investigated, the size distribution of standing water bodies is increasingly dominated by larger, more interconnected networks of flooded areas associated with aquaculture. Much of this expansion has occurred in areas directly adjacent to the Bangladeshi Sundarbans.