Abstract
Pen aquaculture is the main form of aquaculture in some shallow lakes in eastern China. It is valuable to map the spatiotemporal changes of pen aquaculture in eutrophic lakes to assess its effect on water quality, thereby helping the relevant decision-making agencies to manage the water quality (WQ) of lakes. In this study, an automatic approach for extracting the pen aquaculture area was developed based on Landsat data. The approach integrates five algorithms, including grey transformation, discrete wavelet transform, fast Fourier transform, singular value decomposition and k-nearest neighbor classification. It was successfully applied in the automatic mapping of the pen aquaculture areas in Lake Yangcheng from 1990 to 2016. The overall accuracies were greater than 92%. The result indicted that the practice of pen aquaculture experienced five stages, with the general area increasing in the beginning and decreasing by the end of the last stage. Meanwhile, the changes of nine WQ parameters observed from 2000 to 2016, such as ammonia (NH3-N), pH, total nitrogen (TN), total phosphorus (TP), chlorophyll a, biochemical oxygen demand (BOD), chemiluminescence detection of permanganate index (CODMn), Secchi disk depth (SDD) and dissolved oxygen (DO), were analyzed in the lake sectors of Lake Yangcheng, and then their relationships were explored with the percentage of pen aquaculture area. The result suggested that the percentage of pen aquaculture area exhibits significantly positive correlations with NH3-N, TN, TP, chlorophyll a, BOD and CODMn, but significantly negative correlations with SDD and DO. The experimental results may offer an important implication for managing similar shallow lakes with pen aquaculture expansion and water pollution problems.
Highlights
China is the largest inland fishery producer in the world with a proportion of 54.5% of the global inland fishery production in 2012 (FAO, 2014)
The rapid expansion of pen aquaculture led to a serious water pollution problem and became one of the major driving factors of eutrophication in most of the shallow lakes in eastern China [3,5,6,7,8,9,10]
The nutrient levels in the lake were much higher than the surface water quality criteria of Class III (total nitrogen (TN) of 1.0 mg/L; total phosphorus (TP) of 0.05 mg/L) for a drinking water resource, according to the Environmental Quality Standards for Surface Water (GB 3838-2002) (MEPPRC 2002). It has aroused a major concern from the Ministry of Agriculture and Environmental Protection of China and local governments, and they made some legislations to restrict the pen culture area and urged farmers to remove a certain amount of pen aquaculture in order to improve the water quality and protect the water from further deterioration and eutrophication
Summary
China is the largest inland fishery producer in the world with a proportion of 54.5% of the global inland fishery production in 2012 (FAO, 2014). The nutrient levels in the lake were much higher than the surface water quality criteria of Class III (total nitrogen (TN) of 1.0 mg/L; total phosphorus (TP) of 0.05 mg/L) for a drinking water resource, according to the Environmental Quality Standards for Surface Water (GB 3838-2002) (MEPPRC 2002) It has aroused a major concern from the Ministry of Agriculture and Environmental Protection of China and local governments, and they made some legislations to restrict the pen culture area and urged farmers to remove a certain amount of pen aquaculture in order to improve the water quality and protect the water from further deterioration and eutrophication. Developing an approach that is able to automatically determine thresholds and extract pen facility information, using long time-series remote sensing images, is important to map the lake area used for pen aquaculture and to monitor its spatiotemporal change over a large area. The crabs are fed with artificial pelleted feeds supplemented with vegetable feeds (e.g., corn, wheat and pachyrhizus) and feeds of animal origin (e.g., trash fish) [11]
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