Environmental effects of the distribution of fishery resources in Wailingding marine ranching, Zhuhai

【Objective】 The aim of this study was to explore the environmental effects of the distribution of fishery resources in the Wailingding marine ranch in Zhuhai, which could provide theoretical basis for the protection, proliferation and scientific management of coastal fishery resources in the South China Sea. 【Method】 Ten survey sites were set up on Wailingding Island, and marine ecological environment and fishery resources surveys were carried out in spring（April） and autumn（September） of 2020. Temperature, salinity and pH were measured on site using thermometers, salinity meters and pH meters respectively, and dissolved oxygen concentration was measured on site using iodine titration. Dissolved inorganic nitrogen concentration was determined by naphthylethylenediamine spectrophotometry and cadmium column reduction method, phosphate concentration was determined by phosphomolybdenum blue spectrophotometry, and chlorophyll a concentration was determined by spectrophotometry. Phytoplankton density was obtained by quantitative counting using the whole-piece counting method and converted by water volume, and zooplankton density was expressed as individual numbers. The fishery resource survey was conducted using a bottom trawl single trawler. Variance inflation factor（VIF） was used to test the collinearity of environmental factors, and variables with high collinearity were screened out. Generalized additive model（GAM） analysis and model evaluation were performed to analyze the relationship between the spatiotemporal distribution of fishery resources in Wailingding marine ranching and the marine environment. 【Result】 The collinearity test results showed that the VIF of salinity, temperature and dissolved oxygen concentration were all ＞10, indicating serious collinearity. Finally, chlorophyll a concentration, dissolved oxygen concentration, depth, pH, dissolved inorganic nitrogen concentration, phosphate concentration, phytoplankton density and zooplankton density were retained to construct the GAM model. The results of influencing factor screening showed that dissolved oxygen concentration was the most important factor affecting the catch per unit fishing effort（CPUE）, with a contribution rate of 25.3%, followed by chlorophyll a concentration（24.5%） and zooplankton density（16.3%）. The contribution rate of depth to CPUE was the lowest, at 6.3%. The results of GAM analysis showed that the fishery resources of the Wailingding marine ranching in Zhuhai were mainly distributed in the sea areas with dissolved oxygen concentration of 6.700-8.100 mg/L, chlorophyll a concentration of 0.500-1.200 mg/m³, zooplankton density of 230.000-450.000 pcs/m³, and depth of 6.500-10.000 m. CPUE showed a trend of decreasing first and then increasing with the increase of dissolved oxygen concentration, increasing with the increase of chlorophyll a concentration, increasing first and then tending to be gentle with the increase of zooplankton density, and decreasing with the increase of depth. The spatial distribution of fishery resources in the Wailingding marine ranching in Zhuhai showed a certain regularity in spring and autumn, mainly concentrated in the eastern and southern waters of the islands and reefs, and the CPUE in autumn was higher than that in spring. In spring, fishery resources were mainly distributed in the southern and eastern waters near the coast of Wailingding Island. In autumn, the fishery resources were more concentrated in the eastern coastal waters, and the CPUE level in this area was relatively high. 【Conclusion】 The fishery resources of the Wailingding marine ranching in Zhuhai are regulated by various environmental factors. Dissolved oxygen concentration is the most critical driving factor, followed by chlorophyll a concentration, zooplankton density and depth. The fishery resources of the Wailingding marine ranching in Zhuhai are higher in autumn than in spring, and are mainly concentrated in the eastern and southern waters of the islands and reefs. There is a seasonal trend in fishery resources, and the summer fishing moratorium system has a positive effect on the ecological restoration of the ranching.