Fish density increases from the upper to lower parts of the Pearl River Delta, China, and is influenced by tide, chlorophyll-a, water transparency, and water depth

Abstract

In order to assess the spatial distribution pattern and tidal difference of fish density estimated acoustically as well as identify influences of physiochemical variables on fish density, 12 acoustic surveys were carried out in the Pearl River Delta during November 2014 using the Simrad EY60 split-beam echo sounder. Results of these surveys did not reveal a consistent tidal pattern over the longitudinal extent of the Pearl River Delta. The sites near river mouth, Shenwan (28.6 km), Nansha (19.9 km), Yamen (9.7 km), and Nanshui (0 km), provided strong evidence for tidal differences in densities and acoustic size distributions with higher density and smaller fish being acoustically detected at flood tide. However, the flood tide and the ebb tide samplings gave a similar result in terms of mean density and size distributions in upstream sites Sanshui (139.0 km) and Jiujiang (93.1 km). Fishes were unevenly distributed in the Pearl River Delta. Average fish density in the investigated areas during November 2014 was 136.15 fish/1000 m3, with a minimum density of 5.89 fish/1000 m3 in Jiujiang at ebb tide and with a maximum density of 344.08 fish/1000 m3 in Yamen at flood tide. Overall fish density tended to increase from upstream to river mouth. Spatial distribution of fish was positively associated with chlorophyll-a and negatively associated with transparency, water depth, and distance to river mouth. This study contributes to the knowledge of the current state of fish resources and can be useful for proper resources conservation and effective ecosystem management in the Pearl River Delta.