Dynamics of medusa abundance in the East China Sea
XU Zhao-Li **Key and Open Laboratory of Marine and Estuary Fisheries, Ministry of Agriculture of China, East China Sea Fisheries Research Institute, Chinese Academy of Fisheries Sciences, Shanghai 200090, China
Based on data from four seasonal oceanographic cruises in the East China Sea (23°30′-33°N,118°30′-128°E) in 1997-2000, distribution and the seasonal changes of medusa abundance and their dynamics are discussed in this paper using qualitative and quantitative methods such as stepwise regression. Results show that the medusa represent the third largest group, following after Copepoda and Thaliacea for its total abundance, in zooplankton of the East China Sea. Medusa abundance peaked in spring (5.37 ind./m3), and it was the second highest in autumn (2.23 ind./m3), then in winter (1.82 ind./m3) and the lowest in summer (0.88 ind./m3) with the distribution of abundance in nearshore being larger than offshore. Medusa abundance was due to the contribution of two dominant species, Muggiaea atlantica and Diphyes chamissonis, as these neritic species occur in conditions of low salinity. It was found that the medusa abundance in spring showed a linear correlation with surface and bottom salinity. Salinity is the main dynamical factor affecting the medusa abundance, while water temperature acts as a subordinate factor. The area of highest abundance was located at the edge of the coastal waters, in the area of convergence of the Taiwan Warm Current and Changjiang Diluted Waters. Medusa play an important role in the ecosystem and fisheries in nearshore regions of the East China Sea, including seasonal fluctuations in Zooplankton abundance.