Dynamic distribution of Nemopilema nomurai in inshore waters of the northern Liaodong Bay,Bohai Sea

Since the late 20th century,giant jellyfish blooms have occurred in successive years in the northern part of the East China Sea and the Yellow Sea and resulted in ecological disasters in these areas.The main genera in bloom were Nemopilema and Cyanea spp.with N.nomurai as the dominant species.Fishery resources decreased sharply and normal fishing was seriously affected in the sea area where giant jellyfish bloomed.Massive aggregations of N.nomurai had occurred in Liaodong Bay,part of the Bohai Sea since the end of the 20th century in successive years.These blooms had serious impacts on the structure of fishery resources of Liaodong Bay.The dynamic distribution of juvenile and adult N.nomurai and the dominant ecological type reported,juvenile medusae,were analyzed based on the sampling data obtained during the anchor drift net surveys in inshore waters within a 10m isobath of the northern Liaodong Bay,Bohai Sea from late May to mid to late July from 2008 to 2011 combined with sampling data from trawl fishery resource surveys within a 20m isobath of northern Liaodong Bay and fishing pier interview surveys in August every year.In our study,many N.nomurai juvenile medusae were found in the inshore waters of Liaodong Bay in June,concentrated around the 5m isobath in coastal estuaries of the Shuangtaizi River.The average catch density of N.nomurai was 52.8 ind·net-1·h-1 in early and mid June,and 46 ind·net-1·h-1 in late June.The highest catch density,667 ind·net-1·h-1,was found within the 5m isobath in coastal estuaries of the Shuangtaizi River.During July,the amount decreased markedly with the medusae increasing to adult size.The average catch density of N.nomurai was 5.1 ind·net-1·h-1 in early July and 1.8 ind·net-1·h-1 in mid to late July.After June the higher density area moved to deep waters.Few adult medusae of N.nomurai were found within the 5m isobath in inshore waters of northern Liaodong Bay in mid August,while a high-density area of adult medusae was found in the waters 10—20 meters in depth in northern Liaodong Bay.The sea surface temperatures and salinities in areas inhabited by N.nomurai in June were 17.7—27.3℃ and 24.3—31.9 Practical Salinity Unit,respectively.The sea surface temperatures and salinities of high density areas with juvenile medusae of N.nomurai in June in Liaodong Bay region were 20.4—24.4℃ and 24.7—31.6 PSU,respectively.Coastal estuaries of the northern Liaodong Bay were one of the possible nursery areas of N.nomurai.When the juvenile medusae of N.nomurai grew to adults,they swam into deep waters from shallow waters.In contrast to the adults,the relatively high temperatures and low salinity area of coastal estuaries was an optimum environment for growth of juvenile medusae of N.nomurai.