Annual variations in the occurrence of the scyphomedusa Aurelia aurita population in Uwa Sea, western Shikoku, and their predation impact on mesozooplankton

In Uwa Sea, western Shikoku, the scyphomedusa Aurelia aurita occurs in the plankton from January to October, and forms dense aggregations in inlet waters in July onward. The population biomass of A. aurita in the Uwa Sea was estimated by means of composite airphotographs and groundtruth net surveys in 2001-2003. The biomass showed marked geographical variations. The annual maximum biomass ranged from 5.02 x 10^3 ton in 2003 to 7.86 x 10^3 ton in 2002. The gastric pouch contents revealed that the medusae ate mesozooplankton, mainly copepods. Under average medusa density (i.e. 0.56 indiv. m^<-2> or 42.2 g wet weight m^<-2>), the daily predation impact was estimated to be 1.1% of mesozooplankton biomass or 12.4% of mesozooplankton production rate