Author(s): Hans Ulrik Riisgrd
The invasive ctenophore, Mnemiopsis leidyi, was recently observed for the first time in Danish waters where it was widely distributed during the summer of 2007. In certain areas, including Limfjorden, it exhibited mass occurrence. In this study, we assess the abundance of M. leidyi in Limfjorden in the late summer of 2007 and give a preliminary evaluation of the predation effects of this ctenophore. On two cruises in August and September, M. leidyi was found in every net sample from 9 locations in Limfjorden. The population densities were high, up to more than 800 individuals m-3 in the innermost part, but body lengths were small (5 to 15 mm). The bio-volumes were very high (100 and 300 ml m-3) in the central parts of Limfjorden and are even greater than those from the Black Sea, where the greatest mean bio-volume was about 184 ml m-3 in the autumn of 1989 when the zooplankton and fish stocks collapsed. The possible ecological consequences of the mass occurrence of this ctenophore are discussed.