Spreading and reproduction limitations of the American comb jelly Mnemiopsis leidyi in the Baltic Sea

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Based on the information on salinity and temperature needed for survival and reproduction, areas within the Baltic Sea have been identified where the American comb jelly (Mnemiopsis leidyi) could potentially survive and reproduce. In addition, physical drift modelling has been used to investigate the potential dispersion of Mnemiopsis, with the assumption that the invasion occurred via advection from the Kattegat through the Danish Straits into the western Baltic Sea. In the low saline surface layer, the dispersion is mainly driven by the wind; in the lower saline layer, the dispersion is controlled by the baroclinic flowfield and bottom topography. Model runs showed that natural spreading via deep-water currents from the Bornholm Basin where it currently occurs, towards north and east, is limited by topographic obstacles and low advection velocities. However, if Mnemiopsis were transported in the ballast water of a ship to the northern latitudes it might survive but would not reproduce at a sufficient rate to establish a new population. Thus, as a result of the combined effects of low salinity and low temperature it is not probable that Mnemiopsis could establish permanent populations in the central or northern Baltic Sea. It seems that Mnemiopsis could only reproduce at higher rate in the southern parts of the sea. If under climate change the salinity in the Baltic Sea further decreases, the establishment of Mnemiopsis would be even less probable in the north although water temperature would increase.