Mnemiopsis leidyi in the Baltic Sea – distribution and overwintering between autumn 2006 and spring 2007

Abstract

In autumn 2006 the first observations of the West Atlantic comb jelly Mnemiopsis leidyi in Northern Europe were reported from the North Sea, the Skagerrak and the south-western Baltic Sea. Here we report on the further spread of this invasive ctenophore from the south-western towards the central Baltic Sea up to the south eastern Gotland Basin during autumn/winter 2006 and spring 2007. The abundances were generally low (max. 4 ind. m -3 ). While M. leidyi was located in the entire water column in Kiel Bight, it was found exceptionally below the halocline in the deep stratified central Baltic basins. Data of a weekly sampling program at a near shore sampling location in Mecklenburg Bight between January and May 2007 showed that up to 80 % of the individuals were juveniles, smaller than 1 mm total body length and that M. leidyi survived the winter in the Southern Baltic Sea, even if abundances dropped down to <1 ind. m -3 in February. A first assessment of the physiological demands of this species versus the environmental conditions of the Baltic Sea showed that the successful establishment of this ctenophore is probable in the south-western and central Baltic Sea.