Evidence of extraordinary marine species in the SW Baltic Sea

The presence of an extended salinity gradient of the Baltic Sea reveals a variety of species occur at the limit of their physiological tolerance and preference, i.e. in areas and habitats not representing their marine or fresh water origin. Hence, the Baltic Sea is known for its high share of non-indigenous species, which have established. In this study, we compiled extraordinary sightings of transient, non-native or potentially range expanding species, such as jellyfish, squid, fishes and marine mammals in the SW Baltic Sea for a period from 2001 to 2018. We focused on jellyfish, squid, fishes and marine mammals. Hydrographic conditions, such as water temperature and salinity, obtained from a high spatio-temporally resolved hydrodynamic Baltic Sea model, covering a daily resolved 40-year time series were linked to the sightings of these extraordinary species. Our hydrodynamic modelling results demonstarted that changes in the occurrence of exceptional species reflect the dynamics of water mass exchange between the Kattegat/Skagerrak and the SW Baltic Sea. Our analyses show that these changes could be related to the presence of anomalously high saline water masses. This documents that the hydrographically highly dynamic SW Baltic Sea needs special attention for monitoring of non-indigenous species, as high saline and warm water intrusions are more frequent than currently believed and ii) can be linked to sightings of exceptional species in the SW Baltic Sea.