Stowaway invaders

Cargo ships transport an estimated 10,000 marine species in ballast water worldwide. This study uses 22,550 GBIF-mediated occurrences to predict the spread of 18 non-indigenous species (NIS) along shipping routes and weigh the factors that might make potential habitat suitable for them in the Baltic Sea and Northeast Atlantic. The study finds that temperature and sea-ice concentration determine habitat suitability for 61 per cent of species, while salinity surprisingly accounts for only 11 per cent. This finding cuts against current practices for ships exchanging ballast water, where salinity remains the standard parameter for assessing NIS risk. The study also identifies potential NIS hotspots in shallow parts of the Baltic, where ships should avoid exchanging ballast water, pointing instead to the middle of the North Sea as a “cold spot” more suitable for ballast- water discharge with lower risk for spread of NIS.