Fish transport in ships’ ballast tanks seen from a NW European perspective

Abstract This study addresses the importance of ballast water (BW) transportation as a vector of fish transfer in NW Europe. BW and/or sediments from ships arriving for repairs at a shipyard in Poland were sampled for the presence of fishes. The results were compared with earlier data from ships calling at German ports, mainly Hamburg. They are historical, but also link to today, enabling data comparison over time. Sixteen fish taxa represented by 146 identified specimens, as well as not-identified fish larvae and eggs, were found in 13% of the studied ships of different types, mainly in container and general cargo ships sampled in Germany. Although only two species non-indigenous to the sampling area were detected (i.e. Allosmerus cf. elongatus and the hybrid Cottus perifretum × C. rhenanus), both surveys demonstrated the ability of fish to be transported in ballast tanks. The first evidence for the occurrence of freshwater fishes in BW, representing cyprinids, cottids, and percids—including the most numerous ruffes (Gymnocephalus cernua), the presence of which supports the suggested BW-mediated introduction of this species to the Laurentian Great Lakes, is provided. Pelagic and benthopelagic species were noted primarily in BW, while typically benthic fish were sampled both from BW and sediments. The literature on fish found in BW or suspected to have been transported in BW has been complemented by the present work. Our data, combined with data from other European BW studies, identified Gobiidae, Osmeridae, Clupeidae, and Gasterosteidae as the most common families associated with ships’ BW.