Invasive species have been known to be transported to other countries on board ships or in ballast. In a new study, researchers have concluded that they can also travel across oceans by riding on ocean currents as well.
The first comprehensive inventory of Mnemiopsis leidyi in European waters has been published in the journal Global Ecology and Biogeography, and this brings to light evidence that we have been underestimating the role of ocean currents as pathways for invasive jellyfish and other drifting organisms in the seas.
"To explain the invasion of alien species in marine ecosystems, everybody is focused on transport in or on ships. That is important, but does not explain the whole phenomenon,” said lead author Dr Cornelia Jaspers, Biological Oceanographer at the GEOMAR Helmholtz Centre for Ocean Research Kiel and at the Technical University of Denmark in Lyngby.
As part of their research, the team co-related all reliable data on the occurrence of the invasive American comb jelly in European waters since 1990 with the prevailing currents in European waters, taking into consideration their flow directions, strength and stability.
“The models showed that the steady flow pattern of the southern North Sea closely links the region with much of northwestern Europe, including the Norwegian coast and even the Baltic Sea,” according to the Institute's press release.
This connection means that non-native animal species, like the jellyfish, can be spread over long distances in just a short period of time. Those that reach ports in the southwestern North Sea can quickly find their way to Norway and the Baltic Sea.
"Using the imported sea walnut as example, we were able to show that these species can travel up to 2,000 kilometres within three months," said Hans-Harald Hinrichsen, physical oceanographer at GEOMAR.
In fact, after a very cold winter season in early 2010, this jellyfish species had vanished from the Baltic Sea and large areas of northwestern Europe from 2011 to 2013. However, a new population established itself after the warm winter of 2013/14.
Dr Jaspers said that “this new population was of another genotype than the first invaders. Thus, within a short time, a new immigration took place, driven by the prevailing ocean currents."
"The study shows that a single gateway, a single port for example, in which ships with invasive species arrive, is enough, to redistribute non-native species across entire regions,” she added.