This ecoregion has 470 non-indigenous (NIS) and cryptogenic (obscure or of unknown origin) species recorded between 1950 and 2022. The annual discovery rate has steadily increased since the early 1990s (Figure 7).
The main vector for primary introductions is shipping, mostly through ballast water and hull fouling, accounting for 53% of NIS introductions. Transport of NIS as contaminants and parasites on animals (primarily associated with aquaculture) is responsible for 18% of introductions while for 12% of cases the introduction pathway remains unknown.
While the importance of ballast water in new NIS detections has increased over time, that of ship fouling and aquaculture (contaminants/parasites) has been more variable during the past two decades without a clear trend (potentially confounded by uneven search effort). Importantly, the rate of detection of new NIS with unknown pathways is higher during the past decade than before (Figure 8).
The observed ecological impacts include significant reductions in the abundance of several important native species and changes to the physical and chemical composition of both sediments and the water column. Additional effects include out‑competition of native commercial species, fouling of aquaculture and fishing gear, and fish kills through toxin production. Some of the examples include the ctenophore Mnemiopsis leidyi, an apex predator, which has been shown to induce community cascade effects in the pelagic foodweb in a fjord in the Skagerrak area. This was evidenced by a five-fold reduction in their target prey, grazing copepods, and doubled biomass of primary producers released from the grazing pressure.
