First insights into marine invasions along the Namibian coast

Background: Alien species are a growing concern due to harmful impacts on the environment and ecosystem services. However, relatively little information exists about alien marine species in Namibia. Objectives: To compile the first list of marine alien and cryptogenic species for Namibia. Methods: A systematic search of the primary literature was combined with searches of historic taxonomic literature, the World Register of Introduced Marine Alien Species (WRiMS), the Global Biodiversity Information Facility (GBIF), the Ocean Biodiversity Information Systems (OBIS), iNaturalist and the grey literature, to identify alien and cryptogenic species that have been reported from Namibia. Results: A total of 26 alien and 12 cryptogenic species have been documented along the Namibian coast. Most species have a Northern Hemisphere native range and more than half were first recorded pre-2000. The primary likely mechanism of introduction has been shipping [i.e., hull fouling (63% of species), ballast water (20%) and ship boring (7%)] but mariculture (10%) has also played a role. The Ikaras Region supports the most alien (23) and cryptogenic (11) species. This likely reflects the presence of the busy Robert Harbour in Lüderitz, the proximity to the South African west coast, which is known to support >60 alien species and the fact that this region is the most accessible and well studied. Conclusion: This study provides first insights into marine invasions in Namibia, but dedicated surveys for alien taxa will improve our understanding of the full extent of invasions along the Namibian coast.