This is VKM’s key message in a risk assessment of the non-native tunicate commissioned by the Norwegian Environment Agency.

Background

Didemnum vexillum is a non-native species originating from the western Pacific Ocean. It was first detected in Norway in November 2020, in Stavanger. As of February 2023, it has been found at five different locations on the West coast of Norway.

The non-native species is considered invasive. It forms carpet-like colonies and can outcompete other species as it grows over them and either suffocates or reduces their growth. The colonies grow quickly and can spread through both sexual and asexual reproduction. It also has few natural predators.

The species can have a major negative impact on biodiversity in Norway should it spread to various vulnerable ecosystems and habitats. Potentially, it can have profound negative effects on marine aquaculture.

Methods

In addition to the environmental risk assessment, VKM has identified and assessed various risk- reducing measures.

The assessments are based on scientific literature and reports and expert opinions of the authors on the relevance of international findings under Norwegian conditions. VKM has also been in direct contact with international scientists knowledgeable about the organism.

Results and conclusions

According to VKM, D. vexillum probably entered Norway through biofouling on larger vessels.

“We assess that spreading over larger distances, of more than 100 km, is predominantly caused by biofouling on large vessels or installations like oil rigs. This is the largest contributor to the likelihood of spreading,” says Johanna Järnegren, VKM’s spokesperson for the project.

VKM finds that it is unlikely that D. vexillum can be completely eradicated from Norwegian waters.

“It is however possible to reduce the negative impacts on valuable ecosystems and habitats if actions to prevent further spread are taken,” emphasises Järnegren.

“We are still in the early stages of establishment, and it is therefore important to monitor the species and inform the public so that further spread with private boats can be prevented, or at least detected at an early stage.

"On a long-term basis we’ll need international collaboration to prevent spread of biofouling organisms across borders and long distances,” says Järnegren.

The assessment has been approved by the VKM Panel on Biodiversity.