Invasive predatory amphipod established permanently in Rauma port, researchers say

The predatory amphipod, a non-native species likely introduced to Finland via ship hulls or ballast water, has been found in large numbers in Rauma and may spread to inland waters, according to the Finnish Environment Institute (Syke).

Researcher Okko Outinen of Syke confirmed that the predatory amphipod (Dikerogammarus villosus), first detected in Rauma’s port, has now established a permanent presence. Surveys conducted in 2024 revealed a significant population in the area, though no specimens were found in other monitored ports, including Helsinki’s Vuosaari, Kotka, Hamina, Porvoo, Turku, or Naantali.

Outinen noted that the species—measuring two to three centimetres in length—could have arrived attached to ship hulls or through ballast water discharges. Its ability to thrive in low-salinity and freshwater environments raises concerns about potential spread into Finland’s inland waterways.

Eradication deemed impossible

The large population in Rauma indicates the species is well-adapted to local conditions, making removal impractical, Outinen stated. While natural dispersal is limited, human activity—such as boating—could accelerate its spread to new areas.

Unlike native amphipod species, the predatory amphipod actively hunts other invertebrates. It may also serve as a new food source for fish and diving birds, though its long-term ecological impact remains uncertain.

Unpredictable consequences

Outinen cautioned that invasive species can either integrate harmlessly into ecosystems or disrupt them by outcompeting native species. While Finland’s coastline has seen relatively few marine invasives compared to regions like the Mediterranean, the predatory amphipod’s arrival was anticipated, as it has already spread to most other Baltic Sea countries.

Syke has long monitored the species, expecting its eventual appearance in Finnish waters.