Cargo ships bring invasive species into Finland’s Baltic Sea

Invasive species brought by cargo ships are spreading through the Baltic Sea, Yle reports. Researchers from the Finnish Environment Institute have spent the summer surveying species in Finnish ports, including Hanko’s Outer Harbour.

The ships that bring clothing, electronics, cars, fruit and coffee to Finland can also carry unfamiliar animals that threaten the Baltic Sea. Researchers Okko Outinen and Riikka Puntila-Dodd have been collecting samples to determine which species are present and how widely they have spread.

The researchers use nets to collect plankton, a grab sampler for bottom mud, and traps lowered into the sea for crustaceans and fish. In Hanko, they also examined plates that had remained in the harbour water throughout the summer. The plates were covered in algae and small crustaceans.

Samples taken from the mud contained both native Baltic Sea animals and invasive species. The researchers found blue mussels and Baltic clams, which are among the Baltic Sea’s most important species. Baltic clams are sensitive to oxygen depletion and provide food for many fish, so finding large numbers of them is a positive sign for the seabed.

The samples also contained red mud tube worms from the Marenzelleria genus. Native to the river deltas of North America, the worms arrived in the Gulf of Finland in the 1990s in ships’ ballast water. They have since spread across the Baltic Sea, from the coast of Poland to Oulu.

Depending on the location, researchers also often find predatory water fleas, mud shore crabs, round gobies, claw isopods and wandering snails. Some species found in Hanko may be new to Finland, but this will not be known until the samples are examined under a microscope in winter.

Invasive species monitoring has also identified a species previously unknown to science, the translucent spiral snail. Finnish ports have additionally yielded estuary amphipods and predatory amphipods during the 2020s.

Researchers are particularly concerned about the robust predatory amphipod, which threatens native Baltic algae shrimps. These tiny crustaceans play an important role in the marine ecosystem.

Researchers say that finding new species is interesting but also troubling. Invasive species that reproduce aggressively compete with native species for living space.

Because the sea cannot be emptied, invasive species control relies on anticipating new arrivals and gathering information about their habitats, behaviour and distribution.