DNA tests aim to map invasive fish in Finnish ponds

The Natural Resources Institute Finland (Luke) is testing water samples from 99 ponds in Varsinais-Suomi and Uusimaa to detect pumpkinseed sunfish and Chinese sleeper, Yle reports. The method could identify the fish without requiring a direct sighting.

Luke has identified 15,000 ponds in southern Finland where the harmful non-native fish could potentially occur. The samples were collected from randomly selected ponds, most of them in Varsinais-Suomi. Twenty of the ponds are known to contain the fish. Analysis is about to begin, with the first results expected in winter.

Current knowledge of the fish’s distribution is based on public sightings. Pumpkinseed sunfish has the widest known range and has also been reported in Satakunta, Pirkanmaa and Kanta-Häme.

Luke specialist researcher Sanna Kuningas said pumpkinseed sunfish may have been kept in Finland as an aquarium fish or in garden ponds before reaching ponds and flooded quarries farther from residential areas. The fish are likely to have been moved by people, she said, although animals could theoretically carry them to nearby waters. The fish compete with other species for space and food and can displace them.

Eradicating pumpkinseed sunfish has proved difficult. A pond in Paimio was drained in an attempt to remove the fish, while a rapidly dissolving poison was used in Espoo. Kuningas said fishing can limit population growth but cannot remove every fish, as they reproduce readily and young fish are likely to remain.

Anglers reported catching hundreds of pumpkinseed sunfish from a pond in Meri-Pori on social media during the summer. One angler said they caught 250.

Pumpkinseed sunfish is native to North America and Chinese sleeper to northeast Asia. The species were first recorded in Finland in 2014 and 2022, respectively, and both threaten native species.