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During the summer of 2023, it was noticed that Isojoki's small FPMs grow very slowly, and their mortality rate was very high. During two growing seasons, Isojoki's small FPMs have only grown to about 1.5 mm in size. In the same time, the small FPMs grown in the Ähtävänjoki grew to about 5.5 mm in length. The temperature of the river water in Isojoki is very low in the summer, and because of this there is not enough food available in the river for the small FPMs. The low temperature is due to the high amount of spring water. Small FPMs do not have time to get enough reserve food during the summer, which is why they grow very slowly or die during the winter.

By the LIFE Revives project, it was decided to transfer the main part of the surviving little FPMs to Ähtävänjoki to grow, because according to the latest estimate made in 2024, there are still thousands of small FPMs alive. Breeding takes place on perforated plates and gravel boxes, from which the small river pearl mussels cannot escape. Perforated plate (Buddensiek plate) refers to a plastic board with holes drilled into it (usually 50 pieces). A dense mesh net is placed on both sides of the plate and plastic sheets are placed on both sides of the net. A chamber remains between the plastic sheets, from which the little critters cannot escape. From the perforated plates, the small shoots are transferred to gravel boxes for further rearing. Gravel boxes are sturdy food-grade plastic. They consist of a box and a tight lid. Holes have been made in the box and lid and the holes are covered with a dense mesh steel mesh. The mesh size of the steel mesh is small enough for small particles to be washed away. The boxes have a volume of 4.5 liters and are filled with gravel, humus and wood material suitable for small animals. Among the gravel, the weeds grow for several years. Perforated plates and gravel boxes have been used.

During the LIFE Revives project in Ähtävänjoki, the Isojoki small FPMs have time to grow to be clearly larger (20-30mm) than the small FPMs grown in Isojoki (5-10mm). At the end of the project, all the small Isojoki river FPMs that were growing in the Ähtävänjoki will be planted in the restored areas of the Isojoki before the end of the LIFE Revives project.

The first summer's mussels are kept at the old fish farm in Hjulfors, after which they are moved deeper into the river to grow in gravel boxes. The survival and growth of the Isojoki mussels transferred to the Ähtävänjoki river will be monitored throughout the time they are in the Ähtävänjoki river.

Author and photos: Eero Mäenpää, LIFE Revives project coordinator in EPOELY

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