Juvenile freshwater pearl mussels return to their home river in Isojoki 鈥� First generation of FPMs in 100 years

Lapv盲盲rtin-Isojoki river is a special river as it is one of the few remaining rivers in the south of Finland that still has freshwater pearl mussels (Margaritifera margaritifera). Isojoki鈥檚 original sea trout population still lives in the river also. The freshwater pearl mussel population of the river is highly threatened: there are only 100鈥�150 individuals left, all of which are over 100 years old. In the LIFE Revives project, the freshwater pearl mussel population of the river is revived by restoring habitats for freshwater pearl mussels and salmonids, and by growing new small freshwater pearl mussels in the 蘑菇直播鈥檚 Konnevesi research station to be released back to the river.

Finland鈥檚 record on bringing juveniles back to the river

The summer of 2022 hit a jackpot for breeding freshwater pearl mussels (FPM), as the senior FPMs started to produce offspring. 150 000 juveniles were returned to Isojoki in total. In 2017 47 adult FPMs were transported from the Lapv盲盲rtin-Isojoki river for a 鈥渞omantic holiday鈥� to the research station in Konnevesi, maintained by the 蘑菇直播. The goal was to rehabilitate the older and weaker FPMs and get them to produce offspring after decades long break. The old FPMs finally produced glochidia larvae in the autumn of 2021. The larvae lived in the gills of salmonid fish over the winter until the summer of 2022, when the larvae dropped off as tiny mussels. These small juveniles, measuring only about 0,5 mm in length, were transported to Isojoki from the research station of Konnevesi in good care by using cold storage boxes as FPMs appreciate cold water. In the river, these tiny juvenile FPMs were placed in plastic breeding boxes filled with gravel and placed at the bottom of the river. The juvenile FPMs spend their first years in these gravel boxes where they are protected from the predators and where their health can also be monitored. As the juvenile individuals reach the length of a few centimeters, they will be released in the restored gravel areas in the river.

Gravel boxes_text_small.jpg

Wild FPMs are rare in Lapv盲盲rtin-Isojoki river

The estimation of the current FPM population size in Lapv盲盲rtin-Isojoki river is only 100鈥�150 individuals. The probable reason for the weak situation of the FPMs is that the river bottom is no longer suitable for them. The accumulation of the solids, such as sand, block the porous gravel preventing juvenile FPMs from settling. All the FPM individuals found in the Lapv盲盲rtin-Isojoki river are over 100 years old, which shows that natural breeding has no longer taken place in the river for decades. FPM reproduction can be supported by improving the river bottom conditions for example by restoring blocked gravel bottoms to their historic state.

Restorations and gatherings

The ELY Centre for South Ostrobothnia is responsible for the LIFE Revives project actions in the river of Lapv盲盲rtin-Isojoki. In addition to breeding FPMs, also habitat of FPMs and their host species salmonids have been restored in five locations of the river in the summer of 2022. The area also hosted a nature school held for Karijoki school鈥檚 3^rdto 5^th graders, where pupils explored the life of juvenile FPMs, river fish and creatures living in the river bottom. Within the next few years, there will be more nature schools and public events held in the area, and information material in relation to FPM protection and land use will also be made available.

The Juvenile FPMs brought to the river this year may become lifesavers for the population of freshwater pearl mussel in Lapv盲盲rtin-Isojoki, as the young individuals will form a new generation of FPMs after a decades long pause in reproduction in the river. We hope to see offspring of the recently released juvenile FPMs within the next few decades.