RMK returned 1000 freshwater pearl mussels to their home river for further growth 27.06

The employees of RMK’s Fish Rearing Department recently brought one thousand one-year old young mussels from Põlula Fish Farm’s lab back to their home river in Lahemaa. These one-year old mussels are expected to live for more than a hundred years but their precedents have not been given this chance for almost fifty years.

Since there is a huge risk that barely visible freshwater pearl mussels can be killed at the bottom of the river, they are growing on special plates in the river water. The employees at RMK’s Fish Rearing Department and the Environmental Board clean them of sediments once or twice a week.

RMK began to propagate freshwater pearl mussels in artificial conditions two years ago, as at one point their natural reproduction had stopped, and they didn’t have any known offspring that reached adulthood for at least the last fifty years. Thus, freshwater pearl mussels face the very real threat of extinction in Estonia.

Since propagation of freshwater pearl mussels is a complicated process, the employees of Põlula Fish Farm have to do a lot of difficult and precise work when growing the mussels. First, they need to catch at least 120 finger-length trout, whose gills the freshwater pearl mussels are attached to, from the mussels’ home river. The fish will then be transported to a quarantine building at Põlula Fish Farm, where the larvae transform and fall off the fish gills. In nature they would fall off to the bottom of the river, but since the development of freshwater pearl mussels has stopped, the scientists grow them on special plates in the lab and later in the river.

Plates with freshwater pearl mussels. Every plate has a hundred tiny mussels. Photo: Kristiina Viiron.

Once a month, the employees of the fish farm bring the mussels water from their home river and detritus (dead particulate organic material) from the spring of the nearby wetland. Additionally, in the lab, the mussels are fed a special seaweed mix which enables the 0.4 millimetre mussel larvae to grow to 1 millimetre long by late autumn. According to the Freshwater Pearl Mussels Specialist at RMK’s Põlula Fish Farm, Katrin Kaldma, growing the mussels in the lab is vital because mussels with a smaller diameter might not survive the winter in river water. Twice a year, the employees also check on the state of mussels in the river and they microscopically separate the dead young mussels from the live ones.

“Last winter, we learned that the very small mussels don’t survive in the river water,” Kaldma notes. “The large mussels had a better winter – 80% of the mussels who had turned two this spring out of the 250 mussels survived. Less than 15% of the smaller and younger mussels survived.”

A thousand young mussels, who were brought to their home river last week, are a great addition to the four hundred mussels already growing in the river. They will also grow on the plates and special boxes in the river water until they reach an age between seven and ten, where they will be inhabited in the natural environment.

This year, the trout infected by the freshwater pearl mussels have already been caught and the employees at Põlula Fish Farm are currently gathering the larvae that have fallen off from the trouts’ gills. They are hoping to gather around 7000 of them.

Besides propagating the mussels artificially, the employees are also trying to improve the state of the river and make the environment more suitable for the mussels. For this, the sediment input will be decreased, the water flow will shape the river bed, etc.

The international projects Ranniku LIFE and LIFE Revives support the propagation of mussels. In order to revitalise the population, over 15,000 young mussels will be raised and released into Lahemaa river.

Further information:
Katrin Kaldma
Freshwater Pearl Mussels Specialist at RMK’s Põlula Fish Farm
+372 509 4419