RMK collected over 27,000 mussel larvae this year 19.10

RMK Põlula Fish Rearing Department collected over 27,000 larvae of freshwater pearl mussel this year, which was significantly more than expected.

Half of the collected larvae have been released back into their home river in Lahemaa, the other half is growing in the aquarium and incubator at the department’s laboratory.

The rods are attached to plates used to grow freshwater pearl mussels until they are released back into the river. Photo: Katrin Kaldma

To reproduce, female freshwater pearl mussels lay their larvae into water, which will then attach themselves to the gills of juvenile trouts. After the larvae become mature, they drop off their host and fall to the bottom of the river. Since most of the larvae dies at the bottom of the river, more young mussels are grown with the help of people.

“We were able to collect several times more mussel larvae this time because we had been somewhat unsuccessful at catching fish that were infected with larvae,” said Katrin Kaldma, RMK Freshwater Pearl Mussels Specialist. Kaldma added that they had expected to catch about 8000 larvae. One-year-old trout are most susceptible to mussel larvae infections, but only a small number of them were caught this year. Most of the caught fish were two summers old. “Theoretically, older trouts are less likely to get infected as effectively, but practice shows that they were also infected with larvae,” said Kaldma. “When larvae infect fish with bigger gills, more mussels develop.”

Freshwater pearl mussels recently released from a fish’s gills on graph paper. Photo: Katrin Kaldma

The caught fish were brought to the Põlula fish farm to collect mussel larvae. Some of the collected larvae were placed in the incubator, while others were placed on special plates into the aquarium or river. The matured mussels will be released back into their natural habitat in a few years. By that time, it is likely they will survive at the bottom of the river.

Freshwater pearl mussels recently released from a fish’s gills on graph paper. Photo: Katrin Kaldma

RMK began growing freshwater pearl mussels nearly two years ago. Since then, tests have been carried out to determine the best method to keep mussel larvae alive. Last winter proved that bigger larvae are more likely to survive the winter. “We also plan to carry out comparative tests this winter to gain new knowledge,” said Kaldma. All mussels that are currently placed on plates in the river will soon be resettled in locations that were determined more suitable for growth last winter. “We will also examine how well the mussels have been growing in the river since spring and whether there are any differences between locations.”

Kaldma said it is crucial that salmonids can spawn in the river without falling prey to illegal fishermen as this determines the number of juvenile trouts in the river. “The more juvenile trouts in the river, the better the chances for mussel larvae to reproduce,” Kaldma said. “Unfortunately, current monitoring shows that not all fish reach the river to spawn.”

The revival of freshwater pearl mussel populations and their habitats has been carried out since September 2021 with the help of the Finnish, Swedish and Estonian joint project (LIFE Revives – LIFE20 NAT/FI/000611). RMK also grows mussels with the help of the Ranniku LIFE project, which is an Estonian-Finnish joint project.

The restoration of freshwater pearl mussel populations will be discussed at the RMK nature conservation conference which is held on 3-4 November at the University of Tartu Narva College.

Further information:
Katrin Kaldma
RMK Freshwater Pearl Mussels Specialist
+372 509 4419