Pearl mussels

Freshwater pearl mussels

The purpose of the project was to enable the upstream migration of trout from Ljusnan to Valvtjärnsbäcken in order to improve the reproductive potential of the threatened freshwater pearl mussel in the creek.
Valvtjärnsbäcken is a forest tributary of River Ljusnan with one of Sweden's remaining stocks of the endangered and protected species of freshwater pearl mussel. Inventory of the creek has confirmed that the population is aging and that the mussel stocks have declined over the past 20 years. This is probably due to loss of reproduction, which ultimately will mean that the stock vanishes  if no restoration efforts are performed.

The reason for the reduced reproduction of the mussels is most likely that trout has had no possibility to migrate upstream in the creek because the stream has been, for a distance of about 300 m, fully covered with large masses of blasted stone. Trout is vital to the fresh water pearl mussels as for a few months the mussel lives as parasite in the trout's gills and later it further develops to a mussel.

In the project, the stream was cleared from the blasted stone deposited in Valvtjärnsbäcken. The creek was designed in the best possible way for both trout and freshwater pearl mussel in terms of depth and bottom structure. The restoration work was done respecting the surrounding nature. After completion of the project, information signs were put up on the site to avoid any new rock material to be dumped to the creek and to provide information about the restoration and the protected freshwater mussel.