"We are in the middle of another hot summer. After 2015 and 2018, many waters of the river pearl mussel are drying up again. We are fighting for the mussel’s survival, but if suitable waters are not available in the future, we have no chance." The words of thomas speierl sound dramatic. The head of the district of upper franconia’s fisheries advisory service and his staff have pulled out all the stops to conserve the river pearl mussel (margaritifera margaritifera) in the drei-lander-corner in the district of hof. But even the dedicated team can do nothing against the drought.
Some of the pearl creeks fell dry during the last heat period. With a relocation of mussel stocks, the experts have taken the last possible measure in the past few weeks. Now it depends on whether the river pearl mussel can survive in the new water.
Some factors must work well together for the relocation to be successful: the mussel waters, which are poor in lime, must have very good water quality, the temperature load must not be too high and the water stretches must have clean, loose stream gravel.
Host fish important
In addition, host fish must be present for reproduction and the occurrence of natural enemies of these host fish must be low. "River pearl mussels and brown trout have high demands on water quality, especially their early stages of development are sensitive to sediments. The river pearl mussel is also endangered by dwindling stocks of its host fish, the brook trout", female district president henry schramm around the rough challenges.
Because the reproduction of the river pearl mussel is also very complex and only works in bavarian waters in conjunction with the brook trout. Their tiny fry (glochidia), which cannot be seen with the naked eye, need the brook trout as host fish, on whose gills the larvae live for about a year. With a coarseness of about half a millimeter, the young mussels then fall off and develop hidden in the gravel beds of the waters.
Cooperation with the czech republic
Only in the adult stage, with a now hard shell, do they return to the surface of the water bottom after about seven to ten years and become sexually mature at the age of 15 years. If reproduction is successful, up to four million mussel larvae can be released into the water. Up to 1000 glochidia can settle on the gills of the brown trout.
In order to reduce the brook trout population, the district has used hatchery boxes with young fish in the upper franconian pearl mussel waters over the past three years in coordination with its czech colleagues. Because during a stock survey in spring the experts found another drastic decline of the brook trout. Speierl sees two causes: the increasing warming and drying out of the brook due to the persistent heat periods, and increasing pressure from predators such as otters.
If the brook trout is missing, the river pearl mussel has no chance of reproducing. Czech mussel expert ondrej spisar is trying to "infect" brook trout with mussel larvae at the fishery training institute in aufseb, to artificially multiply them. The young mussels obtained in this way are later released into suitable water – perhaps the last chance to preserve the river pearl mussel.