Juvenile Pearl Mussels

in Lower Saxonie's only Margaritifera population juvenile mussels grow up again

In former days there have been 5 water systems in Lower Saxony with some 20 brooks that have been inhabited by Margaritifera margaritifera. The total population was several million mussels. In 1939 only 50.000 animals of these had been left in 2 brooks. In 1965 the population had further declined to 5.000 mussels and in 1985 just 3.000 animals were left. These few remainders lived in one small section of one brook. By that time juvenile mussels were found in very small numbers only, resulting in an over-aged population.
From the 1970s conservationists from the administration of Lower Saxony, from Universities and nature conservation societies as well as private persons joined forces to save the last surviving Margaritifera mussels. All attempts to support the population with cultured juveniles or by releasing infected host fish into the brook turned out to be not successful. The habitat of the juvenile mussels was too severely impounded by large amounts of sand that entered the water course from arable land and was transported downstream consequently.
During the 90th and up to the year 2004 large areas within the entire catchment of the brook could be purchased on the base of a national conservancy project. By changing the land use the intrusion of sand into the water system could be stopped. At the same time rights to dam the brook or to maintain fish ponds were purchased from private people within the valley. Furthermore some sections of the brook were reconstructed. Altogether this resulted in a considerable improvement of the habitat allowing juveniles to grow up again.
Since 4 years by now juvenile mussels have been found again in large numbers. Today the number of mussels is more than twice as high compared to 1985, the majority of mussels today is less than 20 years now!
Pictures below were taken in July 2003 and show some snapshots of the recent development of the brook:
Reconstructed part running through bog land
The water is coloured brown due to humin from the moor. The sediment mainly consists of gravel and coarse sand. Fine sediments and silt that could clog the interstitial are almost totally missing..
Part of the brook that has not been reconstructed. It is much oversized in cross-section due to dredging resulting in thick soft layers of mud. 
A few minutes of searching were enough to find a hand full of juvenile mussels!
Purchasing arable land within the catchment, reducing the number of fish ponds, installing facilities to collect and remove the drifting sand, purchasing the right to maintain weirs and removing mud and silt from a water mill pond has led to an improvement of habitat quality that was enough to allow a natural recruitment of juvenile mussels sufficient to allow a recovery of the population for the first time world wide

In 2004 the eldest of these juveniles had reached an age of about 14 years and were found to be gravid themselves for the first time. A circle is closing .....

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