Occurrence depth of juvenile freshwater pearl mussels (Margaritifera margaritifera) in a river bed tested by experimental mesh tubes

Sediment depth preference was tested on 1-year-old freshwater pearl mussel juveniles. Mesh tubes allowing vertical migration with a possibility to leave hyporheic space, filled with 1-2-mm granulometry size substrate and populated with groups of juveniles, were placed in the natural river bed of the Tepla Vltava River, Czech Republic. The depth position of the juveniles was recorded after 1- and 2-month exposure during the summer and after 8 months including wintering. The juveniles showed a tendency not to stay on the sediment surface, but to penetrate into a very shallow depth, and to accumulate and stay 2-3 cm deep in the substrate in the summer period. No juvenile was found below 8 cm sediment depth, neither in the summer nor after wintering. In the experimental tubes, various survival rates (0-100%) were recorded in close relation to substrate oxygenation development at the tested sites. Oxygen decrease was accompanied by juveniles trying to escape the hyporheal.