Physical microhabitat requirements for Margaritifera margaritifera and the influence of hydro- and morphodynamics on mussel bed stability

Abstract: Margaritifera margaritifera is an endangered freshwater pearl mussel with population declines throughout its Holarctic range. Boulder-bed channels with little input of fine sediment in previously channelized streams are abundant in Sweden. Hence, restoration efforts for M. margaritifera and its host fish (Salmo trutta) are carried out by reintroducing large boulders and creating mussel beds with suitable gravel. In order to improve the restoration methods currently in use I investigated M. margaritifera microhabitat preferences and the stability and suitability of introduced gravel aimed as habitat for mussels. Also, since monitoring of the success of intra-river translocation of mussels is rare or insufficient, movement of mussels after translocation was tracked. Mussel abundance and microhabitat data were collected at a site with a stable M. margaritifera population. At a site where mussel translocation was planned, substrate stability was investigated by surveying erosion and deposition of 0.4x0.4 m painted gravel squares placed on the channel bed. Erosion and deposition were analyzed in relation to physical parameters and morphological features. Mussels were PIT-tagged to track their movement after translocation. Depth, water velocity and the median grain size (D50) affected the mussel density, whereas substrate stability was mainly determined by proximity to large boulders and D50. However, large variations in stability for gravel close to upstream boulders suggests complex hydraulic patterns; thus, high boulder densities are not necessarily desirable when restoring previously channelized streams for the freshwater pearl mussel. The results have implications for restoration of mussel beds and show the importance of understanding how large grains and their configuration affect surrounding hydrodynamics and shear stress.