Fish species composition and host fish density in streams of the thick-shelled river mussel (Unio crassus) - implications for conservation

1. Recruitment and dispersal of freshwater mussels of the Unionoida is strongly linked to their host fishes on which the larvae develop into juvenile mussels. Host limitation has been suggested as one of the potential causes for the decline of the formerly widespread thick-shelled river mussel (Unio crassus), although quantitative data analysing the importance of this factor are still missing. 2. In this study, the status of host fish populations was assessed in nine U. crassus streams within the Danube drainage system situated in southern Germany. In particular, links between recruitment patterns of U. crassus populations, fish species composition and quantitative data of host fish densities in functional and non-functional streams were established. 3. Fish species composition and richness ranged between three and nine species with no difference in presence data between functional and non-functional streams. Despite this high variability, functional U. crassus streams with high proportions of juvenile mussels had significantly higher (mean: 40 Ind. per 100m(2)) densities of primary hosts (Phoxinus phoxinus, Squalius cephalus, Gasterosteus aculeatus) than non-functional streams (mean: 8 Ind. per 100m(2)). In contrast, no relationship was found between the numbers and densities of juvenile mussels and densities of poor hosts as well as of non-hosts. 4. The results of this study indicate that the recruitment status of U. crassus strongly depends on the density of primary host fish species. Host limitation is likely to play a major role in a lack of juvenile recruitment and consequently in the decline of central European U. crassus populations. Therefore, future conservation for this endangered mussel species should more thoroughly incorporate management of host fish species and consider both the requirements of mussels and their hosts in habitat restoration. Copyright (c) 2014 John Wiley & Sons, Ltd.