Species conservation may be complicated in symbiotic interactions. Sub-lethal impacts of human activities on one species may be lethal to other species on which they depend. We investigated the impact of two aspects of eutrophication – changes in phytoplankton and oxygen levels – on the interaction between a freshwater fish that is endangered in several countries, the European bitterling, Rhodeus sericeus, and the unionid mussel hosts that they require for spawning. Reduced concentrations of algae and oxygen each led to a greater proportion of bitterling embryos being ejected prematurely from the mussel hosts. Reduced algal concentrations also led to reductions in mussel ventilation rates and to mussels spending less time with their valves open. Bitterling mortality may have been due to three causes: direct effects of environmental conditions on embryonic survival, active expulsion by mussels when stressed and through responses by mussels that created internal conditions that bitterling embryos could not survive. Our study shows that the sub-lethal effects of pollution on one species can have lethal effects on another species with which it interacts. Such interactions need to be considered in conservation programmes.