Timing, growth and proportion of spawners of the threatened unionoid mussel Margaritifera margaritifera: influence of water temperature, turbidity and mussel density

Abstract

Anthropogenic disturbances often cause decline and extinction of threatened species. The present study investigated how gravid freshwater mussels, Margaritifera margaritifera, were affected by turbidity and water temperature, and by mussel density. At an early date of mussel spawning, there were lower proportions of gravid mussels in streams with evidence of mussel recruitment than in streams without mussel recruitment. At a late spawning date, this pattern was reversed. Higher water temperature in streams without recruitment was probably responsible for this difference. The combination of high water temperature and turbidity may be one reason for reduced growth of gravid mussels in streams without recruitment. There was a positive relationship between adult mussel density and the proportions of gravid mussels. Early gravidity may lead to early release of larvae, early infestation on the host fish and an earlier start of the benthic phase, which may reduce survival rates. Clear-cutting of forests and global warming are factors that are likely to cause increased turbidity/sedimentation and water temperatures in streams. One restoration measure that reduces sediment input and water temperatures is maintaining or restoring riparian zones, but these are long-term measures that require many years before they have an effect in streams.