Potash mining effluents and ion imbalances cause transient stress in adult common roach, Rutilus rutilus

A present ecological issue causing secondary salinization in different countries is the discharge of effluents by the potash mining industry. In Germany, the River Werra is used as a sink for potash mining discharges containing high concentrations of ions, predominantly Cl^-, K⁺, Na⁺, and Mg²⁺ resulting in a strong decline of the biodiversity and abundance of local species. However, hardly anything is known about the acute and chronic physiological effects of high concentrations and imbalances of ions being prevalent in potash mining effluents in fish. Therefore, the stress response and selected immune and growth parameters were investigated in standardized laboratory experiments. A native freshwater fish species, Rutilus rutilus, was exposed to concentrations of the high currently allowed (HT) and lowered future thresholds (LT) and three different ion solutions (containing high Mg²⁺ (Mg), high K⁺ (K) and high Mg²⁺+K⁺ (Mg+K) concentrations) for four different exposure times (24 h, 7 d, 21 d, 8 wk). Tank water (additionally after 9 and 12 h) and plasma cortisol, glucose and protein, hematocrit and hemoglobin were determined after each exposure time. Furthermore, plasma lysozyme and head kidney leucocyte respiratory burst activity (only after 21 d) were evaluated as well as growth parameters. A transient stress response was induced in almost all groups. Tank water cortisol was elevated after 9 h in HT, LT and Mg+K and in HT after 12 h, whereas glucose concentrations increased after 24 h in all exposure groups except K. HT led to enhanced hematocrit and hemoglobin content after 24 h. Plasma protein, immune system and growth were not affected in any group. None of the ion solutions induced acute toxicity but most triggered typical acute stress reactions. Rather the sum of high ion concentrations than single ions challenged the fish. Even though the effects observed in adult roach were only transient and indicate acclimatization under laboratory conditions, adverse effects observed in the river are evident and further research on physiological endpoints including reproductive parameters and impacts on younger life stages seem to be needed to scientifically base protective thresholds.