Secondary salinization of freshwater ecosystems is of increasing global concern. One of the main causes are the effluents of the potash mining industry containing high concentrations of major ions (Cl, Na, Mg, K). In Germany, the ongoing discharge of effluents into the River Werra led to a strong impoverishment of the biodiversity and abundance of local species. Young cohorts of many freshwater fish are completely absent suggesting reproductive failure under these conditions. Therefore, the aim of the study was to experimentally investigate the effects of high concentrations and imbalances of ions that are prevalent in potash mining effluents on reproductive traits of native freshwater teleosts. Sperm motility parameters of the common roach, Rutilus rutilus, and European perch, Perca fluviatilis, were assessed as well as fertilization rate, egg size, hatching, malformations and mortality of embryonic and larval stages of roach. Concentrations of the permitted thresholds (HT) and future thresholds (LT) as well as three ion solutions containing high Mg (Mg), high K (K) and both in combination (Mg + K) were tested. Curvilinear velocity and linearity of perch spermatozoa were elevated with potentially adverse effects on fertilization success. Sperm motility parameters and fertilization rate of roach were not affected. However, egg sizes of roach were increased in all groups due to the osmotic action of ions and in LT, premature hatch was observed. Furthermore, all groups comprised a higher number of malformations including pericardial edema and spine curvatures and group HT exhibited a higher mortality rate compared to control. The results clearly demonstrated that particularly the sum of high concentrations of ions, as prevalent in HT and LT, rather than individual ion species exerts detrimental effects on early development of roach potentially increasing overall mortality under natural conditions. These results emphasize that currently permitted and future thresholds are exceeding tolerated ion concentrations.Copyright © 2020 Elsevier B.V. All rights reserved.