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Environmental pollution including mutagens from wastewater effluents and discontinuity by man-made barriers are considered typical anthropogenic pressures on microevolutionary processes that are responsible for the loss of biodiversity in aquatic ecosystems. Here, we tested for the effects of wastewater treatment plants (WWTPs), weirs and other stressors on the invertebrate species Gammarus pulex at the population genetic level combining evolutionary ecotoxicology, body burden analysis and testing for exposure to mutagens. Exposure to chemical pollution alone and in combination with the presence of weirs resulted in a depression of allelic richness in native G. pulex populations. Our results suggest that the input of a mutagenic effluent from a WWTP resulted in a strong increase in private alleles over the affected populations. In addition, the presence of weirs along the river disrupted the migration across the river and thus the gene flow between G. pulex upstream and downstream. This study provides strong evidence that the assessment of genetic variation including private alleles together with the contamination of mutagenic and nonmutagenic chemical pollution offers new insights into the regulation of genetic population structure and highlights the relevance of emerging anthropogenic pressures at the genetic level.

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In the summer of 2014, reports on heavily diseased and dead Atlantic salmon (Salmo salar, Linnaeus 1758) came from different Swedish rivers along the Baltic Sea coast. Descriptions of diseased fish included apathy, wounds and fungal infections (mycosis). Due to reoccurring summer outbreaks, with severe outbreaks in the rivers Torneälven and Mörrumsån (2014 and 2015) and the rivers Ume-/Vindelälven and Kalixälven (2015), and also reoccurring late autumn mass mortalities due to mycosis, the Swedish National Veterinary Institute (SVA) and the Finnish Food Safety Authority Ruokavirasto (formerly Evira) initiated an examination of the health issues of the salmon in the summer of 2016 (ICES, 2018; SVA, 2017). It was noted that outbreaks varied considerably between regions and time. Summer mortalities were noted in Sweden and Finland, whereas autumn mycoses were also reported from other Baltic