Human impacts on the distribution and genetic diversity of Eurasian minnows (<i>Phoxinus</i>: Leuciscidae) in the Rhenish Massif

Introductions of non-native species, a key driver of biodiversity loss, may be underestimated due to cryptic species. In the Rhenish Massif (western Germany), anthropogenic introductions of Eurasian minnows ( Phoxinus ) caused discrepancies in genetic origin between historical samples from the Ahr and present-day specimens from the adjacent Sieg basin. This study assesses the autochthony of current Phoxinus populations in Ahr and Sieg and reconstructs introduction pathways using cytochrome c oxidase I ( COI ) barcoding and stakeholder surveys. We find that Ahr populations, free from intentional stocking, are almost exclusively inhabited by Phoxinus clade 5b ( P. csikii ). In the Sieg, Phoxinus populations exhibit a zonation pattern: clade 5b dominates mountainous regions, while clade 10 ( P. phoxinus ) is prevalent in foothill regions. Stakeholder surveys revealed substantial historical introductions from the Main/Neckar basin (clades 5b and 10) and Lower Rhine (clade 10), with rarer instances of Rhône (clade 12, P. septimaniae ) and Weser (clade 11, P. cf. morella ) minnows introduced as contaminants in brown trout ( Salmo trutta ) stocking. Combined findings indicate clade 5b's autochthony in the Ahr and mountainous Sieg, while clade 10 is suggested to be native to the Sieg's foothill regions, establishing the Sieg basin as a natural contact zone for these clades.