Artificially heated water bodies represent unusual habitats in temperate regions and form a refuge for exceptional fish communities. The Gillbach, a tributary of the river Erft in Germany, receives thermally polluted cooling water from a power plant. Here, we present data on the composition of the fish community in the Gillbach and found a high abundance of invasive species from all over the world, mostly introduced by releases from home aquaria. We found a species composition that is dominated by invasive species containing the same species as 15 years ago. We focused on guppies (Poecilia reticulata) and determined population size using the mark-recapture method. Furthermore, we investigated the lower thermal tolerance limit (CTmin) to determine if Gillbach guppies have already adapted to colder conditions compared to ornamental and Venezuelan wild type fish. We caught guppies of all sizes, and densities of 3.6 adult guppies per square meter were comparable to densities found in their natural distribution area, pointing toward a self-sustaining population in the Gillbach. The CTmin varied between populations and was significantly lower in ornamental and Gillbach guppies compared to guppies from Venezuela. Despite differences in CTmin and their well-known potential to adapt to new environments, guppies originally stem from the tropics, and a further spread will likely be restricted by low winter temperatures. Thus, P. reticulata in the Gillbach might not represent a threat for local fauna in Central Europe, but provide a unique semi-natural experiment for various questions related to local adaptation of invasive species, as well as ecological interactions with indigenous species.
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