Crayfish invasions at a long-term ecological research site formerly occupied by the noble crayfish Astacus astacus

Invasive crayfish, alongside their associated pathogens, have accelerated the decline of their Eurasian native counterparts, restricting their distributions to small populations nearing extinction. Using past sampling records, as well as conducting two field sampling campaigns with baited traps and environmental DNA (eDNA) in 2021 and 2023, we studied the distribution of invasive crayfish and their associated pathogen (Aphanomyces astaci) in the Rhine-Main-Observatory (RMO), a long-term ecological research (LTER) site in central Germany. This area of 1,058 km² was naturally occupied by the native endangered noble crayfish Astacus astacus exclusively. We only captured one native individual and detected two positive eDNA signals to A. astacus in the northeastern part of the RMO, suggesting a retraction of its historical range to a few headwater streams. In contrast, since 2000, four invasive crayfish species have been documented in large parts of the lower basin, with the signal crayfish, Pacifastacus leniusculus, progressing upstream. The upper stretches of the basin exhibited no traces of either invasive crayfish nor signs of the pathogen A. astaci, so that reintroduction programmes could be considered following careful assessment. Management actions are needed in potential contact zones where native and invasive populations can coexist, to protect the almost relict distribution of A. astacus in this German river basin.