The last hideout: Abundance patterns of the not‐quite‐yet extinct mayfly Prosopistoma pennigerum in the Albanian Vjosa River network

The mayfly Prosopistoma pennigerum (Müller, 1785) (Insecta: Ephemeroptera) once occurred in many European river networks. However, observations decreased in the last decades and the species can be considered largely extinct throughout Europe due to river alterations. Only three extant populations are known from Cabriel (southern Spain), Volga (Russia) and Vjosa (Albania) rivers. We recorded the species along a 150 km stretch in the Vjosa River in three sampling seasons (spring 2018, fall 2018 and fall 2019), counting up to 302 P. pennigerum per m2, the highest recorded abundance for the species to date. Moreover, we detected traces of environmental DNA in a newly designed targeted eDNA assay. In our modelling approach we define the species' niche in a theoretically available niche space given by the Vjosa River network and predict a high probability of presence (θ) in downstream located sections of this river. Expected abundances (λ) could be related to a set of environmental variables, importantly to higher discharge and increased sediment dynamics. Simultaneous occurrence of larvae of different sizes at individual sites suggests an asynchronous life cycle, which may be advantageous to cope with the highly dynamic river hydrology. The P. pennigerum population in the Vjosa is of key importance for the species' global survival. A robust population of the almost extinct mayfly, Prosopistoma pennigerum, occurs in the last free‐flowing river network, the Vjosa. We observed the species with a targeted environmental DNA approach and standard kicknet sampling along a > 150 km stretch of the Vjosa main stem and in its major tributaries. Prosopistoma pennigerum's asynchronous life cycle strategy and morphology grant resilience and resistance, at population and individual level, and are linked to the Vjosa's free‐flowing hydrologic dynamics.