Distribution of fish species in the upper Po River Basin (NW Italy): a synthesis of 30 years of data

Freshwater fish is the most diverse group of vertebrates but, unfortunately, also one of the most threatened. While some well-known, charismatic species have been subject to important conservation efforts, many others have long been neglected. This paper aims to analyse the distribution over time of seven freshwater fish species and one lamprey in the upper Po River basin (NW Italy), an important biodiversity hotspot. Six of them are native species listed in Annex II of the Habitats Directive (Lampetra zanandreai, Protochondrostoma genei, Chondrostoma soetta, Telestes muticellus, Sabanejewia larvata, and Cottus gobio), while the other two are key invasive species (Silurus glanis and Misgurnus anguillicaudatus). Data from four regional fish population monitoring campaigns carried out between 1988 and 2019 were analysed. For each species and monitoring campaign, an average Representativeness Index, measuring abundance and population structure, and an Occurrence Frequency were calculated, and then assessed for changes over the time. Of the studied species, P. genei declined the most in the last 30 years, while C. soetta and S. larvata are in a very critical situation, with very few remaining populations. T. muticellus, C. gobio and L. zanandreai seem to have declined only slightly, but their vulnerability should not be underestimated. The two invasive species, on the other hand, show a substantial increase in occurrence as well as range. While some drivers for the decline in the native species vary according to their ecological and biological characteristics, others are more general and linked to the overall degradation of the river environments: habitat alterations, loss of connectivity, excessive water abstraction, pollution, and the presence of invasive alien species. The effects of climate change, such as the rise of inland water temperatures and the alteration of hydrological cycles, must also be taken into account. Diverse and far-reaching conservation efforts are needed to improve the fish habitat and thus also protect the unique biodiversity of this region.