Risks of alien sturgeon species in the River Rhine basin for the reintroduction success of the endangered native European sturgeon

Sturgeons (order Acipenseriformes: sturgeons and paddlefishes) are amongst the most endangered fish in the World. The dwindling status of the wild populations, however, stands in stark contrast to the thriving status of sturgeons in cultivation. Sturgeons are used to produce caviar, meat, and live fish for the exotic pet trade. Through escapees and unwanted releases alien sturgeons are introduced outside their natural ranges, where they may impact the environment and vulnerable native sturgeon populations. Here we present an in-depth inventory of alien sturgeon species in the river’s Rhine and Meuse delta and assess the risks for the native biodiversity, in light of reintroducing the European sturgeon. We based our study on (a) a 2021 inventory of farms, wholesalers, importers, and retailers (e.g., garden centers, pet shops), and consumers; (b) reports on spread of alien sturgeons (>3,000) obtained from anglers’ logbooks, biodiversity databases, dive websites and social media; (c) a literature study on hybridisation and competition between sturgeon species; and (d) an environmental risk assessment using the Harmonia+-protocol. In total, eleven alien Acipenseriformes have been recorded in the Rhine-Meuse delta. Although most (c. 2000) sturgeons were reported from angling ponds, more than 500 other sturgeons were found in 61 other lakes and ponds. In addition, c. 300 alien sturgeons occurred in hydrologically connected waters. Even though the alien sturgeon survival is promising for the reintroduction of European sturgeon, because of the potential hybridization hazard, competition, and transmission of parasites and diseases, strategies for their removal and prevention of further spread are discussed.