Towards an optimised strategy for reintroducing sea sturgeons (Acipenser sturio and A. oxyrinchus) to Europe

The aim of this paper is to identify an optimised strategy for the reintroduction of two anadromous sturgeon species to Europe: the critically endangered European sturgeon (Acipenser sturio) and the vulnerable Atlantic sturgeon (A. oxyrinchus). Restoration efforts began in the 1970s, followed by artificial rearing and release of A. sturio in rivers of the French Atlantic coast and North Sea since 1991, and artificial rearing and release of A. oxyrinchus in Baltic Sea rivers in 2006. This approach was based on the most recent geographical occurrences of both species but may no longer be a viable strategy. We deliver evidence that both species spawned in North Sea rivers into the twentieth century by analysing acipenserid remains and argue that additional factors need to be considered to determine which species has to be reintroduced at which locality. Factors include the increased international interest in sturgeon restoration, the scarcity of A. sturio that limits stocking possibilities and monitoring population developments, the risk of hybridisation and outbreeding of the genetically eroded A. sturio, the limited availability of suitable rivers due to habitat degradation, and the possible poleward (northward) shifts in suitable habitats due to climate change. This paper analyses the various factors and re-evaluates three alternative, theoretical strategies to determine their advantages and disadvantages: (1) prioritising the restoration of only the critically endangered A. sturio, (2) maintaining a strict north–south division of reintroductions for the two species, and (3) restoring a ‘mixed zone’ of sympatric occurrences in Northwest Europe, particularly in North Sea rivers. This re-evaluation emphasizes the need for scientific communities in Europe to closely collaborate in reintroducing sturgeon species.