Stocking programs are conducted in sturgeon species for conservation purposes to avoid their extinction. Juveniles produced in hatchery can present depleted behavioral capacity compared to wild conspecifics, especially unappropriated anti-predator responses.
In this study, predation risk was assessed for two juvenile stages of Acipenser sturio, the last native sturgeon species of Western Europe, thanks to two complementary techniques: diet analysis of a top predator and telemetry tools. Experiments were conducted in two large rivers (Dordogne, Garonne) in the South West of France. After the stocking of 3 months old juveniles, Silurus glanis diet was assessed using direct stomach content examination and molecular analysis of their feces via digital PCR targeting 6 potential preys. Molecular analysis of S. glanis feces indicated a moderate predation on sturgeon, detected in 30% of the fecal samples, a level similar to that observed for Cyprinus carpio and Faxonius limosus. Eriocheir sinensis was detected in 52% of the samples, while Anguilla anguilla was highly consumed (82 % of the fecal samples). Alosa sp. juveniles were also search but not detected in the feces analysis. No diadromous fish were detected in the direct stomach analysis, which highlighted the usefulness of the molecular approach. Eleven-month-old juveniles were equipped with acoustic predation tags and released in the wild to assess their predation rate during their downstream migration. A high predation rate in the Dordogne River with more than 80 % of the individuals predated before they reached the saline estuary was revealed. In contrast, a moderate predation rate was observed in the Garonne River but linked to a peak of water flow the day following the release, transporting rapidly the individuals towards the saline estuary.
Results are interpreted in the light of the literature review of predation risk along the continuum from freshwater to the sea for A. sturio.
 

predation,telemetry,digital PCR,Acipenser sturio