Predation risk in stocking programs: case study on a critically endangered diadromous species, the European sturgeon Acipenser sturio

Juveniles produced in hatcheries for stocking purposes can have compromised behavior compared to wild conspecifics, especially concerning inappropriate anti-predator responses. In this study, we focus on predation risk for Acipenser sturio, the last native sturgeon species of Western Europe. We first carried out a bibliographic study to identify potential predators along the migration journey from freshwater to marine environments. Then, predation risk was assessed for 3-month-old and 11-month-old juveniles, using two techniques: diet analysis of a top predator and telemetry tools. After the stocking of 3-month-old A. sturio, Silurus glanis diet was assessed using direct stomach content and molecular analysis of their feces via digital PCR. Eleven-month-old juveniles raised in captivity were equipped with acoustic predation tags and released in the wild to assess their predation rate during their downstream migration. Both experiments were conducted in two large rivers in the South West of France. Molecular analysis of S. glanis feces indicated a moderate predation on sturgeon (30% of the fecal samples), a level similar to that observed for Cyprinus carpio and Faxonius limosus. Eriocheir sinensis was detected in more than half of the samples, while Anguilla anguilla was consumed in 82% of the fecal samples. Alosa spp. were not detected in the feces analysis. No diadromous fish were detected in the direct stomach examination, which highlighted the usefulness of the molecular approach. Telemetry survey of older juveniles revealed a high predation rate in the Dordogne River, with more than 80% of the individuals released predated. In contrast, a moderate predation rate was observed in the Garonne River (14%), but 58% of the individuals considered non-predated reached the saline estuary in less than 3 days due to a peak of water flow the day following the release. In both rivers, predation occurred mainly within the 2 days of release, 86% and 75% of the predation events for Dordogne and Garonne Rivers, respectively. Our study demonstrates that predation poses a significant risk to juvenile A. sturio, particularly immediately after release. We further propose differential strategies to enhance early survival in stocking conservation programs.