| Summary: | Formerly present in European coastal waters from the Black Sea to the North Sea, European sturgeon (Acipenser sturio) is now only present in the Gironde-Garonne-Dordogne basin and stocking operations are performed to restore this population. During their migration to reach the estuary, stocked fish have to cross a mud plug located in the upper part of the estuary, which presents severe hypoxic events during summer. The goal of this study was to assess the hypoxic tolerance of European sturgeon juveniles by recording behavioural indicators and opercular beat frequency (OBF). Loss of equilibrium appeared at 1.71± 0.05 mg.l-1 and 1.83 ± 0.06 mg.l-1 (mean±SE) at 20 and 25°C, respectively, and was significantly different from altered swimming and death. At 25°C, OBF was increased in both experimental and control groups compared to 20°C. Whatever the temperature, OBF exhibited a similar pattern, with a maximum at 3.8 and 3.3 mg.l-1 for 20 and 25°C, respectively. Loss of equilibrium determines the lower limit of O2 level to preserve the likelihood of fish survival. Behavioural results are discussed in relation with O2 levels recorded in the mud plug. OBF patterns led to the hypothesis that European sturgeon is probably an oxyregulator as other sturgeon species like Siberian sturgeon.
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