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Résumé; Allis shad (Alosa alosa) populations showed important decline among last decades, and it is assumed that the species' distribution area has restricted. In some watersheds, the status of the species is preocupating, however this remains poorly explained. In the Gironde-Garonne-Dordogne system (SW France), its decline leads to the adoption of a fishery moratorium in 2008. Despite this measure, local populations don't show signs of a recovering. One hypothesis consists in the unefficiency of the recruitment process: indeed, young-of-the-year Allis shad are exposed to many pressures during their downstream migration, such as hypoxia (low water oxygen concentration). In the Gironde estuary, hypoxic events are frequent in summertime, and can consist in episodes of several days with water O2 saturations under 30%. In addition, hypoxic events are thought to be even more severe and frequent in the future, according to most of climate change scenarios. Such low O2 availability could threaten young Allis shad, and as a consequence, this could condition their migration success. To our knowledge, if authors have suggested that 3-months old Allis shad show lower resistance to hypoxia at higher temperature, the abilities of younger developmental stages (larval and migrating stages) have never been studied. Here we meant to describe tolerance range of 10, 30, 60 and 90-days old Allis shad to oxythermic stress, by conducting behavioural analysis at individual scale in decreasing water oxygen saturation conditions (from 100 to 30%), at temperatures 18, 22, or 26°C. By this study, we aimed to bring useful information on the importance of oxythermic conditions for Allis shad in both larvae reintroduction purposes and juvenile migration success.

Résumé; Allis shad (Alosa alosa) populations have shown an important decline throughout thelast few decades, and it is assumed that the species’ distribution area has been restricted. In the Gironde-GaronneDordogne system (GGD, SW France), its decline leads to the adoption of a fishery moratorium in 2008. Despite this measure, locally no signs of recovery are observed. One hypothesis consists of the inefficiency of the recruitment process: young-of-the-year Allis shad are exposed to many pressures during their downstream migration, such as hypoxia. In the GGD system, hypoxic events are frequent in summertime (periods of several days with water O2 saturations under 30%). Those events are thought to become more severe andfrequent according to most climate change scenarios. Such low O2 availability could threaten young Allis shadby altering their migration success. If authors have suggested that 3-months old Allis shad show lower resistance to hypoxia at highertemperature, the abilities of younger development stages have never been studied. Here we meant to describe tolerance range of 10, 30, 60 and 90-days oldAllis shad to oxythermic stress by conducting behavioural analysis at an individual scale in decreasing water oxygen saturation conditions (from 100 to 30%), at four temperatures (18, 22, 26°C for 10, 30 days-old ; 22, 26, 28°C for 60, 90 days-old). By this study, we aimed to bring useful information on the importance of oxythermic conditions for Allis shad in both larvae reintroduction purposes and juvenile migration success