| Abstract |
The allis shad Alosa alosa is an anadromous fish species whose populations have shown an important decline throughout the last decades. The main causes put forward to explain this decline are overfishing, habitat degradation and loss, connectivity breaks on migratory paths, and climate change. In particular, the early stages of life of A. alosa are exposed to numerous pressures (search for prey, risk of predation, physical and chemical connectivity breaks, physiological adaptation related to osmoregulation...) which can have an influence on the success of their continental phase and downstream migration. However, this critical phase is very little documented. Given the difficulties to identify and capture allis shad in the wild, ex-situ experiments have been carried out. Young allis shad were reared during their first three months of life in semi-natural conditions (artificial rivers) offering different habitats. The habitat selection and the behaviour of juvenile were analysed basing on direct observations and video monitoring. New information was also acquired on their feeding and their movements in freshwater. Another experiment was carried out in order to assess the tolerance of those organisms to combined stresses of temperature and hypoxia, based on behavioural indicators. This work provides new knowledge on the ecology of the young stages of life of the species and allows discussion on its status, particularly in the Gironde-Garonne-Dordogne watershed where no clear signs of recovery have been detected for the local population despite a fishery moratorium adopted in 2008. This information would be of interest to improve management practices and to anticipate trends in shad populations in the current context of global change. |