Past, Present and Future of the Allis Shad, Alosa Alosa, Assessed By a Two Steps Population Dynamics Modelling Approach

Around 2000, the anadromous allis shad population in the Garonne basin (southwest France) was the largest in Europe. During the first decade of the 21^st century, catches dramatically declined and led to a fishery moratorium in 2008. In a first modelling study, the high estuarine mortalities combined with a demographic Allee effect in the reproduction dynamics were identified as key mechanisms in explaining the population collapse and hampering the stock recovery under the moratorium. At the European scale, these results were then used to parameterize the GR3D (Global Repositioning Dynamics of Diadromous fish Distribution) model. GR3D is a new individual-based model designed to assess both the persistence of any diadromous fish and the evolution of their distribution in a context of climate change. From the observed 1900 allis shad's distribution, we simulated with GR3D the evolution of the distribution until 2100. Across the 1900-2000 period, a substantial gap was noted between the observed and the predicted distributions that should be linked to the increasing anthropogenic forcing. Simulations over the 21^st century strongly suggested that this species could be able to successfully cope with the ongoing climate warming when taking into account its life history characteristics and its dispersal abilities.