Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Journal of Theoretical Biology 837-848 population dynamics biased random walk individual based model partial differential equation 2007 fdi:010040753 ENG Journal of Theoretical Biology 0022-5193 CC:0002489907-0023 4 10.1016/j.jtbi.2007.04.012 https://www.documentation.ird.fr/hor/fdi:010040753 https://www.documentation.ird.fr/intranet/publi/2007/10/010040753.pdf 247 Horizon (IRD) In this paper, we address the problem of modeling fish population movements. We first consider a description of movements at the level of individuals. An individual -based model is formulated as a biased random walk model in which the velocity of each fish has both a deterministic and a stochastic component. These components are function of a habitat suitability index, h, and its spatial gradient Vh. We derive an advection-diffusion partial differential equation (PDE) which approximates this individual- based model (IBM). The approximation process enables us to obtain a mechanistic representation of the advection and diffusion coefficients which improves the heuristic approaches of former studies. Advection and diffusion are linked and exhibit antagonistic behaviors: strong advection goes with weak diffusion leading to a directed movement of fish. On the contrary weak advection goes with strong diffusion corresponding to a searching behavior. Simulations are conducted for both models which are compared by computing spatial statistics. It is shown that the PDE model is a good approximation to the IBM. 036 ; 020