An attempt is made to model the eutrophic ecosystem of Lake Aydat in the Massif Central, France, with emphasis on the two dominant fish species, perch (Perca fluviatilis) and roach (Rutilus rutilus). The preliminary model raises interesting questions of trophic efficiencies and food chain structure. A better understanding of the functioning of the ecosystem has been reached with this model, which includes extraordinary long food chains (of up to nine trophic levels).