This work was made to study the Margaritifera margaritifera (L.) populations, threatened by a Small Hydroelectric Power plant- Hidroeléctrica das Trutas, located in the River Tuela, northeastern Portugal. The characterization of the current situation of the population of M. margaritifera in the area of influence of the Hidroeléctrica das Trutas was performed and a comparative analysis was made taking into account the Environmental Impact Assessment (EIA), developed in 2003. Furthermore, different measures were proposed to minimize the loss of habitat by the freshwater pearl mussel populations in the river Tuela.