This paper proposes a bioeconomic model destined to show how the human activity may alter the natural evolution of biological species. The model is formulated in term of optimal control theory, and then is transformed into augmented dynamic system by meas of the optimal choice of control variables resulting form the application of Pontryagin's maximum principle. In the case of species abundance, we prove that saddle-point stability of the original bioeconomic system is transferred to the augmented system under optimal choice of control variables.