Szukana fraza: [Abstract = "For a system of nonlinear difference equations that models the dynamics of exploited biological populations, a locally optimal periodic solution is constructed. If this solution is unstable, a stabilizing feedback in the harvesting is introduced. The method is applied to an age\-structured population model in fishery as well as to a host\-parasitoid system for which the number of hosts and the number of introduced parasitoids should be minimized."]