In this paper, we present a simple theoretical, steady-state equilibrium, predator-prey model for the joint management of marine mammals and a fish species. As an empirical case, we choose cod and grey seals in the Nordic-Baltic Sea countries, and several benefits and costs related to the latter are considered. We show that the optimal grey seal population is much lower than the actual population, and this result is robust to variations in relevant parameter values. This result can be explained by the fact that the profit from harvesting cod is much higher than the net benefits from grey seals.