In Barents Sea, there are especially three species of zooplankton preying fish which have been of great commercial value: the Northeast Arctic (NEA) cod (Gadus morhua), capelin (Mallotus villosus) and Norwegian spring-spawning (NSS) herring (Clupea harengus). There is a prey-predator relationship between these three species, that is, capelin is predated by the juvenile herring and cod, whereas the cod may also feed on juvenile herring. On the other hand, the fishmen harvest these three species to gain profit as many as possible. Therefore, the harvesting of these three species has both biological and economic value. This thesis seeks to find an optimal policy of harvesting these three species so as to maximize the net present value.