By-catch is an unavoidable problem in fishery, and the occurrence of by-catch may lead to the phenomenon of discarding. Discarding of non-targeted species will not only result in the losses in economic, but may also damage the resource stock. As the interaction between species exists, the economic costs and the degree of damage derive from by-catch and discarding will become increasingly complicated. The research will start from the assumption of possible interactions between species, investigating that under different interactions between species, the possible influence to the optimal decision of fishermen and resource stocks.