In the Eastern Tropical Pacific (ETP), the operation mode of purse-seine fishing can be divided into dolphin sets and non-dolphin sets. Dolphin sets can catch more yellowfin tuna, but cause a lot of dolphin deaths. On the contrary, non-dolphin sets can catch more skipjack tuna, but discard a lot of tuna. Therefore, the choice of the operation mode to the ETP purse-seine fishery is very influential. If the fishermen use more dolphin sets, they are able to catch high quantity and more valuable yellowfin tuna. Conversely, if the fishermen use more non-dolphin sets, they will not hurt dolphins. But the tuna fish caught by this operation mode is smaller in size and the discarding rate is extremely high which will affect the overall profits of the fishermen. The research will investigate how the fishermen make the choice of the operation mode in the dilemma between dolphin sets which cause dolphins-bycatch and non-dolphin sets which result in tuna-discarding, In addition, if managers can ban dolphin sets or non-dolphin sets to maximize the profit of the fishermen and maintain the ecological balance between tuna and dolphins.