Game theory offers an alternative approach to standard means of resource assessment which can be of value in the definition and assessment of policy options for competing parties. Using a Nash co-operative game it has been possible in this paper to show the significance of fishing strategies for southern bluefin tuna which take into account the age distribution of the catch. Assuming Japan and Australia act according to a cooperative game then the optimal fishing strategy was found to involve Australia avoiding the fishing of the very young cohorts and Japan taking a moderate catch in subsequent older age classes but not the oldest of the age classes. Sub-optimal strategies were also presented and evaluated along with an indication of the level of biomass associated with different strategies. It was concluded that harvesting policies need to be developed both according to weight of fish harvested and age class.