This chapter aims to test theoretical models of the emergence and stability of cooperative foraging using empirical data of cooperative sail-fishing activities on Ifaluk Atoll. Evolutionary ecologists have been particularly interested in determining the importance of foraging efficiency in the evolution of cooperative foraging, since natural selection is expected to produce foraging strategies that maximize the gain rate in time and energy limited organisms. Where cooperative foraging has emerged, there has been considerable interest in determining the optimal number of foragers in a foraging acquisition group. The energetic costs of cooperative and solitary fishing were measured using the Energy Expenditure Prediction Program developed by the Center for Ergonomics at the University of Michigan. Solitary fishing activities occurred in the lagoon and were easily monitored because of their high visibility. Less time to invest in acquiring fishing skills is likely to have a greater impact on solitary fishing success than on cooperative fishing success, since solitary fishing is skill-intensive activity.