Where a fish stock straddles or migrates between country A's exclusive economic zone (EEZ) and country B's EEZ, or the high seas, vesting ownership rights in the stock with A does not ensure efficient harvesting of the stock. This problem arises in the case of migratory tuna stocks in the Western and Central Pacific Ocean (WCPO). Four species of tuna reside for only part of the year in the EEZs of coastal states, many of which are Pacific Island Countries (PICs). Most of the harvesting of the stocks is carried out by distant water fishing nations such as the USA, Japan, Taiwan, China and Korea. Problems arise for achieving efficiency and equity in the harvesting of the stocks by disparate countries. The problems are made more difficult by changes in the harvesting levels of one fleet affecting the rents of another fleet through changes in the age distribution of stock. These types of problem are under review by the Western and Central Pacific Fisheries Commission, formed in 2005. Results from an age-structured steady-state bioeconomic model are used to show: the changes in fleet rents and catches of tuna if all fleets form a cooperative grand coalition to deploy fishing effort to maximize rents over the WCPO; the likely non-stability of the grand coalition; and the inferior Nash Equilibrium outcomes if all fleets fish non-cooperatively to maximize their own rents.