In this paper fishery economics and recent theory of international trade under imperfect competition are integrated. For this purpose, international commertial fishing in an open-sea is formulated and its dynamic properties are analyzed under the condition of imperfect competition with two trading countries. The fish stock may become extinct or converge to a steady state level. A sufficient condition is derived for the fish stock to converge to the steady state in relation to the parameters of the fish's biological growth equation, of the harvesting costs and of the demand functions for the fish in the two countries.