Bulk cargoes make up some 80% of the total tonnage carried in seaborne trade, much of it transported through vessel pooling arrangements. Bulk pools have developed as the result of an increasing use of bulk carriers (decrease of tramp shipping), and the growing preference for freight contracts and other long-term shipping arrangements (contraction of the spot market) by the dominant shippers. In bulk shipping, the barriers to entry by developing countries are due more to the nature of the economic organization of chartering 'markets' or 'exchanges', and controls exercised by transnational corporations, than to the institutionalized grouping of shipowners in the liner conferences. Internally within the shipping industry in general, the concentration of power varies and is more pronounced in the cargo liner sector than in the bulk market. However this varies from dry bulk to liquid bulk, and in oil transport, for example, the oil companies play a restrictive role in the development of bulk shipping.