The marine aquaculture sector is growing rapidly. offshore aquaculture installations have been drawing increasing attention from researchers, industry and policy makers as a promising opportunity for large-scale expansion of the aquaculture industry. Simultaneously, there has also been increased interest in both land-based and nearshore aquaculture systems which combine fed aquaculture species (e.g. finfish), with inorganic extractive aquaculture species (e.g. seaweeds) and organic extractive species (e.g. suspension- and deposit-feeders) cultivated in proximity. Such systems, described as integrated multi-trophic aquaculture (IMTA), should increase significantly the sustainability of aquaculture, based on a number of potential economic, societal and environmental benefits, including the recycling of waste nutrients from higher trophic-level species into production of lower trophic-level crops of commercial value. Several of the challenges facing IMTA in nearshore environments, are also relevant for offshore aquaculture: moreover, the exposed nature of the open ocean adds a number of technical and economic challenges. A variety of technologies have been developed to deal with these constraints in offshore environments, but there remains a number of challenges in designing farm sites that will allow extractive species (e.g. seaweeds and shellfish) to be integrated in fed aquaculture systems and be able to withstand the strong drag forces of open oceans. The development of offshore IMTA requires the identification of environmental and economic risks and benefits of such large-scale systems, compared with similarly-scaled monocultures of high trophic-level finfish in offshore systems. The internalizing of economic, societal and environmental costs of finfish monoculture production by the bioremediative services of extractive species in IMTA offshore systems should also be examined and analyzed. The results of such investigations will help determine the practical value of adopting the IMTA approach as a strategy for the development of offshore aquaculture. (c) 2009 Elsevier B.V. All rights reserved.