Abstract: The aquaculture growth required to meet increasing protein demand by a growing world population, predicted to reach 9billion people by 2050, is driving innovation in both siting and culture practice. Limited possibilities for expansion on land and in inshore coastal areas, and technological improvements in farming structures, have led to widespread interest in offshore aquaculture. A gilthead bream (Sparus aurata) model has been developed and integrated with existing shellfish models in the Farm Aquaculture Management System (FARM) model, in order to analyse various aspects of onshore and offshore aquaculture. The FARM model was used to compare the quantitative effects of finfish monoculture with Integrated Multi-Trophic Aquaculture (IMTA) in ponds, in terms of production, environmental externalities, and economic performance. Very clear benefits of IMTA could be seen in the comparison. The same approach was then applied to offshore culture, considering a combination of gilthead in cages and Pacific oyster (Crassostrea gigas) suspended from longlines. For offshore culture, the primary production and diagenesis modules of FARM were switched off, since there are no feedbacks from those processes to the farm area. Except in upwelling areas, the concentration of food drivers for filter-feeding shellfish falls markedly with distance from the shore―simulations with FARM suggest that in food-poor areas, co-cultivation of bivalves with fish can significantly improve shellfish production, and that the distribution of finfish can be optimised to reduce shellfish food depletion in the inner parts of the farm. We calculate the environmental benefits of IMTA both in terms of population-equivalents and the potential for nutrient credit trading. The finfish model integrated in FARM deals explicitly with the metabolic energy cost of opposing offshore currents in cage culture, and a model analysis suggests that gilthead cultivation at current speeds in the range of 0.1 to 0.5ms−1 is optimal. The lower end of that spectrum probably translates into a greater deviation from the fillet quality obtained from wild fish, and above that limit there is a rapid increase of the feed conversion ratio (FCR) and cultivation becomes financially unattractive.