This study was conducted to compare the effects of shrimp and rabbitfish in mono- and polyculture stocked at high biomass on production and environmental conditions in a mesocosm system. Shrimp (14 g) and/or rabbitfish (19 g) were stocked in four treatments with different density but with the same total biomass (236 g m−2), including shrimp monoculture (SM) (17 shrimp m−2), shrimp–fish polyculture (SF) (11 shrimp and 4 rabbitfish m−2), fish–shrimp polyculture (FS) (6 shrimp and 8 rabbitfish m−2) and fish monoculture (FM) (12 rabbitfish m−2). After 10 weeks of experiment, shrimp survival and biomass were low in the treatments where shrimp were dominant (SM, SF), while rabbitfish survival and biomass were high in all the treatments. Shrimp mortality was assumed to be related to an excess of the system carrying capacity (CC). Results suggested that CC is linked to shrimp biomass/density rather than the system eutrophication level. The ecosystem became heterotrophic as daily feed supply was beyond 7 g m−2 per day. This threshold corresponded to the environmental CC of the semi-intensive shrimp culture system. Under these conditions, the combination of high fish biomass and low shrimp biomass appeared as the most valuable in terms of system performances.