Cage aquaculture (mariculture using floating net cages) has expanded rapidly as a strategy for increasing seafood production without additional land conversion. However, the sustainability of this expansion is fundamentally constrained by the environmental carrying capacity (ECC) of receiving water bodies, the maximum biological load an aquatic ecosystem can assimilate without irreversible ecological degradation. This study examines ECC as a determinant of sustainable cage aquaculture development through a descriptive quantitative analysis of 31 peer-reviewed publications (2021–2023). Quantitative descriptive statistics were computed for thematic distribution, annual publication frequency, and key environmental performance indicators including nutrient emission rates, hydrodynamic flushing thresholds, benthic impact metrics, and ecosystem service values of integrated systems. Results show that cage aquaculture generates 25–55 kg nitrogen and 5–15 kg phosphorus per tonne of fish produced, with consequences ranging from localised benthic anoxia to system-wide algal blooms when ECC is exceeded. Offshore sites with currents above 0.1 m/s provide 3–8 times greater self-purification capacity than nearshore systems, substantially elevating ECC. Integrated multitrophic aquaculture (IMTA) with bivalves and macroalgae removes 5–15 kg N and 8–20 kg N per tonne fish equivalent, respectively, improving net environmental performance. A management priority analysis identified hydrodynamic-based site selection and nutrient budget modelling as the two highest-priority interventions (score: 25 each). This study concludes that sustainable cage aquaculture development requires ECC-grounded spatial planning, adaptive monitoring, and integration of non-fed species to close nutrient loops.