A global spatial analysis based on biophysical modelling identifies that vast swathes of the ocean are suitable for marine aquaculture development.
There is a growing sense of ‘ocean optimism’ — that economic development involving the oceans can lead to improved human well-being. Yet there is a simultaneously increasing concern among scientists, policymakers and the private sector about the declining health of our oceans1,2. The recent United Nations Ocean Conference (https://oceanconference.un.org/) on Sustainable Development Goal 14 (life below water) encompassed both these somewhat contradictory perspectives. Writing in Nature Ecology & Evolution, Gentry et al.3 add insight to this scenario, by presenting prospects for areal expansion of ocean aquaculture.
Existing and emerging challenges facing food production on land are indeed providing incentives for expansion into the oceans3,4,5. Food production on land is a major driver of global environmental change, specifically affecting the two planetary boundaries that have already been crossed: biodiversity loss and altered phosphorus and nitrogen cycles6. Agriculture also accounts for 70% of global freshwater consumption, contributes almost 24% of global greenhouse gas emissions3, and today occupies approximately 40% of the Earth’s surface5.
Meanwhile, the demand for food, and in particular animal protein, is expected to increase substantially in coming decades. Seafood is likely to play a particularly important role, owing to its health benefits and potentially smaller environmental footprint7,8. Any significant growth in food supply from the oceans must, however, come from an expansion of aquaculture because capture fisheries can undergo only incremental increases in supply at best9. Gentry et al. add to earlier work10 on spatial challenges for marine aquaculture by providing a necessary first step towards understanding constraints related to how marine aquaculture could expand in the ocean realm.
Gentry and colleagues’ model uses the temperature tolerance of available marine aquaculture species to estimate the location-specific growth potential for fish and bivalve production in coastal waters of less than 200 metres depth. Model constraints include the suitability of areas and additional environmental conditions, such as allowable depth, oxygen concentration, phytoplankton densities and use restrictions from competing activities (for example, the oil and shipping industries and marine protected areas). The authors acknowledge that their model mainly takes biophysical parameters into consideration, implying that national and regional politics and governance will limit or enhance aquaculture’s growth potential. Although the model outcome indicates that appropriation of only a small fraction of available ocean space has the potential to partially solve our food challenge, the extent to which such aquatic food production is connected to both aquatic and terrestrial ecosystems and resources needs careful consideration5.
The diverse aquaculture sector has varying contributions to environmental impacts and social benefits7. About 44 species make up 90% of total global production5; only 8.5% of the total originates from marine finfish species (including salmon) and around 20% consists of marine molluscs (such as mussels and oysters)11. An important difference between these farmed species groups is that marine finfish species are fed, whereas extractive species such as mussels feed on naturally occurring plankton and organic matter.
Gentry and colleagues’ modelling of potential mussel production uses primary production as a proxy for available food. Expansion of these extractive species in the ocean would ease the pressure on feed resources, resulting in a net addition of protein to the global food portfolio. However, these cultures will probably compete for the same resource base (plankton and organic matter) that capture fisheries and overall ecosystem structure depend on12. The extent of this hypothetical side-effect of large-scale open water mussel farming is uncertain, but as fisheries contribute substantially to food security, especially in many low-income countries13, it may need to be considered for the large-scale instalments that Gentry and colleagues anticipate. One should bear in mind that fishing can in itself be a substantial source of greenhouse gas emissions, especially for large demersal and pelagic fish species and invertebrates14, and can also contribute to biodiversity loss through overfishing and habitat destruction15.
Feed development will ultimately constrain whether any fed aquaculture can expand sustainably in the future10,11. Including this factor as a superimposed layer in Gentry and colleagues’ sea-space model would reveal that feed availability and feed costs will prevent further expansions of mariculture long before any ocean space limitations are reached. Current aquaculture production of fish from off-shore ocean systems is still insignificant but is dominated by species such as salmon, groupers, barramundi and cobia, which all require high-quality protein feeds based on fish resources and increasingly agriculture crops, such as soy. As innovations in novel feed ingredients are rapidly emerging, reducing competition with human food resources will be key for sustainability.
Gentry et al. join others4 in pointing out that capital and operating costs can be high for offshore aquaculture. These costs influence species choice and may also affect who will have access to the products of ocean aquaculture. Large-scale operations and better animal survival from improved water quality further off-shore may help to offset cost disadvantages.
The work of Gentry and colleagues shows that space is currently not a limiting factor for the expansion of oceanic aquaculture. Climate change may alter this, but the big challenges facing the near-term expansion of the aquaculture sector lie in the development of sustainable feeds, and in better understanding how large-scale ocean farming systems interact with ecosystems and human well-being.