Offshore aquaculture, also known as open water aquaculture or open ocean aquaculture, is an emerging approach to mariculture (seawater aquafarming) where fish farms are positioned in deeper and less sheltered waters some distance away from the coast, where the cultivated fish stocks are exposed to more naturalistic living conditions with stronger ocean currents and more diverse nutrient flow.Naylor, R., and Burke, M. (2005) "Aquaculture and ocean resources: raising tigers of the sea" Annual Review of Environmental Resources, 30:185–218.Sturrock H, Newton R, Paffrath S, Bostock J, Muir J, Young J, Immink A and Dickson M (2008) Part 2: Characterisation of emerging aquaculture systems In: Prospective Analysis of the Aquaculture Sector in the EU, European Commission, EUR 23409 EN/2. . Existing "offshore" developments fall mainly into the category of exposed areas rather than fully offshore. As maritime classification society DNV GL has stated, development and knowledge-building are needed in several fields for the available deeper water opportunities to be realized.
One of the concerns with inshore aquaculture, which operate on more sheltered (and thus calmer) shallow waters, is that the discarded nutrients from unconsumed feeds and feces can accumulate on the farm's seafloor and damage the benthic ecosystem, and sometimes contribute to algal blooms. According to proponents of offshore aquaculture, the wastes from aquafarms that have been moved offshore tend to be swept away and diluted into the open ocean. Moving aquaculture offshore also provides more ecological space where production yields can expand to meet the increasing market demands for fish. Offshore facilities also avoid many of the conflicts with other marine resource users in the more crowded inshore waters, though there can still be user conflicts offshore.
Critics are concerned about issues such as the ongoing consequences of using antibiotics and other drug pollutions, and the possibilities of cultured fish escaping and spreading disease among wild fish.
Background Aquaculture is the most rapidly expanding food industry in the worldSkladany, M., Clausen, R., Belton, B. (2007) "Offshore aquaculture: the frontier of redefining oceanic property" Society and Natural Resources, 20: 169–176. as a result of declining wild fisheries stocks and profitable business. In 2008, aquaculture provided 45.7% of the fish produced globally for human consumption; increasing at a mean rate of 6.6% a year since 1970.FAO. (2010) The State of World Fisheries and Aquaculture Rome. FAO, 2010, 197p.
In 1970, a National Oceanic and Atmospheric Administration (NOAA) grant brought together a group of oceanographers, engineers and marine biologists to explore whether offshore aquaculture, which was then considered a futuristic activity, was feasible.Hanson, J. A. (Ed.) (1974) Open sea mariculture: Perspectives, problems and prospects. Stroudsburg, PA: Dowden, Hutchinson & Ross. In the United States, the future of offshore aquaculture technology within federal waters has become much talked-about.Rubino, Michael (Ed.) (2008) Offshore Aquaculture in the United States: Economic Considerations, Implications & Opportunities U.S. Department of Commerce; Silver Spring, MD; USA. NOAA Technical Memorandum NMFS F/SPO-103. 263p. As many commercial operations show, it is now technically possible to culture finfish, shellfish, and seaweeds using offshore aquaculture technology.
Major challenges for the offshore aquaculture industry involve designing and deploying cages that can withstand storms, dealing with the logistics of working many kilometers from land, and finding species that are sufficiently profitable to cover the costs of rearing fish in exposed offshore areas.Stickney, R. R., Costa-Pierce, B., Baltz, D. M., Drawbridge, M., Grimes, C., Phillips, S., Swann, D. L. (2006) "Towards sustainable open ocean aquaculture in the United States" Fisheries, 31(12): 607–610.
Technology To withstand the high energy offshore environment, farms must be built to be more robust than those inshore.Cressey, D. (2009) "Future fish". Nature, 458: 398–400. However, the design of the offshore technology is developing rapidly, aimed at reducing cost and maintenance.Mann, C. C. (2004) "The bluewater revolution" Wired Mag. 12.05.
While the ranching systems currently used for tuna use open net cages at the surface of the sea (as is done also in salmon farming), the offshore technology usually uses submersible cages. These large rigid cages – each one able to hold many thousands of fish – are anchored on the sea floor, but can move up and down the water column. They are attached to buoys on the surface which frequently contain a mechanism for feeding and storage for equipment. Similar technology is being used in waters near the Bahamas, China, the Philippines, Portugal, Puerto Rico, and Spain. By submerging cages or shellfish culture systems, wave effects are minimized and interference with boating and shipping is reduced. Offshore farms can be made more efficient and safer if remote control is used,Finfish aquaculture Atlantic Marine Aquaculture Center, University of New Hampshire. Retrieved 7 October 2011. and technologies such as an 18-tonne buoy that feeds and monitors fish automatically over long periods are being developed.
Existing offshore structures Multi-functional use of offshore waters can lead to more sustainable aquaculture "in areas that can be simultaneously used for other activities such as energy production". Operations for finfish and shellfish are being developed. For example, the Hubb-Sea World Research Institutes' project to convert a retired oil platform 10 nm off the southern California coast to an experimental offshore aquaculture facility.Carlsbad hatchery group proposes offshore aquaculture on oil platform North County Times, 19 June 2005. The institute plans to grow mussels and red abalone on the actual platform, as well as white seabass, striped bass, bluefin tuna, California halibut and California yellowtail in floating cages.
Integrated multi-trophic aquaculture Integrated multi-trophic aquaculture (IMTA), or polyculture, occurs when species which must be fed, such as finfish, are cultured alongside species which can feed on dissolved nutrients, such as seaweeds, or organic wastes, such as suspension feeders and deposit feeders.Troell, M., Joyce, A., Chopin, T., Neori, A., Buschmann, A. H., Fang, J. (2009) "Ecological engineering in aquaculture – Potential for integrated multi-trophic aquaculture (IMTA) in marine offshore systems" Aquaculture, 297: 1–9. This sustainable method could solve several problems with offshore aquaculture. The method is being pioneered in Spain, Canada, and elsewhere.
Roaming cages Roaming cages have been envisioned as the "next generation technology" for offshore aquaculture. These are large mobile cages powered by thrusters and able to take advantage of ocean currents. One idea is that juvenile tuna, starting out in mobile cages in Mexico, could reach Japan after a few months, matured and ready for the market. However, implementing such ideas will have regulatory and legal implications.
Space conflicts As oceans industrialise, conflicts are increasing among the users of marine space.Buck BH, Krause G and Rosenthal H (2004) "Extensive Open Ocean Aquaculture Development Within Wind Farms in Germany: The Prospect of Offshore Co-Management and Legal Constraints" Ocean & Coastal Management, 47: 95–122 This competition for marine space is developing in a context where natural resources can be seen as publicly owned. There can be conflict with the tourism industry,Martinez-Cordero FJ (2007) "Socioeconomic Aspects of Species and Systems Selection for Sustainable Aquaculture" pp. 225–239. In: Leung P, Lee C and O'Bryen P (Eds.) Species and system selection for sustainable aquaculture, John Wiley & Sons. . recreational fishers,Grimes J (1999) "Competition for Common Property Space: New Hampshire's Recreational and Open Ocean Aquaculture. Development" Proceedings of the 1999 Northeastern Recreation Research Symposium, GTR-NE-269, pp. 378–383. wild harvest fisheries and the siting of marine renewable energy installations. The problems can be aggravated by the remoteness of many marine areas, and difficulties with monitoring and enforcement.Harte MJ, Campbell HV and Webster J (2010) "Looking for a safe harbor in a crowded sea: Coastal space use conflict and marine renewable energy development" In: Shifting Shorelines: Adapting to the Future, The 22nd International Conference of The Coastal Society. On the other hand, remote sites can be chosen that avoid conflicts with other users, and allow large scale operations with resulting economies of scale. Offshore systems can provide alternatives for countries with few suitable inshore sites, like Spain.
Ecological impacts
The ecological impacts of offshore aquaculture are somewhat uncertain because it is still largely in the research stage.
Many of the concerns over potential offshore aquaculture impacts are paralleled by similar, well established concerns over inshore aquaculture practices.Salmon Aquaculture Dialogue "State of Information" Reports WWF, 2004.
Pollution One of the concerns with inshore farms is that discarded nutrients and feces can settle on the seafloor and disturb the benthos.Black KD, Hansen PK and Holmer M (2004) Working Group Report on Benthic Impacts and Farm Siting In: Salmon Aquaculture Dialogue, WWF. The "dilution of nutrients" that occurs in deeper water is a strong reason to move coastal aquaculture offshore into the open ocean.Simpson, S. (2011) "The blue food revolution" Scientific American, 304(2): 54–61. How much nutrient pollution and damage to the seafloor occurs depends on the feed conversion efficiency of the species, the flushing rate and the size of the operation. However, dissolved and particulate nutrients are still released to the environment. Future offshore farms will probably be much larger than inshore farms today, and will therefore generate more waste. The point at which the capacity of offshore ecosystems to assimilate waste from offshore aquaculture operations will be exceeded is yet to be defined.
Wild caught feed As with the inshore aquaculture of carnivorous fish, a large proportion of the feed comes from wild forage fish. Except for a few countries, offshore aquaculture has focused predominantly on high value carnivorous fish. If the industry attempts to expand with this focus then the supply of these wild fish will become ecologically unsustainable.
Fish escapes The expense of offshore systems means it is important to avoid fish escapes. However, it is likely there will be escapes as the offshore industry expands. This could have significant consequences for native species, even if the farmed fish are inside their native range. Submersible cages are fully closed and therefore escapes can only occur through damage to the structure. Offshore cages must withstand the high energy of the environment and attacks by predators such as sharks. The outer netting is made of Spectra – a super-strong polyethylene fibre – wrapped tightly around the frame, leaving no slack for predators to grip. However, the fertilised eggs of cod are able to pass through the cage mesh in ocean enclosures.Bekkevold, D., Hansen, M., Loeschcke, V. (2002) "Male reproductive competition in spawning aggregations of cod (Gardus morhua L.)" Molecular Ecology, 11: 91–102.
Disease