Aquaculture is considered a viable alternative to wild fish, providing the means to feed an increasing human population and satisfy a greater demand for seafood. Aquaculture involves raising commercially important species of finfish and shellfish at high densities, in contained natural environments or artificial enclosures, fed by local photosynthetic production or external, controlled food supplies.
There are three kinds of aquaculture systems: extensive, semi-intensive and intensive. The level of intensity is determined by the amount of energy and human intervention required, and in the growth and lifecycle of the target species. Extensive aquaculture is considered more efficient and sustainable than intensive aquaculture, although both systems have considerable negative environmental impact.
Wild Atlantic Salmon
Salmo salar (Atlantic Salmon) are native to the Northern Hemisphere, occupying cool water, gravel-bottomed natural habitats (freshwater rivers, streams, lakes, open bays and coastal waters), in tributaries of the North Atlantic Ocean (see Wild-caught Salmon & Natural Astaxanthin under Solutions).
There are three groups of wild Atlantic salmon, representing the North American, European and Baltic stocks, but these have declined significantly, with some populations considered extinct in some regions, although this is likely due to a combination of factors (overfishing, water pollution, climate change). Most salmon eaten today is farmed, representing 70% of the global market. Atlantic salmon are one of the most intensively farmed seafood species in the world.
Estimates vary around how much wild fish it takes to produce 1kg of farmed salmon (various sources say between 1kg and 8kg), but it takes about 10kg of wild fish to produce 1kg of wild salmon (source unable to be verified). To feed farmed salmon, the wild fish must be caught, processed into fishmeal and fed to farmed salmon, a highly inefficient and unsustainable practice compared to the wild salmon simply eating the wild fish in its natural habitat.
Aquaculture consumes more fish than it produces, placing pressure on wild stocks and depleting wild species, an outcome inconsistent with the argument for an industry. To counteract this, farmed fish are fed poultry by-products, or genetically modified grain.
Impacts of intensive aquaculture
Intensive farming requires large numbers of salmon to be confined in a smaller area than would normally be available to a wild population. Farmed salmon excrete waste in their enclosed cages, and these localized, excessively high levels of nutrient loading may cause eutrophication and uncontrolled algal blooms.
Fish excrement and feed waste are carried by ocean currents into the marine environment, creating organic waste, contaminating surrounding water bodies, polluting benthic environments and killing marine life. Caging high densities of fish in enclosed spaces leads to disease and increased parasitic infestations that must be controlled by antibiotics, which transfer to the flesh of farmed fish.
A study published in Nature by Reimer et al. (2016) found that half of all farmed fish have deformed otoliths. Otolith deformity affects predator detection, prey location and navigation. When farmed salmon escape into the environment, they out-compete wild salmon for food and habitat, spread disease and inter-breed. Yet farmed salmon have survival rates 10-20 times lower than wild salmon (ibid.). If farmed salmon breed with wild salmon, how do these physiological abnormalities and lower survival rates transfer genetically and affect offspring?
Destruction of ‘blue carbon’ ecosystems
Mangrove forest loss along coastlines is an environmental impact of the aquaculture industry. Indonesia has lost 60% of mangrove forest in the last 30 years, mainly due to aquaculture. Mangrove deforestation in Vietnam is driven by shrimp farming. An estimated 3,000,000 hectares of wetlands and mangrove forests have been cleared for artificial shrimp ponds.
Commercial shrimp aquaculture has environmental, social and economic costs. Commercial shrimp farming in developing countries is a cash crop, not a subsistence crop. Any profits generated are exported, devastating local communities and economies, degrading coastlines, and creating environments unable to support fishing, farming or forestry.
Wild shrimp larvae are caught to stock artificial ponds using methods resulting in high levels of bycatch (see Bycatch & Incidental Capture of Cetaceans under Impacts), damage to marine habitats and depletion of local fisheries. Chemicals are used to fight disease in shrimp due to over-crowding in enclosed spaces, creating organic waste and contaminating water bodies.
Acoustic harassment devices and cetaceans
Farming fish using open-net sea cages contributes to the deaths of marine mammals and seabirds, an ‘externality’ in the aquaculture industry. Seals, dolphins and seabirds are natural predators of salmon, and may become entangled in the nets, drowning if they cannot escape, or being shot (often legally) by salmon farmers.
Acoustic harassment devices (AHDs) deter predators by emitting a high frequency, intense noise in ranges that are inaudible to fish, but can be heard by marine mammals and seabirds. This kind of prolonged exposure to AHDs or acoustic disturbance can affect cetacean hearing and exclude some cetacean species from vital habitat.
An experimental AHD deployed in the Bay of Fundy, Canada (the location of many salmon aquaculture sites utilising AHDs as seal deterrents), collected data on the relative abundance and movement of Phocoena phocoena (Harbour Porpoise) in relation to an active and an inactive AHD. The results showed that densities of this species decreased in the vicinity of an active acoustic harassment device (Johnston 2002).
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Borthwick, Mark (2020). Welfare issues in farmed Atlantic Salmon. Fish Welfare Initiative. https://www.fishwelfareinitiative.org/salmon-welfare
Dave W Johnston, The effect of acoustic harassment devices on harbour porpoises (Phocoena phocoena) in the Bay of Fundy, Canada, Biological Conservation, Volume 108, Issue 1, 2002, Pages 113-118, ISSN 0006-3207, https://doi.org/10.1016/S0006-3207(02)00099-X
NOAA Fisheries (2021). New England and Mid-Atlantic Geographic Strategic Plan 2020-2023. https://media.fisheries.noaa.gov/dam-migration/ne-ma_geographic_strategic_plan_implementation_plan.pdf
Reimer, T., Dempster, T., Warren-Myers, F. et al. High prevalence of vaterite in sagittal otoliths causes hearing impairment in farmed fish. Sci Rep 6, 25249 (2016). https://doi.org/10.1038/srep25249
Thorstad, E.B., Fleming, I.A., McGinnity, P., Soto, D., Wennevik, V. & Whoriskey, F. 2008. Incidence and impacts of escaped farmed Atlantic salmon (Salmo salar) in nature. NINA Special Report 36. 110 pp. https://www.fao.org/3/aj272e/aj272e.pdf