Review: The environmental and economic impacts of salmon aquaculture

A brief overview of salmon aquaculture

The aquaculture of fish has a long history as a major source of food for humans, and has been practiced worldwide for centuries. Its origins are thought to be in Asia, with China being regarded as the ‘ancestral home of aquaculture’. While aquaculture in general, is still dominated by Asia (China alone accounts for 70% of world aquaculture (FAO, 2002)), the world’s two leading producers of cultured salmon (salmonids) are Norway and Chile accounting for 33 and 31 percent of world production (FAO, 2013).

Although various families of fish have been farmed throughout human history, the practice of salmon becoming an intensively farmed food product began during the 1960s as a result of increased demand for the fish. This began when sea cage culture was first used in Norway to raise Atlantic salmon to marketable size. The success of this practice in Norway lead to the development of salmon farms in Scotland, and in time, Ireland, Canada, the North Eastern seaboard of the USA, as well as Chile and Australia. The family Salmonidae includes a variety of species that fall into two main categories: Atlantic species and Pacific species. The sole species of Atlantic salmon (Salmo salar) accounts for the majority of salmon available on the world market, with more than 99% of Atlantic salmon products available coming from aquaculture production (CSAFE, 2006) . Pacific Salmon species include Sockeye salmon (oncorhynchus nerka), Chinook salmon (oncorhynchus tshawytscha), pink or Humpback salmon (oncorhynchus gorbuscha) and Coho salmon (oncorhynchus kisutch) (Montgomery 2003). In 1980, farmed salmon accounted for less than 1% of the  world supply of salmon, yet by 1998, farmed salmon surpassed the total of wild-caught salmon (Abbors 2000) and is now a $5.4 billion industry that generates almost two million metric tonnes of farmed salmon annually (WWF, 2002). Commercial aquaculture is increasing worldwide at an annual rate of 10% (Walton 2002), and with this intensification of salmon aquaculture, a variety of problems have arisen. The environmental impacts of salmon aquaculture have garnered significant attention from the scientific community as well as conservationist groups. The implications of this increasing development of salmon aquaculture include damaging effects to the surrounding environment as well as significant economic impacts.


The environmental and economic impacts

The environmental impacts attributed to salmon aquaculture are numerous. Many of these problems arise due to the conditions in which the salmon are cultured, with often up to tens of thousands of salmon enclosed in pens for months at a time, offering ideal conditions for the spread of diseases and parasites throughout the enclosed population.

Parasites – Sea lice

The term Sea Lice encompasses 559 species of ectoparasitic copepods belonging to the family Caligidae. Sea lice are defined by their parasitic characteristics on fish and have proved to have significant negative impacts on fish health, both farmed and wild. The species Lepeophtheirus salmonis has a high specificity for salmonids and the presence of the species has proved a major challenge in maintaining the health of salmon farms.

Parasitic sea lice

Parasitic sea lice

Cultured salmon living in such close proximity provide optimal conditions for the transmission of sea lice between individuals, often resulting in a large percentage of salmon farm populations carrying the parasites. Sea lice attach to the host fish and feed upon the blood and mucus of the salmon which can cause substantial changes in mucus consistency and also damage to the epithelium resulting in loss of blood/fluids, electrolyte changes, and cortisol release (Ross et al, 2000). These factors can significantly decrease the immune responses of salmon and make them susceptible to other diseases, also reducing growth and performance (Ross et al, 2000) thus affecting salmon mortality. A primary concern is the transmission of sea lice from farmed salmon to wild salmon populations. Evidence put forth by Krkosek et al, (2007) revealed that sea louse infestation of wild juvenile pink salmon (Oncorhynchus gorbuscha) on the pacific coast of Canada, was a result of transmission via infected farmed salmon, and caused a decline in pink salmon populations, with louse induced mortality of the species being 80%. Krkosek et al theorised that if outbreaks of sea lice continue within pink salmon populations, ‘a 99% collapse in population abundance is expected in four salmon generations’. Parallel infestations of wild salmon have occurred in regions of close proximity to salmon farms in Ireland, Scotland, Norway, British Columbia, and Chile (Costello, 2009). The estimated cost of sea louse infestation to the world industry is €300m a year, taking into account economic losses resulting from decline in fish stocks and the costs controlling sea lice.

The infestation of sea lice in salmon farms have proved significantly problematic in challenging the success of salmon farming and various techniques have been developed by salmon farmers to control these infestations. The separation of year classes within the salmon farms, whereby juvenile lice-free salmon (smolts) are prevented from being in contact with mature lice-infested salmon, prevents the transmission of lice to juvenile populations (Watershed Watch Salmon Society, 2004). The use of Cleaner wrasse (Labridae) in Norwegian salmon farms has shown to be effective in lessening the abundance of sea lice in the farms. Around 5 million wrasse are stocked annually in Norway and a study conducted by Treasurer et al, (2002) concluded that salmon in cages where cleaner wrasse were present had only 1-8 lice attached over the first year in the cage, compared with up to 40 lice per fish in cages absent of wrasse. Another prevention method used is fallowing, which involves the removal of all farmed salmon from a farm and leaving the pens empty for one production cycle (two years), thus breaking the cycle of sea lice and other disease infestation in that farm. Husbandry is also key in prevention of lice infestation, through the maintenance and cleaning of nets and removal of dead or sick fish. Various parisiticides are also in use, and are administered through bath treatments or in-feed treatments (Haya, 2005).  As well as being directly harmful to the health of farmed salmon, sea lice can also act as vectors for the transmission of diseases between individuals and farmed/wild populations.


Disease

As a result of the confined conditions that farmed salmon live, infectious diseases are often transmitted readily throughout the stock, often causing declines in salmon populations. Known diseases that have impacted salmon aquaculture include Infectious Salmon Anaemia (ISA), Infectious Hematopoietic Necrosis (IHN), Furunculosis, Bacterial Kidney Disease, Betanodovirus and Salmon Alphavirus (Crane et al, 2011).

Infectious Salmon Anaemia is considered a major threat to the viability of salmon farming and is now the first of the diseases classified on List One of the European Commission’s fish health regime (European Health Commission, 2013). ISA virus was first discovered in a Norwegian Atlantic Salmon hatchery in 1984, where 80% of salmon in the outbreak died (Plarre et al, 2005). Transmission of ISAv occurs primarily via water and through faeces, mucus and urine dispersion where it enters through the gills and broken skin of vulnerable fish (Thorud, 1988). Transmission can also occur via species of sea louse (Plarre et al, 2005). The virus causes severe anaemia in the salmon (Crane et al, 2011), eventually leading to the death of the individual and often mass fatalities within populations (Plarre et al, 2005). As well as Norway, the distribution of the virus is now known to include Canada, Scotland, Eastern USA, Chile and Ireland (Crane et al, 2011). There is no known treatment for the virus in an already infected salmon and methods of prevention mainly requires the eradication of the entire fish stock if an outbreak is confirmed on a farm. In such events, the economic losses of these farms can be significant (Frederick et al, 2006).

Another major pathogen that has proved harmful to salmonid populations is Aeromonas salmonicida, a bacterium that causes the disease Furunculosis, symptoms of which include internal and external haemorrhaging of the infected fish, swelling of the vents and kidneysboilsulcersliquefaction, and gastroenteritis (Staley et al, 2001). In 2005 furunculosis killed 1.8 million Atlantic salmon smolts at a single commercial salmon hatchery on Vancouver Island (TbuckSuzuki, 2013). It is distributed across the northern hemisphere. The disease is one of the most commercially significant salmonid diseases, occurring in salmonid aquaculture worldwide (Department of Agriculture, Fisheries and Forestry, 2013).

Attempts to control transmission of diseases on salmon farms through the use medication of the salmon stock have proved somewhat effective, yet eradication of infected stocks is still often required. After the first reported outbreak of furunculosis in the 1980s, the disease spread rapidly. In 1988, 32 Norwegian salmon farms were infected. By 1992, this number had jumped to 550 (due to massive escapes in 1988 and 1989) (Pure Salmon, 2013). In addition, it was found furunculosis had spread to more than 74 natural waterways in Norway (Pure Salmon, 2013). In an attempt to eradicate the disease, 20 Norwegian salmon farmers slaughtered the whole salmon population of their farms, thus causing an estimated economic loss of more than $100 million (Pure Salmon, 2013).

Salmon farms have fallen victim to a host of other diseases in recent years. In 2013, GRIEG Seafood, one of Scotland’s five biggest salmon farms announced that 2400 tonnes of salmon with a market value of £8 million died the previous year as a result of Amoebic Gill Disease, caused by the amoeboid Neoparamoeba perurans. Surviving salmon of the disease were so weakened, that the application of sea lice treatment to the farms killed even more individuals. The overall economic loss due to the outbreak of the disease rests at £32.5 million (Herald Scotland, 2013).


Release of waste products into the environment

The abundance of salmon in such concentrated areas, primarily open-net cage farms, can result in significant fouling of the ocean floor causing further negative impacts to the environment, specifically benthic communities. With often up to tens of thousands of salmon enclosed in pens, the waste products of farmed fish can accumulate under the pens, smothering portions of the ocean bottom, contaminating the marine ecosystem and depriving species of oxygen.

A variety of waste products accumulate around salmon farms, including carbon, nitrogen and phosphorous. Dissolved inorganic nitrogen and phosphorous are released through excretion, and inorganic Carbon as CO2 is released through respiration. (Wang et al, 2012). Particulate organic carbon, nitrogen and phosphorous are released through defecation and loss of feed (Wang et al, 2012). In 2000 and 2001, nutrient discharges from aquaculture in the Northeast Atlantic, including Scotland, Denmark, Norway and Ireland were estimated at almost 40,000 tonnes of nitrogen and 6,600 tons of phosphorous (Scottish Executive, 2003). Excess feed and faeces that can accumulate in the sediment surrounding open-net cage farms can lead to eutrophication in the water column and anaerobic conditions in the sediment.

Image credit: Michael L. Webe | SeaWeb Aquaculture Clearing House |

Image credit: Michael L. Webe | SeaWeb Aquaculture Clearing House |

The crowded conditions in which farmed salmon live, along with bad water quality,  induces stress in these fish and contribute to impaired growth and predispose them to disease (Haya et al, 2001). This, in turn, necessitates increased use of medicinals, of which too are released into the surround environment (Haya et al, 2001), which can lead to antibiotic resistance in wild species.  This could have negative impacts on marine environmental biodiversity, and on terrestrial animal and human health as a result of selection for bacteria containing antimicrobial resistance genes (Buschmann et al, 2012). It has also been shown that chemicals such as those used in treatment of salmon farms can have negative effects on the health of the surrounding environment e.g chemicals used in the treatment of sea lice infestation have been found to be lethal to shrimps and lobsters (Haya et al, 2001).

A solution that has been put in place by a small number of salmon farmers to combat the release of biogenic waste into the external environment is through the development of closed-containment systems. The systems physically separate fish from the external environment, by means of the container’s impermeable barrier which prevents the transmission of diseases and parasites and also eliminates escapes and discharges of wastes into the ocean. Though these systems have been shown to be very effective in preventing waste discharge, there is a large resistance in the salmon aquaculture industry to develop these systems, their reasoning being that they are not economically viable (Environment Protection Agency, 1998) (G3 Consulting, 2000). The most successful method of prevention currently used is down to the location of the salmon farms, where areas of high currents can disperse and dilute any released waste products, reducing the negative effects on the environment.


Escapees

It is not uncommon for farmed salmon individuals to escape from sea cages and into the natural environment and when this occurs, it can have negative implications to wild species.

Negative effects of escaped farmed salmon on wild populations have been scientifically documented, the negative effects ranging from ecological interactions to the genetic effects of interbreeding. Interbreeding between farmed salmon and wild salmon individuals have shown to reduce genetic diversity, thus reducing lifetime success of the fish, lower individual fitness and decrease  production over at least two generations (Thorstad et al, 2008) as well as reduced disease resistance and adaptability (Gardner et al, 2003). Due to the differing environments in which farmed salmon and wild salmon originate, the two groups can differ both morphologically and genetically. This can result in escapees outcompeting wild species for food and habitat, thus causing a decline in native populations (Volpe et al, 2000).

The spread of disease and parasites to wild populations is also a major negative impact caused by escapees (Krkosek et al, 2007). The subject of genetically modified salmon is too a major concern in relation to farmed salmon escapes. In recent years, a handful of companies have developed genetically modified salmon that grow several times faster than regular salmon and that also require less feed, thus increasing the amount of annual stock of farms and reducing the amount of money spent on feed (McLeod, 2006). This move has been met with controversy from conservationist groups and ecologists, who are concerned that genetically modified escapees could have detrimental effects to wild salmon populations, due to out competing native species.

While solutions to combat escapees include the regular maintenance of nets and cages and the development of protection zones where salmon farming is prohibited, closed-containment systems would be the most effective prevention against escapees. But as stated, the high economic cost of the development and maintenance of such systems has resulted in a reluctance among the majority of the aquaculture industry (G3 Consulting, 2000).



Concluding thoughts

Through the research conducted in writing this review, it’s been revealed the negative environmental impacts salmon aquaculture can instigate. From the transmission of parasites and pathogens to the implications of escapees on wild salmon populations, it’s clear that a new system needs to be put in place that prevents these events from occurring. Closed-containment systems are likely to be the solution to these problems. In a report published by the Standing Committee on Fisheries and Oceans to the Canadian House of Commons (March 2013), the committee undertook a study on closed-containment systems, and provided significant evidence for the benefits of these systems. Ocean-based solid-wall containment systems and land-based, recirculation aquaculture systems have shown to be extremely effective in the filtration of waste, complete removal of escapes and reduction in the transmission of diseases and parasites between fish.  These systems also offer near complete control over water quality, temperature, oxygenation. This system has no doubt overwhelmingly benefits to the environment but its economic feasibility is a topic of debate due to the systems being high-tech and high-cost endeavours. In the long run, the systems prove beneficial due to the high growth rate and the potential of the systems to stock seven times the density of conventional salmon farms (Standing Committee on Fisheries and Oceans, 2013), both of which will ultimately result in economic gain. If sufficient awareness is raised regarding the positive aspects of closed-containment systems, coupled with reduced costs of the systems as technology advances, they could potentially replace conventional salmon farms in the future.

The aquaculture of salmon provides millions of people around the world with an important food source and source of protein, along with vast employment opportunities and economic gain to countries and communities. The industry is hugely beneficial yet the environmental impacts caused must be reduced drastically before irreversible damage is done to ecosystems worldwide.

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– JK


Header photo credit: http://lightninjaj.blogspot.co.uk/

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• The Standing Committee on Fisheries and Oceans (2013) Closed-containment salmon aquaculture report. Available at: http://www.scribd.com/doc/129210039/Closed-containment-salmon-aquaculture

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