This is a complicated question and one which is particularly hard to answer because there are so many factors involved. 

Essentially, we are faced with the following problem: there are not enough fish in the ocean to meet the growing demand for human consumption (especially for favored fish species) by harvesting from wild populations, as we always have. Overfishing has become a major environmental concern as fish stocks have been collapsing around the world – destroying what were once thriving ecosystems and threatening global food security.

Aquaculture, or fish farming, has been proposed as a promising solution to supplement the supply from fisheries in order to meet the global demand for fish, and other marine species, in a sustainable manner. It is one of the fastest-growing industries and has reportedly increased 12-fold in the last three decades, compared to the fishery industry which has remained static over the same time period. In the next 10 years, an estimated 60% of consumed fish is expected to come from aquaculture.

Salmon is one of the most popular fish on the market. Despite being viewed as a luxury, a staggering 1 million salmon meals are eaten every day in the UK, the majority of which is sourced from the Scottish aquaculture industry. Commercial farming of salmon in Scotland began in 1969 and has expanded to such an extent that it now produces a whopping 179 000 tonnes of salmon per year and contributes around £1.8 billion to the economy every year. 

Direct Fed Species

One of the major concerns around the sustainability of salmon farming is that salmon are what is known as a direct-fed species, rather than mussels or oysters which are indirectly fed species (or filter-feeders). Salmon are carnivorous and fish farmers typically feed their salmon a combination of fishmeal and fish oil which is made from wild-caught populations of oily fish, such as herring capelin and sprat. While the oily wild-caught fish contribute to the high levels of omega-3 fatty oils for which salmon are so well-known, harvesting wild populations to feed farmed populations of fish is not a sustainable solution going forward and does not solve the problem of overfishing but rather perpetuates it. 

However, in countries such as Norway, salmon farmers are looking for more environmentally friendly options to fish meal and are moving towards using more plant-based feed, which is a more sustainable alternative but does not necessarily provide the same high levels of omega-3 fatty acids that the fish oil does.

Sea Lice

While humans have become very successful at agriculture and terrestrial farming, we are still fairly new to aquaculture and one of the biggest challenges to salmon farming is sea lice and other parasitic infestations. With commercial fish farming, thousands of individuals are kept in pens at a much greater density than they would be found naturally. In addition, the pens are generally kept at the same location, albeit with water flow through, but again it is unusual for natural populations to inhabit the same area for extended periods. The combination of the high densities of penned-in fish restricted to the same patch of water for prolonged periods significantly increases the risk of disease and parasite transfer between individuals, and populations of sea lice are far greater in farmed salmon populations than in wild populations. Although, the same is true of any farmed animal.  

There are 3 major concerns with increased parasites levels:

1. They can affect natural populations

Because the fish farming pens are situated along the wild salmon migration route, wild salmon sometimes encounter the farmed salmon which can result in a transfer of parasites. As the density of parasites is much higher in the farmed population it acts as a reservoir for the sea lice and the wild populations that come into contact with the pens may be overwhelmed by the number of parasites and suffer severe population declines because of it.

2. They cause harm to the individual animals and they suffer because of it

Living in a parasite-infested tank is likely to be incredibly stressful to any animal, but sea lice eat flesh and several reports have been released showing salmon being eaten alive by these parasites. All farming practices should prioritize animal welfare, and there are concerns that this is not the case for salmon farming. However, animal welfare and survival rates have been shown to be improving in the Scottish salmon aquaculture industry since it started with advances in technology and medicine.

3. These parasites are treated with chemicals that can affect non-target species

One of the commonly used parasiticides in salmon farming is formaldehyde. It has been banned in the UK but is still used in Scotland. Although formaldehyde breaks down rapidly in water, there are concerns that it does affect the fish, although the effects are highly variable among individuals. The most common problems associated with formaldehyde use are damage to gills and changes in mucous cells. However, effects are being made to replace the use of formaldehyde with better alternatives.

High Volumes Of Organic Waste 

Another concern with salmon farming is the high volume of organic waste which is released by the fish. As the fish are farmed in pens in natural water systems, this spills over into the surrounding environment which can cause problems. One of the effects of waste accumulation is eutrophication in which algal blooms occur and rapidly deplete the water of oxygen and sunlight – killing many aquatic organisms in the process.

Escaped Farmed Salmon

There have also been several salmon escapes over the years, with a particularly bad one in 2016 in which over 300 000 farmed salmon escaped from their enclosures. Escaped farmed salmon present a threat to local wild populations, firstly because of increased competition for resources and secondly because of the genetic consequences of interbreeding between farmed and wild stock. The main concern in terms of genetics is introgression which results in a loss of local adaptations and reduced fitness in subsequent generations in wild populations. If the fish lose local adaptations, they become less suited to their environment and therefore have lower rates of survival which can lead to population declines.

Greener Option?

Although there are problems with salmon farming in Scotland, there is always going to be an environmental impact of food production on the scale that is needed to meet the global demand for food. And all these factors need to be weighed up and considered. For instance, in terms of water, aquaculture uses less water than any terrestrial farmed animals. With the development of Recirculating Aquaculture Systems, which are land-based production systems which purify and reuse water, salmon can be farmed without ever needing to be placed in the ocean which would drastically improve their environmental impact as wastewater could be treated and there is no risk of escapees and parasites infecting wild populations. Also, as vaccines develop there will be lowered mortality rates among salmon and less need for chemicals to treat disease or parasite infections.

Conclusion

Although the industry is not as green as it would like you to believe there is movement towards a greener system. No matter what you eat, it is affecting the environment in some way and the best you can do is be a responsible consumer and be aware of the impact of your choices on the environment. Salmon farming is not as sustainable as we are perhaps led to believe but relying exclusively on fisheries and wild harvest of fish is not a sustainable solution either in the light of the growing demand for seafood. While there are many improvements that can be addressed in Scotland, and lessons that can be learned from other countries, it is an industry which has the potential to be a more sustainable source of seafood – it is just not quite there yet.