Can Aquaculture be Intensive and Sustainable? A Look at the Scottish Atlantic Salmon Aquaculture Industry

29 April 2016, at 1:00am

UK - Scotland is the largest producer of Atlantic salmon in the EU, and the third largest in the world. The importance of salmon aquaculture to Scotlands economy, particularly for coastal and rural communities is not insignificant.

No food production system can operate completely without environmental harm, however technological and operational improvements mean the reduction of inputs like space and feed, and outputs like greenhouse gas emissions or chemicals is possible – whilst still allowing for yield increases.

Achieving such “Sustainable intensification” is no easy task. Nevertheless a recently published paper lead by Dr Tim Ellis (CEFAS, UK) indicates Scotland’s Atlantic salmon aquaculture industry is moving in the right direction.

Using annual Scottish fish farm production survey data dating back to 1979, covering both freshwater (ova to smolt) and seawater (smolt to harvest) production, the scientists were able to build an picture of how Scotland’s salmon aquaculture industry has developed over time.

Sites and Systems

The number of active farms in both freshwater and seawater increased rapidly thought the late 1970’s to the early 1990’s, but declined after 1999, with companies concentrating production at sites that are most suitable to salmon farming. At the same time, the average capacity/size of net pen sites increased. Both freshwater and seawater farms tend to operate either at or near the maximum permitted biomass, as regulated by the Scottish Environment Protection Agency.

Scotland does have some land-based aquaculture systems for both freshwater and seawater though their use has declined, likely due to their running costs. Those that were used for seawater farming have largely been reallocated for broodstock. Broodstock sites have also declined, partly as a result of company consolidations, and partly to maintain broodstock at sites that are disease free.

Biological Performance

The size at harvest has seen increases over the years. Increased smolt size and earlier transfer to seawater from freshwater pens are considered to be one factor for the increase in size, but so is the enhancement of growth rates.

The improvements in growth rates arise from a number of different factors including improved feed formulas, and reduced stress as a result of health management designed to reduce infections (including the use of vaccination) and husbandry improvements such as swim through at net changes.

These same improvements also increased survival rates in both freshwater and seawater system.

Increased survival and increased weight at harvest has in turn, resulted in an increase in yield per recruit since the 1970s. These improvements indicate that salmon welfare might have also improved. Selective breeding has also played a crucial role, with traits such as faster growth rates and high feed-conversation efficiency becoming more prevalent in Scottish farmed salmon.


Since 1989, the number of companies operating in seawater has declined from around 160 to 18 in 2014, with just 6 companies dominating seawater production. The declining trend and increasing domination of the industry by a few key players primarily arises through consolidation, with a number of companies leaving the industry under times of poor trading prospects, market uncertainty, as well as concerns surrounding disease impacts on production. There has also been “a notable increase in the number of Scottish companies coming under foreign ownership”. A similar declining trend has also occurred in the freshwater sector.

Whilst the number of companies has shown a marked decline, employment has, with the exception of a period between 2000 and 2006, increased. Whilst full-time employment dominated both freshwater and seawater sectors, the number of part-time employees increased in the freshwater sector after 1998 when industry switched from having in-house vaccinators to contracting vaccination services.

Environmental Pressures

Escapism can occur for a number of different reasons, including storm damage to pens, predation damage to nets, or accidents during fish transfer. The number of fish escaping from seawater systems is higher than freshwater, but has seen a general decline in the number of reported escape events. Although number of reported escaped fish however has not declined, the development of triploid (sterile) salmon should remove the risk of genetic introgression in wild populations where used.

Following recommendation from the Fish Health Inspectorate, fallowing – where all fish and net-pens are removed from a site primarily to control disease and parasites, has increased in freshwater farms. Mandatory fallowing after outbreaks of notifiable disease was implemented in 1998 to further control disease and parasites. Duration of the fallow depends upon risk classification, and whether disease was confirmed or suspected. Alongside improvements in feeding systems, increased fallowing is also believed to reduce eutrophication impacts of farms.

A number of other improvements have also been made. For example, the introduction of vaccines and improvements in disease control have resulted in the reduction of chemicals to control sea lice and algal fouling, and antibiotic use.

Feed developments to reduce reliance on wild fish stocks whilst still providing sufficient nutrients are also helping to reduce the environmental impact of farms. Given the level of investment in research and development for the aquaculture industry, further reductions in impacts seem likely to occur in the future.