The growing world population is in need of more and more protein, and aquaculture can be seen as a potential solution. However, there have been claims that some fish farming results in a net reduction of marine protein resources due to fish meal in feeds. Trine Ytrestøyl and colleagues from Nofima in Norway researched how salmon farming performs.

The world's population is currently increasing by 80 million each year, and is expected to reach 9 billion by the year 2050.

The Food and Agricultural Organization of the United Nations (FAO) has predicted that 70 per cent more food must be produced globally by 2050 to meet the increase in demand.

The population growth, combined with increased urbanisation and higher per capita income in large parts of the world, changes consumption habits and puts pressure on the available resources.

The per capita meat consumption was 15 kg in 1982, when the world population was 4.5 billion, and is expected to reach 37 kg in 2030.

This will have a large impact on the environment and the available resources of land area, fresh water, and phosphorus, and urgent action to develop food systems that use less energy and emit less greenhouse gases is required.

The global food sector is currently responsible for around 30 per cent of the world's energy consumption and contributes more than 20 per cent of the global greenhouse gas emissions. In addition, land use changes, mainly through deforestation, contribute another 15 per cent of greenhouse gas emissions.

Any method of food production can be evaluated in terms of the influence it has on the environment and how much natural resources are consumed in the process.

Eagle et al. defined ecologically sustainable food production as production that maintains the natural capital on which it depends, and that in principle can continue indefinitely. Well-managed fisheries where the catch is regulated based on stock assessment fulfil this definition.

However, no industrial food production is truly sustainable today, because all such productions depend on non-renewable energy sources such as oil and gas, as well as non-renewable phosphorous sources.

Industrial food productions may be evaluated in terms of energy produced in relation to the input of industrial energy. When the sustainability of food productions is evaluated, the goal should be to maximise the nutritional output for human consumption and minimise the input of resources (organic and inorganic), with the lowest possible impact on the environment.

The nutritional content of food products is easy to calculate, but it is more challenging to quantify the use of natural resources and to assess the environmental effects of different food production systems.

All food production has environmental consequences. Agriculture is the main source of water pollution by nitrates, phosphates and pesticides, and livestock production is a major source of greenhouse gases. Livestock production uses large amounts of fresh water and land areas.

The global meat consumption is increasing by around 3.6 per cent per year and has nearly doubled between 1980 and 2004. It is expected to double again by 2030.

There is also a shift from extensive grazing systems to more intensive production systems that depend on more concentrated feeds and feed ingredients that are traded internationally. More than 30 per cent of the world cereal production is currently used in feed for livestock.

Global food production is also heavily dependent on the use of phosphorus fertiliser. The low phosphorous concentration in soil in large parts of the world makes it a limiting factor for plant growth on entire continents such as Africa and Australia, and in many large countries such as Brazil and India.

Phosphorus is thus essential for global food production, and agriculture consumed almost 90 per cent of the P used in 2010, 82 per cent was used in fertilizers and 7 per cent was used in animal feed supplements. However, the current use of phosphorus is not sustainable.

Phosphorus is not recycled at present, but moves through an open one-way system in which the phosphorus ends up in the ocean.

A meat-rich diet consumes three times as much P as a vegetarian diet, and for a world population of 7.7 billion people, a 20 per cent increase in phosphorous-fertiliser would be required without changes in the world diet, whereas a 64 per cent would be required if the complete world population were to have a diet that resembles the diet in developed countries.

With less space and water resources available on land, growing food in the ocean is an attractive option. Aquaculture now accounts for almost half of the total food fish supply and the percentage is increasing every year.

The rapid growth in the aquaculture industry has raised concerns among consumers, retailers and non-governmental organisations about the environmental impact and sustainability of fish farming.

The dependence of the aquaculture feed industry on fish meal and fish oil and the consequences for wild fish stocks are often used as arguments against the sustainability of salmon production.

Forage fish are often small pelagic fish at lower trophic levels that are important prey for species higher up in the food chain. The majority of the world's fish resources are fully exploited or overexploited.

A further growth in aquaculture must therefore rely on an increase in the use of alternative sources of lipid and protein. There is, however, still a potential for an increased utilisation of discards and by-products from the processing of fishery products for human consumption.

Approximately 25 per cent of the fish meal produced worldwide originates from trimmings, but the potential is larger, considering that around 120 million tonnes of fish are consumed by humans.

If the edible portion is around 50 per cent, there are roughly 60 million tonnes of trimmings and by-products available for the production of fish oil and fish meal. This is three times the amount of forage fish used for this purpose today.

Improved regulation and management of the capture fisheries are necessary for a sustainable and optimal utilisation of the marine production systems.

Farming of Atlantic salmon has been seen as negative due to the use of small pelagic fish in the feed, and it has been claimed that salmon farming reduces the amount of marine protein available for human consumption.

In common with all food production, aquaculture has environmental consequences, and feed production is a major input factor in salmon production.

An understanding of the environmental impact of different feed formulations and how they affect resource utilisation is thus important for making strategic decisions about food production regimes.

Several indicators and methods for measuring the sustainability and production-efficiency of aquaculture productions have been developed, such as the simple fish in/fish out ratio, forage fish dependency ratio, marine nutrient dependency ratio and nutrient retention and nutrient flow models.

More extensive methods such as the ecological footprint model and life cycle analysis (LCA) are also used to assess the sustainability of aquaculture and other food production systems. These methods are complementary and cover different aspects of biophysical performance and resource efficiency.

Evaluating the sustainability of food production methods is complicated, and many aspects must be addressed. There is currently no single method that is robust enough to cover all environmental impacts related to food production, and several methods must be used to evaluate eco-efficiency and sustainability.

The present study shows the retention efficiency of nutrients from feed resources to final product in the Norwegian salmon production, including limiting resources such as the omega-3 fatty acids EPA and DHA and phosphorous.

It is highly relevant to compare the efficiency in commercial scale with experimental data, and this is to the authors' knowledge the first attempt to make such calculations for an entire commercial aquaculture production.

How did Norwegian salmon perform?

In 1990, 90 per cent of the ingredients in Norwegian salmon feed were of marine origin, whereas in 2013 only around 30 per cent. The contents of fish meal and fish oil in the salmon feed were 18 per cent and 11 per cent, respectively, in 2013.

Between 2010 and 2013, salmon production in Norway increased by 30 per cent, but due to a lower inclusion of marine ingredients in the diet, the total amount of marine ingredients used for salmon feed production was reduced from 544,000 to 466,000 tonnes.

Norwegian salmon farming consumed 1.63 million tonnes of feed ingredients in 2012, containing close to 40 million GJ of energy, 580,000 tonnes of protein and 530,000 tonnes of lipid. 1.26 million tonnes of salmon was produced.

Assuming an edible yield of 65 per cent, 820,000 tonnes of salmon fillet, containing 9.44 million GJ, and 156,000 tonnes of protein were produced. The retentions of protein and energy in the edible product in 2012 were 27 per cent and 24 per cent, respectively.

Of the 43,000 tonnes of EPA and DHA in the salmon feed in 2012, around 11,000 tonnes were retained in the edible part of salmon. The retentions of EPA and DHA were 46 per cent in whole salmon and 26 per cent in fillets, respectively.

The fish in/fish out ratio (FIFO) measures the amount of fish meal and fish oil that is used to produce one weight equivalent of farmed fish back to wild fish weight equivalents, and the forage fish dependency ratio (FFDR) is the amount of wild caught fish used to produce the amount of fish meal and fish oil required to produce 1 kg of salmon.

From 1990 to 2013, the forage fish dependency ratio for fish meal decreased from 4.4 to 0.7 in Norwegian salmon farming. However, weight-to-weight ratios such as FIFO and FFDR do not account for the different nutrient contents in the salmon product and in the forage fish used for fish meal and fish oil production.

Marine nutrient dependency ratios express the amount of marine oil and protein required to produce 1 kg of salmon oil and protein. In 2013, 0.7 kg of marine protein was used to produce 1 kg of salmon protein, so the Norwegian farmed salmon is thus a net producer of marine protein.

Several aspects must be addressed when assessing the environmental performance of food production systems.

The input of organic and inorganic resources and the output of both, in terms of nutrients for human consumption and in terms of waste and emissions to the environment, must be quantified.

Life cycle assessment methodology (LCA) is often used to study the environmental efficiency of food production systems. Recycling of nutrients from agro-industrial by-products into animal production is a key factor in increasing the environmental efficiency of food production, and is positive for overall productivity and efficiency.

Mass balance models are more suitable than LCA models for tracking nutrient flows and estimating nutrient retention efficiencies. However, it is essential to have access to accurate data to be able to track the major flows of nutrients in food production systems and estimate how efficiently they are utilised.

The availability of representative data on nutrient composition of the feed, final product, and (in particular) of the parts of the animal that are not consumed by humans is necessary for tracking the nutrient flows when drawing up a resource budget for an entire food production system. An overview of the inputs and outputs of nutrients and energy such as the one presented in this study should be obtained for other food production systems.

The efficiency of a food production system depends also on how much of the final product is actually consumed by humans. The FAO has estimated that 30 per cent of the food produced in the world is not consumed, for various reasons.

In the developed world, retailers and consumers are responsible for most of the waste, whereas in developing countries, losses occur mainly during the harvest and storage of food.

Avoiding these losses will reduce the demand for land, water, and energy, and will reduce the emissions of greenhouse gases. Thus, more focus should be directed towards reducing food losses after the product leaves the farm gate.

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