Feed Conversion Ratio: Tales and Facts

The fish farming industry argues that salmon is an incredibly efficient eater, with a feed conversion rate of about 1.2 to 1 compared with 8 to 1 for land-based animals such as cattle. This, however, is a slightly misleading argument.

The feed conversion ratio (FCR) or feed conversion efficiency (FCE), to define it simply, is a measure of an animal’s efficiency in converting feed mass into increased body mass. By way of example, sheep and cattle need more than 8 kg of feed to put on 1 kg of live weight, their ratio is thus 8:1. The pork industry claims to achieve an FCR of around 3.5:1. Poultry has an FCR of 2:1. Farm raised Atlantic salmon apparently have a very good FCR of about 1.2 to 1. How can this increased efficiency be explained?

According to the fish farming industry (Aquamedia, at http://www.piscestt.com/home/FAQ/Answers/ans8_en.asp): “In an efficient trout or salmon farm, the FCR will be close to 1. This is extremely low when compared with land animals.” The website indicates three reasons for this: the biology of the fish (it’s cold-blooded, so doesn’t need energy to keep warm), the way the fish live (farmed salmon just float about without using much energy) and the high nutrient concentration of fish feed.

As the website states: “It is true that salmonid fish are very efficient converters of feed into energy and building blocks (muscles) for growth, but their feeds are also more concentrated than those for pigs or cattle since they are carnivores and do not need ‘filler’ ingredients such as fibres. Salmonids use oils and fats as their prime energy source and, therefore, this is more concentrated than the carbohydrates needed by land-living animals.” This third reason is indeed the main explanation for the increased efficiency.

The food chain can be simplified as follows:

At the bottom are primary producers: plants (both in soil or in a marine environment, e.g. phytoplankton) convert nutrients from the soil resp. water and light from the sun into food for herbivores (plant-eaters). Herbivores, e.g. cattle, will then convert this food/energy into body mass at a ratio of 8 to 1. However, if fed on grain, which is more concentrated than grass, ruminants for example can convert 7 kg of grain to 1 kg of meat so their ratio improves to 7:1. Humans then eat the beef at a FCR similar to that of pigs, i.e. 3.5 to 1. It is not just that humans are twice as good as cows at converting food, it is that our omnivorous diet includes high-energy  food such as meat in which the plant food has already been “concentrated” into higher-energy food such as meat. In fact, instead of consuming 350 grams of beef for 100 grams of human muscle, we would need to eat 2.8 kg of grass to achieve the same weight gain (hint for people trying to lose weight: eat low-energy and high-bulk food!). Pigs and chickens are better than sheep or cattle as their diet also includes grain and meat (e.g. worms in the case of chicken).

What happens is this: generally, each so-called trophic level (i.e. the position it occupies in a food chain) relates to the one below it by absorbing some of the energy it consumes, and in this way can be regarded as resting on, or supported by the next lower trophic level. Food chains can be diagrammed to illustrate the amount of energy that moves from one feeding level to the next in a food chain – this is called an energy pyramid. The energy transferred between levels can also be thought of as approximating to a transfer in biomass, so energy pyramids can also be viewed as biomass pyramids, picturing the amount of biomass that results at higher levels from biomass consumed at lower levels.

The higher up on the trophic pyramid (moving from a herbivorous through an omnivorous to a purely carnivorous diet), the lower the conversion ratio because most of the conversion work has already been done on the trophic level below. One of the foods with the most concentrated energy available would be fats, e.g. oil – this is contained in the food fed to farmed salmon, so no wonder their FCR is so good: the food they eat is highly concentrated. But to compare like with like, we should have to calculate how much conversion was required on the lower levels.

For example, the primary producer in water is phytoplankton, which gets eaten by zooplankton (e.g. krill) – conversion rate unknown. Small non-carnivorous fish eat the plankton (Tilapia fish typically has a ratio of 1.6-1.8 to 1) and then get eaten by larger predator fish such as salmon higher up on the pyramid. To raise carnivorous species like salmon on farms, we have to catch a lot of forage fish and grind them into fishmeal and fish oil as feed. How much? According to most estimates, it takes five pounds of wild-caught sardines, anchovies, and other forage fish to produce a pound of farmed salmon – it’s just that those five pounds are concentrated into oily pellets of 1.2 pounds, which then fed to the salmon will give a FCR of 1.2. If salmon had to feed on grass or even grain, their FCR would not look that good!

There is thus the fundamental issue of whether or not farming carnivorous species such as salmon is actually sustainable. Unlike herbivorous species (like tilapia and carp) – which require minimal inputs of fishmeal – harvesting of wild fish (for example, sardines, whiting and anchovies) and krill for fishmeal in unsustainable amounts is required to produce the feed in order to farm salmon.

This article is written by kris

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One thought on “Feed Conversion Ratio: Tales and Facts

  1. Pingback: Plenty of Fish in the Sea? | Jason Schreiber

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