Feed conversion ratio (FCR) is calculated from the number of kilos of feed that are used to produce one kilo of whole fish.

Two additional terms are used by the farmer, the biological FCR and the economic FCR. Biological FCR is the net amount of feed used to produce one kg of fish, while the economic FCR takes into account all the feed used, meaning that the effects of feed losses and mortalities, for example, are included.

Farms reporting a low FCR normally have good management practices in place, with no overfeeding and very low, if any, mortalities.

Overfeeding or underfeeding will increase the FCR. As a consequence, many farms are equipped with monitoring systems, for instance underwater television cameras, as well as devices for controlling the supply and delivery of feeds.

The basic principle in feeding is that the fish should be fed exactly to satiation. If they are fully fed, the fish are not stressed and they provide high quality food for human consumption. This requires the farmer to have the skills to judge precisely how much feed to give. As aquaculture has evolved, many devices have been developed to help the farmer in judging how much feed to provide. Special computer software for stock management and surveillance provides accurate guidelines for the best feeding regimes.

In an efficient trout or salmon farm, the FCR will be close to 1. This is extremely low when compared with land animals. There are three reasons for this; the biology of the fish, the way the fish live and the high nutrient concentration of fish feed.

Fish have a low body maintenance requirement. Fish are poikilothermic (cold-blooded) animals (their body temperature is equal to that of its environment) and thus they do not use feed to maintain body temperature as warm-blooded animals do (e.g. poultry, pigs and cows).

For a fish, floating in the water consumes less energy than standing and walking on legs.

The salmonid fish are very efficient converters of feed into energy and building blocks (muscles) for growth and their feeds are also more concentrated than those for pigs or cattle, since they 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.

Finally, land animals have a far higher feed capacity than fish and therefore they can grow well on less concentrated feed. This characteristic represents an environmental adaptation since salmonids are carnivorous (meat eating) while most farmed land animals are basically herbivorous (plant eating).

Research work by the major feed companies indicates that the FCR for certain species can be as low as 0.8, so there is further room for fish farming to become even more productive, without compromising its environmental or welfare standards.