The relatively unchanging environment of traditional extensive fish culture
 appears to be ecologically benign
but is it really better than intensive aquaculture
when feed resource utilisation is taken into account?

Reasons for intensifying aquaculture production

The past 20 years have seen the development from the most extensive traditional aquaculture practises to the most intensive, technically advanced systems.  This parallels similar advances seen in terrestrial animal production, albeit over a somewhat longer timescale.

In true extensive culture, livestock are kept in an environment that is little changed from its natural habitat, where the relationship between species is regulated through culling (harvesting), restocking and the introduction of new stocks or species.  The first step in intensification entails fertilising the system, to boost natural (plant) food production.  In aquaculture, such semi-intensive systems are analogous with fertilised arable or grazing lands.  Control of all stages in the culture cycle is often a feature of such systems.  Further intensification is represented by the addition of an external feed source, to supplement natural food production. 

Additional improvements require other intensification steps, or the optimisation of the efficiency of individual intensity levels.

In all culture systems, the word "intensive" is not synonymous with "overcrowded"; it only implies the introduction of an external food source to increase production efficiency.  This is the primary means for all meat production in the industrialised world and an increasingly common strategy in less developed countries, as more efficient production becomes a necessity.

While the development of efficient extensive culture has been slow, there have been rapid advances in intensive aquaculture systems.  In 1972, the Norwegian salmon industry used 1.9 kg of animal protein to produce 1.0 kg of salmon that contained 0.18 kg of protein, a figure that indicates a protein retention of only 9%. 

By 1996, by using more efficient methods and the best feeds available, salmon growers had reduced this input to 0.4 kg of (mainly animal) protein per kg of salmon produced.  This equates to an increase in protein retention of 45%.  The best fish farmers now do even better; in experimental trials, up to 66% protein retention has been demonstrated for salmon and trout.  This represents the highest efficiency achieved by any one-step animal culture system.

In the culture of un-domesticated Atlantic halibut (Hippoglossus hippoglossus), a  technology that is still developing, protein retention of 46% has already been achieved.

The culture of carnivorous and non-carnivorous species fish species

Animal protein is used to produce animal protein in the culture of carnivorous fish on the basis of higher acceptability of the food by the fish and the value of the final product for human consumption.  Fishmeal is the primary source of animal protein in fish feeds.  However, using state-of-the-art nutritional and feed production know-how, it is now becoming possible to reduce the high proportion of fish protein in the feed rations with no detriment to the cultured species or to human consumers.

One means is by the culture of single cell protein (SCP), on a natural gas substrate  (methane), using microbial (bacterial) fermentation.  As much as 50% of the animal protein requirement in salmon diets has been successfully replaced by the latest generation of SCPs.

Another feed option for carnivorous fish culture may lie in the use of concentrated plant proteins.  In experimental trials, using Atlantic salmon, soya protein concentrate has been successfully used for as much as 75% in of the protein requirement.  In trials involving rainbow trout, 100% of the fishmeal has been replaced by soya protein concentrate.

This shows that some plant protein feed sources can be sufficiently upgraded to enable substitution (partial or even total) for animal proteins.  In this way, carnivores can use plant proteins just as efficiently as non-carnivores, moving them one step lower in the food chain, allowing their growth on substantially lower primary energy input.  Such techniques represent a considerable step in food production technology, resulting from our increasing knowledge of the precise and minute nutritional requirements of carnivorous species in culture systems. 

By such means, carnivorous fish can be cultivated to produce at least
twice as much animal protein as they consume!