Aquaculture and Agriculture;
a comparative perspective

If one compares the development of extensive and intensive terrestrial animal culture with aquaculture, it appears illogical at first sight to culture carnivorous fish such as salmon. Carnivorous animals, such as dogs, are not kept primarily for their meat production but to help the hunter and protect the herd. Historically, mankind has always cultured plants and plant eaters (herbivores) for food.

Extensive and intensive terrestrial cultures have both developed to a stage where they exert a huge, indeed dominant, impact on the global environment.  Cattle and other ruminant livestock such as sheep and goats graze 50% of the planet's land area.  Ruminants, along with pigs and poultry, also eat feed and fodder raised on 25% of the earth's cropland (Durning and Brough, 1991).  The overriding majority of terrestrial plants and animals consumed today are cultured and very little terrestrial food is hunted or gathered.  The natural balance of nature that preceded the onset of agriculture has been all but obliterated in the process.

The opposite is true for the marine environment.  Over 90% of the world's total harvest of fish and shellfish are caught through traditional wild fisheries and < 10% are cultured.  However, for a number of reasons, many of the wild fisheries stocks are threatened and aquaculture is seen as a means of sustaining these in some cases, through restocking or seeding.  It is predicted that wild fish catches will also become increasingly augmented by cultured production in the course of time.  That said, the fish meals used in most European feeds come from sustainable stocks of industrial fish, primarily caught in the North Atlantic.  At the present time, fish culture only exceeds the wild catch in freshwater and, to a lesser degree, in brackish water.  The long tradition of freshwater culture is based primarily on non-carnivorous species.  The sector continues to expand; production has doubled over the last ten years, to 12 million tons p.a. globally and is now twice the wild freshwater catch, which has been stable at about 6 million tons p.a. for some years and is possibly at its sustainable limit.

The sources of nutrients and energy that are used to produce different aquaculture species are various, as are the possibilities for their alternative use.  The following table compares the efficiency of the production of common carp, channel catfish, marine shrimp, rainbow trout, Atlantic salmon and seabass.  It is worthy of note that, possibly contrary to popular belief, production efficiency is highest in the most intensive culture systems, using rainbow trout and Atlantic salmon, and less efficient in semi-intensive or extensive production systems for marine shrimp and common carp.

FCR = Feed Conversion Ratio, the quantity (weight) of feed required for the quantity of (wet) weight gain

Fortunately, cultivation techniques developed, primarily in wealthy countries, to increase the production efficiency for high value species, can be successfully transferred to the cultivation of lower value species.  For example, in the case of breeding programmes, which are very costly, each step in the right direction is an improvement for all time.  In salmon farming, improvements arising from selective breeding include faster growth and greater disease resistance.  Societies that are less able to meet such research costs can benefit from the application of such technology transfer, resulting in improvements to the efficiency of the traditional, low-cost farming methods.

Aquaculture and Agriculture, relative efficiencies

It is very interesting to compare the relative efficiencies of agricultural and agricultural meat production. The amount of dietary protein and energy retained by farmed salmon is approximately twice the retained by chicken and pigs, which are the most efficient terrestrial converters. Furthermore, the level of waste impact is also correspondingly low.

The relatively unchanging environmental of traditional, extensive fish culture appears ecologically benign, but is it really better than intensive aquaculture when feed resource utilisation is taken into consideration?