Terrestrial and aquatic food production

Animals, whether they are primary consumers, (the animals lowest in the food chain), secondary consumers, or even higher up the chain, all ultimately rely on the primary energy sources for life, the sun.

This energy is first converted into plant biomass by the process of photosynthesis.  The higher an organism is up the food chain the lower will be its maximum achievable biomass (total population weight), since energy and therefore biomass is lost at each step.

About 70% of the sun's energy reaching the earth surface falls on the oceans whilst 30% falls on the world's landmasses.  Vitousek et al.  (1986) estimated the amount of organic dry matter produced by the primary production of terrestrial and aquatic plants.  He then estimated the proportion that each contributes directly to human consumption and, more indirectly, via the feeding of domestic animals.

Land plants produce 132 billion tonnes of dry organic matter per annum (p.a.).  According to Vitousek, about 0.8 billion tonnes p.a. of this is used directly for human consumption.  Thus total direct human consumption of organic matter is about 0.6% of terrestrial primary production.  A further 2.2 billion tonnes p.a. of plant dry organic matter is fed to domestic animals.  Domestic animals are thus primary consumers, which convert their intake of primary plant production into just 0.2 billion tonnes p.a. of meat. 

Aquatic plants produce 92 billion tonnes p.a. of dry organic matter, of which a mere 0.02 billion tonnes are harvested for direct human consumption.  This represents < 0.02% of aquatic primary production.  The wild fish caught through fisheries are considered to be secondary consumers in the aquatic food chain.

Humans are thus, along with livestock, the important direct consumers of terrestrial organic matter.  A large amount of plant organic matter is also used for purposes other than human food. Examples are timber for burning or shelter and clothing. 

In contrast, the primary production of the oceans is far less well utilised.  One other major difference between the terrestrial and marine eco-spheres is in the levels of culture that they support.  This is why aquaculture would appear to have so much more potential as a future source of energy and protein than other current technologies.

based on “Aquaculture and the Environment; New Perspectives” -  
by AKVAFORSK(Norway) and Biomar AS