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The range of production in aquaculture
The value of organic wastes
John E. Bardach and Michael T.
This is the old United Nations University website. Visit the new site at http://unu.edu
Contents - Previous - Next
The range of production in aquaculture
The value of organic wastes
John E. Bardach and Michael T. Santerre
East-West Resource Systems Institute, EastWest Center, Honolulu, Hawaii, USA
The use of organic residues in aquaculture is best discussed with awareness of the following facts about aquatic biology: (i) Limitations to biological production in fresh, brackish, or ocean waters are predominantly dissolved nutrients and/or food as well as shelter or substrate. Seasonality and intensity of the input of solar energy are also important. Iii) In aquaculture (and incidentally in fisheries), what is eventually to be harvested has to be contained or concentrated.
Among these inputs, nutrients and/or food can be supplied, at least in part, by organic materials or residues. As aquacultural practices increase in magnitude and hasten the flow of materials and energy through the systems, compared to natural conditions, it stands to reason that fertilization wastes can, under certain conditions, save both monetary and caloric inputs. Likewise, judicious use of agricultural or organic industrial wastes as feed materials can lower the cost of growing aquatic animals. it is the purpose of this paper to describe the use of such practices and to discuss certain ecological, economic, and managerial conditions that determine or limit them.
Organic waste utilization in aquaculture can either be extensive, with wastes occurring naturally or being added, with little or no further management of production (1), or it can be highly intensive under conditions where extraneous feeding and fertilization with inorganic plant nutrients play a major role in augmenting animal protein yield from the system. Aquatic animal husbandry is pursued in embayments, ponds, rivers, lakes, raceways, in brackish water, fresh water, and full-salinity ocean water. Many variants of extensive and intensive fish culture rely importantly on the growing together of a few compatible species (polyculture) to make fullest use of the various types of food present (plankton, bottom fauna) in a body of natural or managed water (Figure 1). Feeds and/or inorganic or organic fertilizers may also be added. Shellfish - molluscs and prawns or shrimp - can also be grown in polyculture, but managerial practices involving several species of fish exclusively are more advanced than those that combine the husbandry of invertebrates and fishes.
Figure. 1. Feeding Niches of Chinese Carp Species: 1, grass carp (Stenopharyngodon idellus) eating vegetable leaves; 2, bighead carp (Anistichtys nobilis) eating zooplankton; 3, silver carp (Hypophthalmichtys molitrix) eating phytoplankton; 4, mud carp (Cirrhinus molitorella) eating benthic animals, detritus, fish faeces; 5, common carp (Cyprinus carpio) on diet similar to that of mud carp; 6, black carp (Mylopharyngdon picous) eating molluscs. (From Bardach et al. )
The rationale of polyculture is obviously to divert as much as possible of the attainable biomass into channels that are useful to man, compared to those that prevail in a wholly natural food web. It should be pointed out that stocking fish in a lake does not necessarily increase the productivity of the lake, only the productivity of harvestable (i.e., desirable) organisms. In a lake supporting a complex food web ending in only a few harvestable fishes, and many other non-desirable organisms (for human consumption), stocking with desirable fish fingerlings, as well as control of undesirable species, especially predators, diverts the biomass of the "non-suitable" species into production of desirable fish biomass. It should be added, though, that monoculture, especially of molluscs in brackish water and of predatory fish such as trout, salmon, or groupers, can furnish high annual yields that are commercially attractive in spite of substantial inputs: for these species, the food is extraneously supplied.
A brief comparison of materials flow in ponds versus cage culture seems appropriate here. The food webs of manured, fed, and fertilized polyculture ponds are exceedingly complex in comparison with cage monoculture systems.
The principal pathways of nitrogen in ponds are illustrated in Figure 2. Possible nitrogen inputs to intensively managed polyculture ponds may be many and varied: atmospheric nitrogen, and nitrogenous substances present in in-flowing water, in inorganic fertilizers, in organic wastes, and in feeds. Similar diversity is also present in losses of nitrogen from ponds. Measurement of nitrogen flow between the various nitrogen pools (e.g., dissolved nitrogen pool, bacterial nitrogen pool, etc.) is a difficult task, requiring sophisticated te