Biological productivity as an index of water quality and production potential of cultured organism needs prime consideration for site selection. Productivity in terms of qualitative and quantitative aspects of plankton and benthos are treated separately, in this manual. In the present discussion we shall look first at primary production and then show this influences secondary production in water bodies. We shall explain first certain concepts to elucidate aspects of productivity and then the energy flow in an ecosystem and the importance of this information in aquaculture. Finally we shall briefly refer to the methods of measuring productivity.
We have had occasion to look at the various physical and chemical and certain biological characteristics of water bodies and their influence on fish production and aquaculture in particular. I specially refer to the exercise on correlating fish production and dissolved solids (conductivity) and depth of water bodies, using Rider's (Ryder et al., 1974) morpho-edaphic index, MEI, (Handerson & Wellcome, 1974). It is quite obvious that there is a strong correlation between fish production and the amount of dissolved solids (inversely correlated with depth of the water body), in fresh waters except generally at the extreme high level. This has to be so because fish production is based on the productivity of water which is again, correlated directly with the amount of available nutrients and minerals, most often reflected by the dissolved solid content.
Before we enter into a discussion on productivity, it would be helpful to look at the concepts of the ecosystem, habitat and ecological niche, and food cycle in water bodies, including food chain and trophic structure. These terms are of specific interest to aquaculture and productivity in general and therefore deserve some consideration.
