Aquaculture Water Quality for Small-Scale Producers - Aquaculture and the Environment: A Shared Destiny - page 126

under realistic, idealized situations. But there is a necessity to find practical solutions
applicable for commercial aquaculture.
The problem of the academic extension is an important issue in aquaculture if we consider
that small-scale aquaculture provides almost half of the worldwide inland fish production
(FAO, 2010). Kawarazuka (2010) commented “aquaculture and small-scale fisheries can
improve the food intake”. Mohanty et al. (2010) mentioned that at least 90 % of the people
involved in aquacultural practices works in small fisheries.
Facing the problem of the continuous rising world population, it seems to be clear that the
good use of all natural resources (including small freshwater bodies), must be done. So there
is an important, unattended practical knowledge field to be considered. Technologies for
small-scale fish producers are ready available (Van Gorder, 2003), and the current
knowledge on aquatic science can be helpful for the producer with the goal to optimize its
own resources (time, land, water and energy).
The objective of the present chapter is to present selected topics on water quality
management, especially if they can be carried out by low-cost technologies in small systems.
Due to the close relationship among water quality and yields, we hope it can be helpful as a
practical guide for fish producers based on scientific principles.
2. Overview of water quality in aquaculture
Water is the physical environment where fish develop, growth and reproduce. The dynamic
of the mass and energy involved on an aquaculture system is complex, because bacteria,
algae and fish growth together in the pond (Wheaton, 1982). The main energy input comes
from sunlight, and nutrients of the system are commonly provided by pelletized fish feed.
So the transformation of the elements carried out by autotrophic and heterotrophic
organisms change the physical/chemical/biological composition of the water (Hargreaves,
1998).
Temperature and pH are fundamental for the aquatic living organisms, due to the intimate
relationship between them and the velocity of its biochemical processes. Oxygen (O 2 ) and
carbon dioxide (CO 2 ) are important molecules because they are involved in photosynthesis
and respiration processes. Nutrients like nitrogen are essential on biological metabolism:
when is ingested as protein or amino acid, it can be incorporated by the organism as
functional proteins or in structural tissues. But when is excreted by the fish as ammonium
(NH 3 ), in certain circumstances it can be toxic and even lethal in high doses. Another
nutrients, like phosphorus, potassium and calcium are also important, and they lack are
usually diagnosed by deficiency.
Besides the physical and chemical factors, the biotic component also can change the water
composition. For example, algae can consume of produce oxygen and carbon dioxide,
depending on light presence or absence. The nitrogen, phosphorus and potassium can be
used and assimilated by unicellular organism, incorporating them as biomolecules.
Moreover, the nitrogen can be also used to produce energy on chemoautotrophic
biochemical cycles (Hargreaves, 1998).
So, the biological impacts of water quality over the cultivated species could be analyzed
under physical, chemical or biological perspectives. For practical purposes, a quick