Aquaculture Water Quality for Small-Scale Producers - Aquaculture and the Environment: A Shared Destiny - page 128

Where [H + ] is the concentration of hydrogen ions. Fish and other vertebrates have a pH
blood value near to 7.4. The contact between environmental water and fish blood is only
separated by one or two cells of the gills. An ideal pH for an aquacultural system must be
near to 7. The lethal limits are below 5 and above 10, for most of the fish species.
There is an important relation between fish respiration and pH. In the gills the gaseous
interchange of O 2 instead CO 2 occurs. This interchange can be difficult if pH is not optimum.
The effects are called Bohr and root. So even if we have enough oxygen in our system, if pH
is not adequate fish could not breathe (Wurts & Durborow, 1992).
pH, like temperature, is always changing. For example: In afternoon, the oxygen
concentration decrease and the phytoplankton photosynthesis stops by the absence of
sunlight. The concentrations of O 2 and CO 2 began to change, and the pH can vary due its
intimate relationship with CO 2 equilibrium (Wurts & Durborow, 1992). Another important
factor to take into account is that cinematic of certain types of bacteria can change in low
pH, so the mass and energy transformations of the pool carried out by unicellular organism
can influence the environment (Ebeling et al., 2006).
2.1.3 Dissolved oxygen
According to Rumei et al. (2003), dissolved oxygen is the most important manageable
variable in aquaculture. The oxygen is necessary to glucose breakdown and energy release
inside fish cells. However, its diffusivity and availability on water is mediated by
temperature, elevation and salinity (Boyd, 1998).
Low concentrations of oxygen can produce negative impacts on fish health, like poor
growth performance, low feeding rate, and increase risk on potential diseases or even fish
death. These impacts are specific for every fish species. The particular oxygen requirements
depend principally on the fish biology. For example, trout needs a high quantity of oxygen
(about 7 ppm), but catfish (bottom, detritivore fish) can survive with only 0.5 ppm of oxygen
(Akinwole & Faturoti, 2007).
The presence of dissolved oxygen on aquaculture water ponds depends on physical
factors (salinity, temperature and altitude), algae photosynthesis or artificial supplying
(Boyd, 1998). Oxygen consumption depends on the carry capacity of the systems. The fish
biomass is an important factor to be considered in a system, because the food supplied to
the fish plays an important role in water ponds biogeochemistry (Timmons, 2002;
Hargreaves, 1998).
2.1.4 Ammonia
The ammonia is a nitrogen compound excreted by fish through gills and faeces. So the
amount of ammonia is in direct relation with the amount of feed input on the pond.
Ammonia can also be produced in pond by organic material decomposition driven by
bacterial activity (Durborow et al., 1997a).
The ammonia is presented in two forms: the high toxic un-ionized ammonia (NH 3 ), and the
non-toxic ionized ammonia (NH 4 + ). They are in chemical equilibrium driven by temperature
and pH. The sum of NH 3 and NH 4 + is called Total Nitrogen Ammonia (TAN). In general,