For simplicity in calculations and because of incomplete information on the time dependence of production and consumption rates, the mass balance is often simplified to consider steady-state conditions in which ACCUMULATION becomes zero, and equation 1 can be written as:
INFLOW + PRODUCTION = OUTFLOW
+ CONSUMPTION (2)
And the INFLOW and OUTFLOW terms may be expressed as the product of the concentration of the substance being observed and the flow rate:
where Q is the flow rate (m3 d"1), Cl is the influent concentration (g m~3), and C0 is the effluent concentration
In a fully mixed rearing impoundment, the effluent concentration is approximately equal to the concentration at any point within the impoundment and is the concentration to which the target species is exposed. The production and consumption functions depend on the species, size, and condition of the fish and on the environmental conditions to which they are exposed. In addition, these rates fluctuate through diel cycles in response to activity levels associated primarily with feeding and digestion. In general, there is little quantitative information on how the various factors affect the rates of oxygen consumption and metabolite production, and for purposes of design, it is common to relate these rates to feeding levels (13,14):
SOLIDSprod = FEED X SOLIDSrc (8)
where DOcons is the rate of oxygen consumption by fish within the impoundment (g d-1), FEED is the amount of feed applied to the impoundment (gfeed d-1), DOrc is the oxygen consumption per unit of feed applied (g gf~ed), and similarly for carbon dioxide (C02), total ammonia nitrogen (TAN), and solids productions. Feeding rate is often expressed as a proportion of the fish biomass applied per day, and tables are available from feed manufacturers for various fish species and ages. Approximate values used for the consumption of oxygen and production of carbon dioxide, ammonia, and solids per unit mass of feed (g gfeed-1) are 0.20, 0.28, 0.03, and 0.30, respectively (13,14). These values can be useful as a first approximation, but detailed calculations must be based on values specifically developed for the species of interest and for the feed and culture system being used.
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