Since fish feed is the primary source of N in fish hatchery effluents, nutritional mass balance models have been developed which partition the fate of dietary N into fish body accretion, recovered wastes, and unrecovered wastes. A basic nutritional mass balance model for calculating N lost from a fish culture facility is described by:

N_ho = N_fed - [N_s + (N_te - N_tb) + N_m]

where:

N_ho = N of hatchery origin in effluent

N_fed = N in feed fed

N_s = N in removed tank solids

N_te = N in fish at the end of the growth period

N_tb = N in fish at the beginning of the growth period

N_m = N in dead fish removed from the system

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