Based on field observations of Melard and Philippart (1980),
where growth rates of Nile tilapia declined at DO concentration
of 3 mg L−1, Soderberg (2006) suggested that minimum DO
tension for flowing-water tilapia culture should be 60 mm Hg,
equivalent to 2.9 mg L−1 at 32.7 °C, the average temperate of
the present study (conversion based on Colt, 1984). If oxygen
supply to fish is limited by diffusion through the gills, it may not
be possible to define one single incipient oxygen concentration
below which fish growth starts decreasing. The incipient
concentration would depend on the balance between oxygen
supply to the fish, determined by the oxygen gradient across the
gills, the gill surface area (and therefore body weight) and the
blood–water distance; and oxygen demand by the fish, determined
by the amount and composition of the feed and the
activity level of the fish. In the present study, fish were fed to
satiation with a high-protein diet. Their requirements for oxygen