The effect of free ammonia nitrogen, pH and supplementation with oxygen on the growth of South African abalone, Haliotis midae L. in an abalone serial-use raceway with three passes

The farming of abalone, Haliotis midae L., can be intensified in serial-pass systems, but water re-use increases the concentration of NH3 (free ammonia nitrogen, FAN) and reduces water pH. Changing the percentage dietary protein from 33% to 26% reduced the concentration of FAN (F-42,F- 252= 2.79; P<0.0001) in a serial-pass system and did not reduce weight gain (F-1,F- 12=1.09; P=0.31) or length gain (F-1,F- 12=1.08; P=0.31). Low water pH was the most important variable to contribute to a reduction in abalone growth (weight gain: F-1,F- 19=64.5; P<0.0001; r(2)=0.76; length gain: F-1,F- 19=41.9; P<0.0001; r(2)=0.67). In addition, supplemental oxygen (103% saturation) improved length gain (t=3.45, P=0.026) in abalone exposed to an average FAN concentration of 2.43 +/- 1.1gL(-1)) and an average pH value of 7.6 +/- 0.13, relative to a treatment with no oxygen supplementation. Thus, in an abalone serial-use raceway with three passes, FAN was not the first growth-limiting variable. It is suggested that future studies should examine the major causes of reduced water pH in serial-use systems and their effect on the growth and health of H. midae.