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

Abstract

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 (F42, 252 = 2.79; P < 0.0001) in a serial-pass system and did not reduce weight gain (F1, 12 = 1.09; P = 0.31) or length gain (F1, 12 = 1.08; P = 0.31). Low water pH was the most important variable to contribute to a reduction in abalone growth (weight gain: F1, 19 = 64.5; P < 0.0001; r2 = 0.76; length gain: F1, 19 = 41.9; P < 0.0001; r2 = 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 ± 1.1 μg L−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.