Effect of sediment on growth and acute ammonia toxicity for the school prawn, Metapenaeus macleayi (Haswell)

The effects of sediment on growth, survival, food conversion efficiency and acute ammonia toxicity were determined for the school prawn, Metapenaeus macleayi, a species which normally buries in sediment during the day. Survival of prawns in 70-litre acrylic aquaria was high (90–100%) regardless of the absence of sediment (bare plastic) or the type of sediment (mud, fine sand or coarse sand) or whether the prawns were confined in mesh cages. Growth (weight gain) and food conversion efficiency for confined prawns were significantly lower than for non-confined prawns in aquaria with or without sediment. Prawns grew 11–22% faster in aquaria with sediment than in aquaria without sediment although the type of sediment had no effect on growth. In a 96-h acute toxicity experiment, mortality of prawns increased with increasing ammonia concentration and, at a concentration of 31 5–32 mg total ammonia-N (TAN)/I, was higher in aquaria without sediment (30.0 ± 5.8%; mean ± s.e., n = 3) than in those with sediment (6.7 ± 6.7%). Emergence of prawns from the sediment was affected by time of day and ammonia concentration. During the day, emergence increased from 7.3 to 79.2% with increasing ammonia concentration (< 0.05 to 52.8 mg TAN/1), but most prawns were emergent during the night regardless of ammonia concentrations. In aquaria without prawns, ammonia concentrations and pH were always lower in water extracted from the sediment than in the water column. The effects of different arrangements of sediment, filtration and aeration on ammonia concentrations in aquaria are described in relation to conducting and interpreting ammonia toxicity experiments with prawns.