Using ammonia nitrogen excretion rates as an index for evaluating protein quality of prawns in turbot (Psetta maeotica) nutrition

Abstract

Total ammonia nitrogen excretion rates were measured in Black Sea turbot (averaging 88.8 g) to compare the protein quality of prawns, the natural food of turbot, with that of the protein source commonly used in the feed industry, i.e. anchovy meal. Two different prawn species (Baltic prawn, Palaemon adspersus, and rockpool prawn, Palaemon elegans) were offered to fish as wet feed at 17.5 +/- 0.5 degrees C and a salinity of 17 ppt. Furthermore, for determination of the effects of wet feed and dry feed on the peak times of ammonia nitrogen excretion rates, another experimental group of fish was offered a commercial dry diet with anchovy meal as a single protein source. The ammonia nitrogen excretion rate in both groups fed prawns peaked 3 h after feeding, while the peak of the excretion rate of fish fed the dry diet was delayed up to 6 h after feeding. Cumulative ammonia nitrogen excretion rates as well as the excretion as a proportion of ingested nitrogen were significantly lower (P < 0.05) in fish fed prawns than those in fish fed the dry diet. Significantly lower excretion levels in the prawn groups might be a reflection of the protein quality of these species, which may be higher than that of the anchovy meal for turbot nutrition.