Digestibility and amino acid availability of three protein-rich ingredient-incorporated diets by Murray cod Maccullochella peelii peelii (Mitchell) and the Australian shortfin eel Anguilla australis Richardson

In this study, the apparent dry matter (ADM), protein (PD) and energy (ED) digestibility, and the amino acid availability (essential, EAAA; non-essential, NEAAA; total, TAAA) of diets incorporated with one of three protein-rich ingredients (soybean meal, shark meat meal waste and meat meal) were evaluated for Murray cod Maccullochella peelii peelii (Mitchell) and the Australian shortfin eel Anguilla australis Richardson. The reference diets (RDs) used for Murray cod and shortfin eel had 50% and 45% protein, and 10% and 15%, lipid respectively. The test diets consisted of 30% ingredient and 70% RD, and digestibility estimations were made using chromic oxide as a marker. In both species, the highest ADM and PD of the test diets was observed for shark meat meal (73.1 ± 1.58 and 87.5 ± 1.27) and soybean meal (70.6 ± 0.82 and 86.5 ± 0.49) diets respectively. The PD of meat meal-incorporated diets was the lowest and, in shortfin eel, significantly so compared with all the experimental diets. For any one species, the ED of the diets did not differ significantly. The above observations were also reflected in dry matter and nutrient digestibility of ingredients. Significant differences (P < 0.05) in EAAA among diets were evident in both species. In shortfin eel, TEAAA for the meat meal-incorporated diet (50.5 ± 4.25) was significantly lower than for all the other diets. In Murray cod, TEAAA, TNEAAA and TAAA (89.7%, 82.1% and 85.2% respectively) were significantly higher (except for TNEAAA) for the meat meal-incorporated diet than for the reference diet (84.5%, 77.6% and 80.6% respectively), and all essential amino acids of this diet were available in excess of 82%. The results indicate species differences in the utilization of ingredients. The present data on PD and EAAA, combined with previously published data, indicate a close correlation between these two parameters, suggesting that PD may provide a fairly reliable indication of the amino acid availability.