Estimating digestible protein requirements of silver perch, Bidyanus bidyanus Mitchell

In this study, we estimated requirements for digestible protein, using intact protein sources, at one digestible energy content. Using digestibility data for silver perch (Bidyanus bidyanus Mitchell) for a large number of ingredients, we formulated a 'summit' diet to contain between 1.4 and 1.8 times the 'expected requirements' for digestible essential amino acids (based on requirements for channel catfish, Ictalurus punctatus Rafinesque). A 'diluent' diet was formulated to contain 0.4-0.5 times the expected requirements of digestible essential amino acids. Both 'summit' and 'diluent' diets contained similar digestible energy (14.7 MJ digestible energy kg-1 for the summit and 13.4 MJ digestible energy kg-1 for the diluent). Six diets were prepared with the following amounts of summit-diluent diets: 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100. A practical diet widely used by commercial farmers was also included as a control. Ten juvenile fish (2.1-2.6 g) were stocked into each experimental 70-L acrylic aquarium, and each dietary treatment was randomly assigned to five replicate aquaria. Fish were fed twice daily to apparent satiation for 54 days. Final individual fish weight ranged from 4-15.5 g. Results were analysed using intersecting linear regression analysis. The optimum digestible dietary protein for diets with 13.4-14.7 MJ digestible energy kg-1, after which protein deposition did not increase significantly, was 28%. Although this study did not determine requirements for individual amino acids, for diets with the digestible energy content used here, requirements for individual amino acids obviously did not exceed the content in the 28% protein diet. These contents are useful as an estimate of 'recommended levels' for silver perch diets with 13.4-14.7 MJ digestible energy kg-1. The proximate composition of fish was affected by diet. Whole body protein and moisture increased, whereas lipid content decreased with increasing dietary protein content (and increasing protein-energy ratio and decreasing lipid). Fish size was also affected by diet; however, the changes in whole carcass proximate composition also occurred for fish fed diets 60:40, 80:20 and the summit diet which were a similar final weight.