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Notes: The effects of different processing techniques on apparent digestibility coefficients (ADCs) and performance of silver perch Bidyanus bidyanus were evaluated. Results of a factorial digestibility trial, in which three diets (SP35, 95LC1 and 95LC2) and three processing methods (cold pelleted, steam pelleted and extruded) were evaluated, indicated that extrusion, but not steam conditioning, significantly improved ADCs of dry matter (DM) and energy. ADCs of DM and energy of cold-pelleted diets were statistically similar to steam-pelleted diets, and ADCs of nitrogen were unaffected by processing method. No interaction was found between diet type and processing method for DM, energy or nitrogen ADCs. A performance trial indicated that feed intake, weight gain and specific growth rate of fish fed steam-pelleted diets was greater than those of fish fed extruded diets. Feed conversion and digestible protein efficiency was better in fish fed extruded diets. The results indicated that extruded diets were better utilized than steam-pelleted diets; however, voluntary intake of extruded diets may have been limited. Fish fed cold-pelleted SP35 exhibited inferior performance compared with fish fed steam-pelleted or extruded SP35. Reduced performance of fish fed this diet may relate to poor utilization of digestible protein or reduced palatability. Diets for silver perch with similar formulations to SP35 and 95LC2 should be steam pelleted.

Notes: 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.