Performance of juvenile Murray cod, Maccullochella peelii peelii (Mitchell), fed with diets of different protein to energy ratio

The results of a 56‐day experiment on juvenile Murray cod, Maccullochella peelii peelii, an Australian native fish with a high aquaculture potential, of mean weight 14.9 ± 0.04 g, fed with five experimental diets, one a series of 40% protein content and lipid levels of 10, 17 and 24% (P₄₀L₁₀, P₄₀L₁₇ and P₄₀L₂₄), and another of 50% protein and 17 and 24% (P₅₀L₁₇ and P₅₀L₂₄) lipid are presented. The specific growth rate (SGR) (% day^?1) of fish maintained on different diets ranged from 1.18 to 1.41, and was not significantly different between dietary treatments, except P₄₀L₁₀ and the rest. However, there was a general tendency for SGR to increase with increasing dietary lipid content at both protein levels. The food conversion ratio (FCR) for the 40% protein series diets were poorer compared with those of the 50% protein diets, and the best FCR of 1.14 was observed with the P₅₀L₁₇ diet. The protein efficiency ratio (PER), however, was better in fish reared on low protein diets. The net protein utilization (NPU) also did not differ significantly (P > 0.05) in relation to dietary treatment. As in the case of PER the highest NPU was observed in Murray cod reared on diet P₄₀L₂₄ and the lowest in fish fed with diet P₅₀L₂₄. The carcass lipid content reflected that of the diets, when significant increases in the lipid content was observed in relation to dietary lipid content at both protein levels. However, body muscle lipid content did not increase with increasing dietary lipid content, and was significantly lower than in the whole body. The fatty acids found in highest concentration amongst the saturates, monoenes and polyunsaturates (PUFAs) were 16 : 0, 18 : 1n‐9 and 22 : 6n‐3, respectively, and each of these accounted for more than 60% of each of the group's total. The muscle fatty acid content was affected by the dietary lipid content; for example the total amount (in μg mg^?1 lipid) of monoenes ranged from 72 ± 5.1 (P₄₀L₁₀) to 112 ± 10 (P₄₀L₂₄) and 112 ± 2.8 (P₅₀L₁₇) to 132 ± 11.8 (P₅₀L₂₄) and the n‐6 series fatty acids increased with increasing dietary lipid content, although not always significant. Most notably, 18 : 2n‐6 increased with the dietary lipid level in both series of diets.