Growth and protein turnover in Atlantic salmon ( Salmo salar L.); the effect of dietary protein level and protein particle size

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In modern fish feeds, the protein sources consist of denaturated finely ground ingredients. From the literature, it has been reported that use of coarsely chopped, but not denaturated, fish as the dietary protein source gave better growth performance and protein utilisation. Growth, feed utilisation and protein turnover using two different fish meal particle sizes (micro- or coarse-grounded) at three dietary protein concentrations (30%, 35%, and 45%) were studied in individually tagged Atlantic salmon in a 3-month growth experiment. At the end of the experimental period, 14C- l-lysine was injected intraperitonally and dorsal muscle samples were taken at 2- and 4-h post-injection. Incorporation of 14C- l-lysine into muscle protein, RNA, DNA and water soluble protein was analysed from samples of muscle tissue. Only small effects on growth rate, feed conversion rate, protein and energy retention, and nitrogen and fat digestion were found. During the growth experiment, large individual variations in growth rates were observed, which did not correlate to the initial body weight. The total RNA content expressed as RNA amount per unit of DNA (RNA:DNA ratio) did not reflect the specific RNA activity, and individual growth rate was not correlated to the specific RNA activity or RNA:DNA ratio and only poorly to the relative incorporation rate of amino acids. Growth rate was, however, correlated to the relative efficiency of protein synthesis. The results indicate that the protein catabolism is more important for net protein deposition and growth than protein anabolism.