Influence of dietary digestible energy content on growth, protein and energy utilization and body composition of growth hormone transgenic and non-transgenic coho salmon (Oncorhynchus kisutch)

Triplicate groups of size (mean initial weight, 12.9 g)- and age (1.5 years old)-matched growth hormone transgenic (T; construct OnMTGH1) and non-transgenic (NT) pre-smolt coho salmon held in running, 8-11 degrees C well water on a natural photocycle, were each fed to satiation twice daily one of four dry diets for 84 days. The diets had similar ratios of digestible protein to available energy (21-23 MJ/kg dry diet), identical essential amino acid balance, and supplemental vitamin and mineral levels, but different digestible energy (DE) content (i.e., 15, 17, 19 or 21 MJ/kg dry weight) by varying alpha-cellulose, anchovy oil and pre-gelatinized wheat starch concentrations. The intent was to determine whether the two genotypes of fish differed with respect to their abilities to ingest food to satisfy their daily energy needs. Moreover, for T fish, which characteristically have high and unregulated growth hormone production, we were interested to determine whether the physical distention of the stomach and other aspects of their digestive physiology would allow similar available protein and energy intake and, consequently, fish growth across a wide range of dietary DE content. All diets supported excellent growth in T coho, but only diets with >= 17 MJ DE/kg supported growth in NT coho. Specific growth rates (SGR) in weight of both coho genotypes were directly related to dietary DE but were uninfluenced when DE was >= 17 MJ/kg. Values for SGR for T coho fed diets with 17-21 MJ DE/kg were 6-13-fold higher than those of NT counterparts. Dry feed and gross energy intakes were directly related to dietary DE in NT coho, whereas the opposite was true in T coho when the values were expressed on a body weight basis. Values for feed efficiency were positively correlated with dietary DE in both genotypes and significantly elevated by each DE increment in T fish. Gross energy and available (digestible) energy utilization values were uninfluenced by dietary treatment in T fish. Percentages for gross (PPD) and available protein deposited were unaffected by diet treatment in T fish, but PPD in NT coho was significantly depressed in fish fed the diet with lowest DE. In T coho, apparent digestibility coefficients for gross energy and organic matter were directly related to dietary DE, whereas the opposite trend was found for protein digestibility. Terminal percentages of body lipid content were directly related to dietary DE content in T salmon. (c) 2005 Elsevier B.V. All rights reserved.