| Summary: | Trials were conducted to investigate the effect of species, fish size and dietary protein/energy ratio on the efficiency of feed use for growth (biomass) in salmonids. To gain insight on efficiency of this conversion and how the above factors affected the efficiency of this conversion, several approaches were used. First, the efficiency of conversion of feed to growth was investigated by simple growth assays. Weight gain, feed efficiency (weight gain/feed intake) and composition of the gain was species specific. Diet composition had no affect on weight gain or composition of gain. However, feed efficiency decreased (P < 0.05) with decreasing digestible protein/digestible energy ratio (DP/DE) for juvenile lake trout, Atlantic salmon and chinook salmon and rainbow trout, while no effects were observed for postjuvenile fish (P > 0.05). There were species differences (P < 0.05) in nutrient and energy digestibility. Digestible nitrogen (DNRE) and energy retention (DERE) efficiency also differed among species (P < 0.05) and diet affected DNRE (P < 0.05) but not DERE (P > 0.05). The energetics of growth was investigated by empirical approaches of partitioning of metabolizable energy (ME) utilization for non-productive (maintenance) ' vs.' productive aspects of growth. The maintenance requirements (MEm) and the cost of growth (kg) and of protein (k p) and lipid deposition (kf) were estimated by either linear or non-linear regression models as well as by a multivariate analysis. K g averaged 0.63, irrespective of species, fish size and diet. K p varied from 0.53 to 0.81 and it was higher (P < 0.05) for post juvenile salmon compared to post juvenile trout and were not affected by diet (P > 0.05), while kf varied from 0.81 to 0.94, irrespective of species, fish size and diet. Using respirometry, estimates of total heat production (HE), basal metabolism (HeE) and heat increment of feeding (HiE) were obtained and species variation investigated. Estimates of HeE and HiE were used to validate MEm and ...
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