A comparison of the digestibility of a range of lupin and soybean protein products when fed to either Atlantic salmon (Salmo salar) or rainbow trout (Oncorhynchus mykiss)

This study compares the digestibility of a series of lupin and soybean protein products when fed to either rainbow trout or Atlantic salmon. The test ingredients in the study, from one of two key grain resources (lupins: Lupinus angustifolius and soybeans), represented various levels of processing of each grain in order to increase the protein content of the meals. A reference ingredient of enzymatically hydrolyzed casein (EHC) was also included in the study. The rainbow trout (266±18 g) were housed in freshwater tanks (250 l, salinity <1‰, 22.1±1.8°C) and acclimated to the diets for 6 days before faecal collection commenced. The Atlantic salmon (66±10 g) were housed in similar freshwater tanks (250 l, salinity <1‰, 15°C) and acclimated to the diets for at least 6 days before faecal collection commenced. Faeces were collected from each fish species using settlement collection methods. The digestibility of organic matter, phosphorus, energy and nitrogen was assessed using the diet-substitution method, with each test ingredient included in the diet at 30%. Several differences were observed between the two fish species in their capacity to digest nutrients and energy from each of the products. Organic matter and energy digestibility of each of the ingredients was largely reflective of the protein content of each ingredient. Protein digestibilities were generally consistent between the two fish species with only lupin kernel meal having a significantly higher digestibility when fed to Atlantic salmon than rainbow trout and the soybean protein concentrate a significantly lower digestibility. Although limited differences in protein digestibility were noted among the ingredients when fed to rainbow trout, more substantial differences were noted when the same ingredients were fed to Atlantic salmon. The digestible energy value of the range of products examined was generally higher in Atlantic salmon than rainbow trout. A clear difference between the two fish species was their capacity to digest phosphorus from the ingredients, with several of the plant protein ingredients showing differences in phosphorus digestibility between the two fish species. Generally, both series of grain products have excellent potential as feed ingredients for either of these species. However, the digestive capacity of Atlantic salmon appears to more positively respond to the absence of dietary non-starch polysaccharide content than that of rainbow trout. © 2004 Elsevier B.V. All rights reserved.