The activity of the somatotropic axis was analysed in juvenile rainbow trout (Oncorhynchus mykiss) fed either a fishmeal-based diet (FM) or graded levels of plant proteins to replace 50 % (PP50 diet), 75 % (PP75 diet) or 100 % (PP100 diet) of the fishmeal protein. For this purpose, partial cloning and sequencing of the gene encoding rainbow trout growth hormone receptor (GHR) was first accomplished by RT-PCR, using degenerate primers based on the sequences of non-salmonid fish GHR. Growth rates and energy retention were lowered by the PP75 and PP100 diets and a concurrent and progressive increase in plasma levels of growth hormone (GH) was found. However, no changes in hepatic GH binding and total plasma insulin-like growth factor (IGF)-I levels were observed among the four experimental groups. This fact agrees with the lack of changes in hepatic measurements of GHR and IGF-I transcripts. No consistent changes in IGF transcripts were found in peri-visceral adipose tissue and skeletal muscle, but GHR mRNA was up-regulated in the peri-visceral adipose tissue of fish fed the PP75 and PP100 diets, which would favour the lipolytic action of GH. Two specific bands (47 and 33 kDa) of IGF-binding proteins were found in the plasma of all analysed fish, but the sum of the two integrated areas increased progressively with plant protein supply, which might reflect a reduced free IGF availability. Therefore, in our experimental model, the growth impairment could be due, at least in part, to a lowered availability of biologically active IGF (free IGF fraction) rather than to liver GH desensitization or defect in IGF synthesis and release at the systemic and/or paracrine–autocrine level.

The incorporation of an attractant mixture by substitution of an equivalent amount of basal dietary mixture was studied in terms of feed intake and growth performance in European seabass juveniles (mean initial weight : 17 g) fed plant-protein rich diets during 21 days. Three diets had as main protein sources; fish meal, or one of two soy protein concentrates (S and E). To the basal mixture of these last two diets, as well as to a corn-gluten based diet, an attractant amino acid mixture was added at a 2.5% level. Daily feed intake was measured throughout the experimental period. Feed intake and weight gain were highest in seabass fed the fish meal based diet. In those fed the soy protein concentrate diets, the addition of the attractant mixture improved growth significantly; voluntary feed intake and feed efficiency were also ameliorated in thesegroups. In terms of growth, feed intake and protein utilisation, lowest values were found in fish fed with the corn-gluten rich diet, supplemented with the amino acid mixture.