Protein growth performance, amino acid utilisation and somatotropic axis responsiveness to fish meal replacement by plant protein sources in gilthead sea bream (Sparus aurata)

Partial or total replacement of fish meal by a mixture of plant protein sources (corn gluten meal, wheat gluten, extruded peas, rapeseed meal) balanced with indispensable amino acids (IAA) was examined in juvenile gilthead sea bream (Sparus aurata) over the course of a 12-week growth trial. A diet with fish meal (FM) as the sole protein source was compared to diets with 50%, 75% and 100% of replacement (PP50, PP75, PP100). Protein retention was improved with more plant protein supply, and just a slight decrease in the final weight gain was found in fish fed PP50 and PP75 diets. However, in the PP100 group, weight gain was depressed up to 30% mainly as the result of a marked reduction of feed intake. These fish also showed a lower fat gain along with a marked hypocholesterolemic effect. Dietary treatment did not alter the hepatic activity of amino acid catabolising enzymes (alanine aminotransferase (ALAT), aspartate aminotransferase (ASAT), glutamate dehydrogenase (GDH)), although the size of the total muscle free amino acid (FAA) pool was increased by more plant protein supply. The activity of the somatotropic axis also varied among experimental groups, and the up-regulation of circulating growth hormone (GH) levels with a high plant protein supply followed the decrease in growth rates, plasma levels of insulin-like growth factor-I (IGF-I), and liver mRNA transcripts of lGF-I and GH receptors. This catabolic feature evidenced a liver desensitisation to the anabolic action of GH in the PP100 group, and to a lesser extent in the PP75 group. Taken together all these findings, up to 50-75% of fish meal replacement seems to be feasible with IAA supplementation, but further research is needed to fully identify the responsible factors for the depressed feed intake in order to achieve a full replacement in a fish species having high dietary protein requirements. (C) 2004 Elsevier B.V. All rights reserved.