Potential of plant-protein sources as fish meal substitutes in diets for turbot (Psetta maxima):: growth, nutrient utilisation and thyroid status

An experiment was conducted in order to assess the incorporation in diets for juvenile turbot of extruded lupin (Lupinus albus) and heat-treated (RM1) or untreated (RM2) rapeseed meals (Brassica napus) (26 and 40 mu mol glucosinolate/g DM, respectively). The level of incorporation of 30% for each plant-protein, as well as 46% for RM1 and 50% for lupin was tested and compared with a fish meal based control diet. Triplicate groups of turbot (initial body weight of 66 g) were fed by hand with isonitrogenous and isoenergetic experimental diets, twice daily and to visual satiety, during 63 days. Extruded lupin can be incorporated in diets of turbot up to a level of 50% without adverse effects on growth performance and body composition. Rapeseed meal can only be incorporated at levels up to 30%, but a preliminary heat treatment of RM is necessary in order to improve its nutritional quality. In turbot-fed the RM-based diets, plasma T-4 levels were reduced with low dietary content in glucosinolate breakdown products (3.6 mu mol/g), but no decrease in plasma T-3 levels was observed with the higher level of toxic compounds (4.4 mu mol/g). A significant deiodinase type II compensatory effect, leading to an increase of the conversion of T-4 to T-3, was observed in vitro in the liver of turbot fed RM1-based diets. The intake of lupin-based diets also had an effect on thyroid status with an increase of plasma T-3 levels and of deiodinase type I activity in liver and kidney, suggesting an increase in the degradation of rT(3) and in the conversion of T-4 to T-3. (C) 2000 Elsevier Science B.V. All rights reserved.