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 f or juvenile turbot of extruded lupin (Lupinus albus) and heat-treated (RM1) or untreated (RM2) rapeseed meals (Brassica napus) (26 and 40 mu mol gluco sinolate/g DM, respectively). The level of incorporation of 30% for each pl ant-protein, as well as 46% for RM1 and 50% for lupin was tested and compar ed with a fish meal based control diet. Triplicate groups of turbot (initia l body weight of 66 g) were fed by hand with isonitrogenous and isoenergeti c experimental diets, twice daily and to visual satiety, during 63 days. Ex truded lupin can be incorporated in diets of turbot up to a level of 50% wi thout adverse effects on growth performance and body composition. Rapeseed meal can only be incorporated at levels up to 30%, but a preliminary heat t reatment 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 die tary content in glucosinolate breakdown products (3.6 mu mol/g), but no dec rease in plasma T-3 levels was observed with the higher level of toxic comp ounds (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 vi tro in the liver of turbot fed RM1-based diets. The intake of lupin-based d iets also had an effect on thyroid status with an increase of plasma T-3 le vels and of deiodinase type I activity in liver and kidney, suggesting an i ncrease 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.