Practical diet with low fish-derived protein is able to sustain growth performance in gilthead seabream ( Sparus aurata) during the grow-out phase

A 12-week performance trial was undertaken to evaluate the effects of a concomitant replacement of fishmeal and fish oil in a practical diet for gilthead seabream with a complementary mixture of vegetable proteins (soy, peas, corn and wheat) and oils (soybean, rapeseed), in terms of growth performance, feed utilization, apparent digestibility of nutrients and soluble nitrogen (N) and phosphorus (P) excretion. Fifteen homogenous groups of 50 seabream each (mean initial body weight: 180.7 ± 0.4 g) were stocked in 1000-L tanks and fed one of five experimental extruded diets formulated to be isonitrogenous, isolipidic and isoenergetic. A control diet (CTRL) was formulated with practical ingredients to contain 48% protein, 20% fat and 23 kJ/g energy. Two other diets were formulated in order to replace 40 and 60% of fishmeal by increasing levels of selected plant-protein ingredients (PP40FO and PP60FO, respectively). Based on these two last diets, two others were formulated in which fish oil was replaced at a 65% level by a mixture of soy and rapeseed oils (PP40VO and PP60VO). Growth of seabream, expressed either as weight gain or daily growth index was not significantly affected by the replacement at either 40 or 60% of fishmeal by plant-protein sources. At 40% fishmeal replacement level, the further replacement of 65% of fish oil by vegetable oils had no effect on growth performance. However, the concomitant replacement of 60% fishmeal and 65% replacement of fish oil caused a slight reduction in weight gain, but essentially a significant decrease in feed efficiency (FE). Proximate composition of fish was not affected by the various dietary treatments. Replacement of both fishmeal and fish oil had no significant effects on daily deposition of N, fat or P. Soluble P excretion was significantly reduced by the use of plant protein-rich diets. Growth performance of gilthead seabream during the grow-out phase was sustained by a practical dietary formulation containing as little as 13% of marine-derived proteins.