As the world population rises, the demand for protein-rich foods increases. The aquacultural sector can provide protein-rich food for the increasing world population. Several studies have tried to find sustainable and renewable protein sources to substitute fish meal (FM) in fish feed, however, some ingredients as vegetable protein and industrial by-products might have a poor nutritional composition and be inadequate in essential components (Zamani et al., 2020). The aim of this study was therefore to determine the optimal level of FM replacement within compound diets for juvenile rainbow trout (Oncorhynchus mykiss) by single cell protein (Uniprotein Aqua, Unibio).
Five diets were included in the study with a control diet (D1) formulated based on commercial feed for rainbow trout. The four other dietary treatments were formulated by replacing FM with SCP (Uniprotein Aqua, Table 1). All diets were isoproteic (42% crude protein), isolipidic (21% crude fat) and isoenergetic (22.1 MJ/kg) produced by Sparos Lda. (Portugal). Trout (n=720) were individually measured and randomly distributed in 24 tanks (500 L) attached to an IRTAmarTM unit for mechanical, biological filtration and UV treatment, after a two-week acclimation at the Institute of Agrifood research and Technology (IRTA, Spain). The dietary treatments were provided twice a day (0800 and 1300) through automatic feeders (Arvo-tec T drum 2000, Arvotec, Huutokosk, Finland) and feed residuals were collected after two hours to determine daily feed intake. The body weight (BW) and standard length (SL) of the trout were measured in the beginning and end of the trial (83 days). The initial BW was 11.3±0.2 g (mean±SD) and standard length 9.0±0.1 cm. Histological analysis of the liver and foregut-middle intestine was also collected and examined under a light microscope.
The optimal level of FM replacement with Uniprotein Aqua was estimated by quadratic regression to 41% (R2=0.57, P<0.05). The highest somatic growth was estimated for diet 3 compared with diet 5 (Table 1). No differences in total feed intake or feed conversion ratio (FCR) were measured despite the lowest FCR was determined for diet 3 (Table 1). No differences were found between dietary treatments for histology of the liver or intestine. For the liver no melanomacrophage centers or lymphocyte infiltration were found indicating healthy liver conditions for all diets. For the intestine, no inflammatory disorders or enteritis were recorded for any of the dietary treatments.
From this study it can be concluded that the optimal level for replacing FM with Uniprotein Aqua was 41% without affecting performance, growth, or histology of the liver and intestine.