Growth and physiological responses of Nile tilapia, Oreochromis niloticus fed dietary fermented sugar beet bagasse and reared in biofloc system

Sugar beet bagasse (SB) is a common agro-industrial waste used in animal feeds. However, its high crude fiber content makes it difficult for fish to digest. Fermentation processes can improve the nutritional value of SB by-products. The study uses Lactobacillus rhamnosus and yeast (Saccharomyces cerevisiae) to ferment SB waste for use as a soybean meal substitute in Nile tilapia fingerlings. This led to an increase in protein, lipid, ash, and nitrogen-free extracts, while the fiber content decreased. Four experimental isonitrogenous (crude protein: 249.2 g kg⁻¹) and isocaloric (gross energy:18.25 MJ kg⁻¹) diets were formulated as follows: a control diet with 0 % LYFSB (L. rhamnosus and yeast fermented SB), and three other diets with LYFSB replacing soybean meal (SBM) at levels of 10 % (LYFSB_10 %), 20 % (LYFSB_20 %), and 30 % (LYFSB_30 %) based on protein content, over a 70-day period. 180 fingerlings of Nile tilapia (average initial weight of 5.19 ± 0.02 g) were placed at random in twelve circular plastic tanks (60 L capacity) with a stocking density of fifteen fish each. Nitrite and ammonia levels decreased as the replacement levels of LYFSB in tilapia diets increased, while nitrate levels showed the opposite trend. The phytoplankton community includes nine species in the biofloc system and four in tilapia gut content, with Chlorophyceae being the most common class, having the highest number in fish-fed diets (7805.67 organism/ml). The study revealed that LYFSB significantly influenced the zooplankton community, with protozoa dominating in biofloc water and rotifer in tilapia gut. LYFSB_10 % and LYFSB_20 % had the highest bacteria counts. Fish fed with LYFSB_10 % and LYFSB_20 % diets showed the highest values for specific growth rate (2.51 % day⁻¹) and weight gain (24.92 g fish⁻¹). These diets also exhibited increased activity levels of trypsin, lipase, and amylase enzymes. Increasing the inclusion of LYFSB in fish diets as a replacement for FM improved the intestinal and liver health of fish. The diets with LYFSB_10 % and LYFSB_20 % showed the highest levels of globulin, total protein, albumin, and IgM, with no significant differences between them. Fish fed LYFSB_10 % had lower cholesterol and triglyceride levels. Superoxide dismutase (SOD) and catalase (CAT) activities were highest in fish fed LYFSB_10 % and LYFSB_20 %, while malondialdehyde (MDA) activity was lowest in the LYFSB_30 % diet. The study showed that the biofloc system enhanced water quality and increased the efficiency of tilapia fed LYFSB_10 % and LYFSB_20 %.