Aquaculture is one of the fastest-growing food-producing sectors, with rainbow trout (Oncorhynchus mykiss) ranking as the third most produced finfish species in the United States. Formulated diets for trout have traditionally depended on fishmeal (FM) as the primary protein source but rising FM costs and limited supply have driven interest in alternative protein sources, mainly plant-based alternatives. This study evaluated fermented soybean meal (Fermex200) and conventional soybean meal (SBM) as partial and full replacements for FM in rainbow trout diets, focusing on growth, feed utilization, body composition, intestinal health, growth related gene expression, and production costs.
A 12-week feeding trial is underway using 414 rainbow trout at the grow-out stage (79.80 ± 4.21 g; 19.46 ± 0.22 cm). Fish were randomly assigned to six diets in triplicate tanks (23 fish per tank) within a closed recirculating aquaculture system. Diets were isonitrogenous (43% crude protein) and isolipidic (20% crude lipid), with FM replaced at graded levels: 0% (control-FM), 25% SBM (SBM25), 50% SBM (SBM50), 25% Fermex200 (Ferm25), 50% Fermex200 (Ferm50), and 50:50 SBM: Fermex200 (SBM+Ferm). Fish are being fed to apparent satiation, and growth monitored every four weeks.
Preliminary 28-day results showed no significant differences (p > 0.05) in growth and feed utilization (Table 1). The SBM50 group had the highest final weight (FW) and weight gain (WG), while the Ferm25 group had the best feed conversion ratio (FCR). Specific growth rate (SGR) remained consistent across treatments, feed intake and survival did not differ significantly (Table 1). The study and statistical analysis are ongoing, but preliminary findings indicate that up to 50-100% of dietary fishmeal can be replaced with SBM and/or Fermex200 without compromising growth performance in rainbow trout. Data for body composition, fillet quality, intestinal histology, gene expression and production costs will be analyzed at the end of the trial and presented.