Partial replacement of fishmeal with soybean meal (SBM) as a sustainable and economic alternative has been explored in a number of commercially targeted aquaculture species, including freshwater rainbow trout (Oncorhynchus mykiss). However, soy and other plant-based materials contain high levels of saponins and other anti-nutritional factors that trigger subacute enteritis of the distal intestine in many of these fishes. This condition is characterized by increased intestinal permeability, inflammation, and morphological abnormalities leading to decreased feed efficiency and impaired growth.

In rainbow trout, including SBM above 20% of the diet has been shown to induce soy-enteritis, placing a physiological threshold on the level that can be substituted into a standard aquaculture diet. Previous research has examined a number of mechanisms to combat this enteritis, including manipulation of the gut microbiome, ingredient processing to remove anti-nutritional factors, and antioxidant and probiotic additives. One unexplored approach is inclusion of trimethylamine oxide (TMAO) in aquaculture feeds. TMAO is a universal cytoprotectant, accumulated in numerous species as a protein and membrane stabilizer. Here, we test the ability of TMAO to enhance enterocyte stability and suppress the inflammatory HSP70 signal thereby combatting soy-induced enteritis and leading to increased feed efficiency, retention and growth in freshwater rainbow trout. Further, we examine whether marine fish solubles, a rich source of TMAO, can be used as an economically practical means of administering this additive, enabling its application on the commercial scale.

Farmed rainbow trout (Troutlodge Inc.) were stocked at a mean initial weight of 40 g and n=15 per tank into triplicate treatment tanks. Tanks were fed one of six diets prepared on a digestible nutrient basis providing 40% digestible protein, 15% crude fat, and meeting ideal amino acid concentrations. Diets included a fishmeal 40 control (% of dry diet), SBM 40, SBM 40 + TMAO 3 g kg^-1, SBM 40 + TMAO 10 g kg^-1, SBM 40 + TMAO 30 g kg^-1, and SBM 40 + 10% fish solubles. Tanks were fed twice daily to apparent satiation for 12 weeks and fecal, proximate, histological and molecular analyses conducted.

The results of this study will be discussed as well as the utility of including TMAO to enable higher utilization of U.S. soy products in salmonid aquafeeds.