Dietary acidification, decreasing the pH of the diet, has been used in the aquaculture industry to inhibit microbial growth and promote fish and gut health. However, research has shown that acidification also has the ability to enhance growth and digestibility metrics across a variety of aquaculture target species, including rainbow trout, red drum, and barramundi. Currently, typical commercial aquafeeds exhibit a pH of approximately 6.0 and are not designed to maximize the biological potential of the fish based on pH.
After feeding, fish must decrease stomach pH to facilitate digestion. This drop is initiated by parietal cells in the stomach epithelium that secrete protons (H+) into the stomach lumen and bicarbonate ions (HCO3- ) into the bloodstream. Termed the alkaline tide, this process is energetically expensive and can last up to 48 hours in trout. In this study, we examined the effect of diet pH on growth performance and postprandial gut pH, blood chemistry, and metabolism in rainbow trout .
A 12-week feeding trial was conducted with rainbow trout (Oncorhynchus mykiss , Troutlodge Inc., Sumner, WA) . Experimental diets (n=10) were formulated at 45% crude protein, 15% fat with amino acid supplementation to meet or exceed the known nutritional requirements of this species. This practical trout formulation expressed a mash pH = 5.7 without modification. Experimental diets ranged from pH = 3.5 to 6.8. Diet pH was modified through the addition of hydrochloric acid (diets 1-6, pH 3.5 - 5.7) or sodium hydroxide (diets 7-10, pH 5.9 - 6.8). Diets with a pH below 5.6-5.7 resulted in a decrease in growth and feed in take. Diets with a pH below 5.3 resulted in a decrease in feed efficiency. Additional results examining digestibility, feed evacuation rate, and the physiological and metabolic state of the fish post-feeding will be presented and discussed as they relate to the performance data. It remains unclear what drove the discrepancy between the negative results seen in this study and the benefits of dietary acidification published in the literature for the same or other fish species.