Trout are a major aquaculture target species in the United States. Diet directly influences production efficiency and diet developments provide a practical and manageable means of enhancing farm performance. In this study, we focused on the effects of diet pH on energy expenditure in trout.
Trout require an acidic stomach environment for digestion. To achieve a low pH in the stomach, the fish must spend energy pumping H+ into the stomach lumen and, consequently, HCO3- into the blood. If the fish were able to obtain these H+ from a low pH diet, it is possible that they would not need to spend the energy sourcing them from the body. For this project, the metabolism of stomach, gill, and muscle tissue was measured to determine the effect that diet pH has on energy expenditure before and after feeding in rainbow trout (Oncorhynchus mykiss).
Ten experimental diets with a pH ranging between pH = 3.9 and 6.8 were fed to rainbow trout over twelve weeks. There were three tanks per diet with thirty fish per tank. During the trial, gill, stomach, and muscle samples were collected before and after feeding and frozen at -80℃ for later analysis. Each sample was then homogenized, and the supernatant was collected. The electron transport system (ETS) microplate assay was used to measure metabolic activity with a spectrophotometer.
It was found that in rainbow trout gill, fish fed a diet pH of 3.9 and 4.7 had a lower ETS activity than diets with a higher pH (Figure 1). These results were similar to oxygen consumption measured at the whole animal level and support our hypothesis that energy used for digestion would decrease with a decrease in diet pH. This may be explained by a reduced acid-base disturbance with feeding and decreased need for ion transportation at the gill. ETS activity was also seen to increase 1 to 6 hours after feeding and then decrease at 24 hours. This verifies the ability of the ETS assay to measure “specific dynamic action” (a predictable metabolic increase after feeding).