Analysis of feeding effects of EP on growth and digestion in cultured bluefin tuna

Abstract

To develop an extruded pellet (EP) appropriate for the cultured bluefin tuna Thunnus orientalis, we compared the feeding effects of EP and raw fish feed on their growth and digestion. EP-fed tuna had slower growth compared to raw fish-fed tuna after a decrease in pepsin activity, and growth was restored to the same level as that of the control when protease activity increased resulting from a bloated pyloric caeca. EP feeding delayed the intestinal arrival time of food compared to raw fish feeding, because food is retained for longer in the stomach. This corresponded with the time of expression of digestive-related factors in both groups, resulting in delayed expression of digestive enhancer (cck) and digestive suppressor (pyy) in the EP group compared with the control group. The expression levels of Ghrelin, a growth related factor expressed when nutrients are absorbed into the intestine, were also significantly higher in the EP group than in the control group. Moreover, expression levels and times of growth hormone (gh) were not significantly different between EP and raw fish feed. Therefore, bluefin tuna has the capacity to adapt their digestive physiology in response to EP in order to grow e-ciently.