Antarctic krill meal is a nutritional powerhouse which could minimize the over-reliance on traditional marine ingredients and add functional value in sustainable feed formulations for high trophic level fishes. To validate this, we carried out a 10-week study to evaluate the potential benefits of Antarctic krill meal inclusion in low fish meal feeds for rainbow trout. Five isonitrogenous experimental feeds with graded levels of high quality Antarctic krill meal (Aker QRILL - 0, 2.5, 5, 7.5 and 10%) and uniformly low fish meal content (15%) were fed to triplicate groups of fish, twice daily, to visual satiation. At the end of the trial, the effect of krill meal inclusion level on growth, feed use, carcass composition, tissue metrics, digestive enzyme activities, plasma metabolites, metabolic rates / scope, thermal tolerance limits and microanatomy of digestive organs was examined in the rainbow trout juveniles.
Regardless of the inclusion level, krill meal improved feed intake in rainbow trout. Correspondingly, thermal growth coefficient and specific growth rate was significantly higher in all the krill meal fed groups, without any difference in feed conversion efficiency. With respect to whole body composition, protein and ash content increased with krill meal inclusion. Daily nitrogen gain was high in the krill meal fed groups linked to high nitrogen intake. However, protein retention, protein efficiency ratio and nitrogen loss did not differ between the dietary groups. Daily lipid gain was significantly higher in KM2.5 and KM5 groups, whereas lipid retention decreased at high krill meal inclusion levels. Concerning phosphorus dynamics, whole body P content, daily P gain and P retention was high in the KM5 dietary group, consistent with higher intestinal alkaline phosphatase activity.
Krill meal inclusion was found to enhance intestinal trypsin activity. Particularly, KM5 dietary group showed coherently higher total protease, trypsin and leucine aminopeptidase activities. Likewise, intestinal lipase activity was significantly higher in the KM5 and KM10 groups. Intestinal microvilli height and density was also apparently higher in KM2.5 and KM5 fed fish. But intestinal amylase activity was found to decrease with krill meal inclusion. Plasma total protein and globulin levels, and hepatic total antioxidant capacity were high in KM5 and KM7.5 groups; whereas plasma triglyceride levels and aspartate aminotransferase activity were high in KM2.5 and KM7.5 groups. Irrespective of the inclusion level, krill meal elevated plasma albumin levels potentially indicating improved fish welfare. Conversely, hepatosomatic and viscerosomatic indices decreased in the krill meal fed groups, possibly linked to reduced fat deposition in the hepatocytes.
MO2max showed no difference between dietary groups. But, MO2rout showed significant decrease in KM5, KM7.5 and KM10 indicating improved energy homeostasis. The highest metabolic scope was observed in the KM5 dietary group. Dietary krill meal increased CTmax and also improved CTmin at high inclusion levels. Overall, 5% krill meal inclusion was found to be optimum to achieve improved feed intake, growth, digestion, carcass composition, antioxidant status, metabolic scope and thermal tolerance in trout fed low fish meal diets.