# Dietary Supplementation with Whole-Fat or Defatted Antarctic Krill Powder Improves the Growth Performance, Body Coloration,  and Immune Capability of Red–White Koi Carp (Cyprinus carpio var. koi)

## TL;DR

Adding Antarctic krill powder to koi carp diets improves growth, color, and immunity, but too much can harm the liver.

## Contribution

The study identifies optimal replacement levels of whole-fat and defatted krill meal in koi carp diets to enhance performance and immunity without liver damage.

## Key findings

- Replacing fish meal with krill meal improves growth rate and reduces liver and visceral indices in koi carp.
- Defatted krill meal increases antioxidant activity and carotenoid content more than whole-fat krill meal.
- Excessive krill meal causes liver damage, with recommended replacement levels of 20-30% for whole-fat and 10-20% for defatted krill meal.

## Abstract

Replacing fish meal with varying levels of whole-fat or defatted Antarctic krill meal can offer the following advantages: (1) enhanced SGR and WGR, and reduced HSI and VSI, thereby improving the growth performance of red-and-white carp; (2) increased carotenoid content in the scales and skin, along with enhanced expression of TYR for improved body coloration; (3) augmented lysozyme and superoxide dismutase enzyme activities, with better antioxidant effects observed in the defatted krill meal replacement group; and (4) substitution of whole-fat krill meal significantly downregulates the expression of LPL. Meanwhile, elevated levels of whole-fat replacement groups lead to a reduction in the liver AKP enzyme activity, as well as an increase in the number of lipid droplets. However, the defatted replacement groups at high levels exhibit a significant increase in the vacuolated degeneration of liver cells and various degrees of liver injury. It is recommended that the replacement level of whole-fat krill meal in compound feed, with a fish meal supplemental level of 500 g/kg, should be between 20 and 30%, while the replacement level of defatted krill meal should be between 10 and 20%.

This study was conducted to investigate the effects of replacing fish meal with either whole-fat or defatted krill powder on the growth, body color, immunity, and related gene expression of red–white koi carp. A total of 630 red–white koi carp with an initial body mass of 13.5 ± 0.05 g were randomly divided into seven groups with three replicates per group and 30 fish per replicate. The control group was fed a basic diet (C0). The other six diets were supplemented with different levels of whole krill meal or defatted krill meal as replacements (10% whole fat, 20% whole fat, 30% whole fat, 10% defatted, 20% defatted, and 30% defatted) in the experimental groups, named W10, W20, W30, D10, D20, and D30, respectively, for a total duration of 60 days. The growth, body color, immunity and gene expression indexes were measured in the koi after completion. The results indicate the following. (1) Compared with C0, the experimental groups of koi showed a significant increase in the specific growth rate (SGR) (p < 0.05), while the hepatosomatic index (HSI) and viscerosomatic ratio (VSI) decreased. Additionally, there was a significant increase in the relative expression level of insulin-like growth factor (IGF-1) in both the liver and muscle (p < 0.05). (2) The experimental groups of koi carp exhibited a significant increase in the carotenoid content in the scales and skin, as well as an elevated relative expression level of the tyrosinase (TYR) gene in the muscle (p < 0.05). (3) The lysozyme (LZM) and superoxide dismutase (SOD) activities were significantly increased in the experimental groups compared to C0 (p < 0.05). Additionally, the SOD activity was significantly higher in the defatted groups than in the whole-fat groups (p < 0.05). Furthermore, the liver alkaline phosphatase (AKP) activity was significantly lower in the D20 and D30 groups compared to the other five groups. (4) The expression level of LPL was significantly lower in the liver and muscle of the whole-fat group compared to the defatted group (p < 0.05). Tissue section observation revealed that the hepatocytes in the W20 and W30 groups exhibited a reduced size and an increased lipid droplet count, while the vacuolar degeneration of the hepatocytes increased in the D30 group. In conclusion, replacing fish meal with whole-fat or defatted krill powder or defatted Antarctic krill meal significantly improves the growth performance, body color, and immunity of red–white koi carp. However, excessive addition of krill meal can easily cause liver damage. The recommended replacement level for whole krill powder is 20% to 30%, while defatted krill powder should be replaced at a level of 10% to 20%.