Protein Sparing Effect of Lipid and L-carnitine in Diets for Largemouth Bass (Micropterus salmoides)

A 3×2 (protein/lipid ratio×L-carnitine) factorial experiment was conducted to evaluate the protein sparing effect of lipid and L-carnitine in diets for largemouth bass,Micropterus salmoides. Two hundred and seventy fingerlings with average body weight of (3.27±0.21) g were randomly divided into six groups with 3 replicates each and fifteen fingerlings per replicate. Six isocaloric experimental diets were formulated with three protein/lipid ratios (39.9%/7.4%,30.3%/13.9% and 21.5%/19.5%) crossing two L-carnitine levels (0 and 1 g/kg),and were fed to the fingerlings for six weeks. The results showed as follows: weight gain rate was 686% when fish were fed the 39.9%/7.4% (protein/lipid) diet but was reduced to 176% when fish were fed the 21.5%/19.5% diet (P0.05). Condition factor of fish fed 21.5%/19.5% (protein/lipid) diet was significantly lower than that of fish fed 39.9%/7.4% (protein/lipid) and 30.3%/13.9% (protein/lipid) diets (P0.05). FCR was increased significantly from 1.15 to 2.33 when the dietary lipid level was increased from 7.4% to 19.5% (protein level was decreased from 39.9% to 21.5%) (P0.05). Body moisture content was lower in the fish fed 21.5%/19.5% than that in the fish fed 39.9%/7.4% diet (P0.05). When the dietary lipid level increased from 7.4% to 19.5% (protein level decreased from 39.9% to 21.5%),the body crude lipid content increased from 5.6% to 11.5% while body crude protein content decreased from 15.6% to 10.4% (P0.05). The liver lipid,muscle lipid and hepatosomatic index reached the highest values when fish were fed 21.5%/19.5% (protein/lipid) diet. The percentage of lymphocytes was significantly increased from 38.30% to 48.41% (P0.05),and the percentage of granulocytes was significantly decreased from 51.75% to 42.14% (P0.05) when dietary lipid level was increased from 7.4% to 19.5% (protein level was decreased from 39.9% to 21.5%). It was concluded that more dietary lipid addition with the aim to spare dietary protein slowed the growth of largemouth bass,and led to a fatty liver and immune suppression. Moreover,though the addition of 1 g/kg L-carnitine in diet had no effect on all the parameters tested relevant to fish growth performance,an interactive effect of dietary protein/lipid ratio with L-carnitine was observed in the percentages of granulocytes and lymphocytes with the peak values found in the fish fed 21.5%/19.5% (protein/lipid) and 0.1% L-carnitine diet.