Effects of Graded Levels of Carbohydrate on Growth and Survival of Largemouth Bass, Micropterus salmoides

Excessive carbohydrates (CHO) in diets for largemouth bass (LMB), Micropterus salmoides , are suspected of accumulating glycogen in hepatocytes, which may result in liver dysfunction. This study evaluated the effect of graded levels of dietary CHO on growth, survival, and liver histology of LMB. One hundred feed‐trained advanced fingerling LMB (128.5   ±   21.5   g) were stocked into each of nine 3400 ‐ L polyethylene tanks. Tanks were randomly assigned one of three experimental diets containing different CHO levels (13, 19, or 25% of diet). The extruded diets were approximately isonitrogenous (42% crude protein) and isocaloric (3   kcal/g energy). There were three replicate tanks per dietary treatment. Bass were fed to apparent satiation twice daily for 148   d. Survival was significantly higher ( P   ≤   0.05) for fish fed the 13 and 19% CHO diets (89 and 90%, respectively) compared to those fed the 25% CHO diets (82%). Average harvest weight of fish fed the 13% CHO diet (380   g) was significantly greater ( P   ≤   0.05) than for fish fed other diets. Average harvest weight of fish fed the 19% CHO diet (347   g) was significantly greater ( P   ≤   0.05) than for fish fed the 25% CHO diet (310   g). Specific growth rates (%/d) were significantly higher ( P   ≤   0.05) in fish fed the 13 and 19% CHO diets than in fish fed 25% CHO diet. Feed conversion ratios for fish fed the 13 and 19% CHO diets (2.3 and 2.4, respectively) were both significantly lower ( P   ≤   0.05) than in fish fed the 25% CHO diet (3.6). There were no significant differences ( P   >   0.05) in condition factor, protein efficiency ratio, hepatosomatic index, or liver glycogen concentration among fish fed the different experimental diets. Overall, mean blood glucose levels in fish fed the 13 and 19% CHO diets (61.0 and 71.2   mg/dL, respectively) were significantly lower ( P   ≤   0.05) than in fish fed the 25% CHO diet (87   mg/dL). Histopathological examination of livers from fish fed the three diets was used to score the degree of vacuolization of hepatic tissues (0   =   normal, 1   =   slight, 2   =   mild, 3   =   moderate, and 4   =   severe). Regression of vacuolization scores on dietary CHO levels was statistically significant ( P   ≤   0.05) and indicated a direct positive relationship between liver vacuolization and dietary CHO level ( R 2   =   0.57). These data indicate that LMB grow faster and use feeds more efficiently when CHO are maintained at <20% of diet. CHO levels >20% negatively impacted liver histology, but a liver tissue analyses did not document glycogen accumulation.