THE SUITABLE STARCH TYPES AND LEVELS IN THE DIET OF BROWN TROUT (SALMO TRUTTA) JUVENILES

To investigate the effects of starch on the growth, feed utilization, digestive enzyme activity, liver biochemical indexes and liver histology of brown trout juveniles, six isonitrogenous and isolipidic diets were formulated with corn starch and cassava starch at three levels of 5%, 10% and 15%, respectively, to feed brown trout juveniles with an initial body weight of (0.50±0.03) g for 84 days. The results indicated that the weight gain rate (WGR) first increased and then decreased, while the feed coefficient ratio (FCR) showed an opposite trend with the increasing dietary starch level (P<0.05). The 10% cassava starch group had the highest WGR (518.8%) and the lowest FCR (1.32). There were no significant differences in the survival, viscera-body ratio, condition factor, and body composition among the groups (P>0.05). The crude protein retention decreased with the increasing starch level, and the 15% starch group had significantly lower values than that of the other groups (P<0.05). While the lipid retention first increased and then decreased with the increasing dietary starch level, and the 10% cassava starch group showed significantly higher lipid retention than that of the 10% corn starch group (P<0.05). The starch types and levels did not significantly affect the activities of pepsin, gastric amylase, liver alanine aminotransferase, liver aspartate aminotransferase, and the contents of total cholesterol and triglycerides in liver (P>0.05), while the activities of intestinal amylase and protease in 15% starch group were significantly higher than those in other groups (P<0.05). The 15% starch groups showed significantly higher liver glycogen content and more nucleus shift and cell vacuolation than the other groups. In conclusion, the suitable diet for brown trout juveniles was suggested to be 10% cassava starch under the experimental conditions.