Studies on food intake, digestion and growth of Oreochromis niloticus

Abstract

Factors affecting the daily food intake, digestion and growth of 0. niloticus were investigated for fish of varying weights. A preliminary experiment showed that daily food intake Increased with increasing feeding frequency up to six meals per day. Further increase in feeding frequency had no significant effect on total daily food intake. This feeding frequency was subsequently used to investigate the relationship between maximum daily food intake and fish weight. Three different models were used to describe this relationship, the best fit being given by Log M = a + b Log W. Maximum daily food consumption was found to vary between 2.4% and 9.1% b.w. for 200g and 10g fish respectively. However these values appear high and are not recommended for commercial culture situations. The satiation meal size (maximum food ingested in a single meal) for fish of different weights was also investigated and varied between 1.27% and 1.96% b.w. for 200g and 10g fish. A time of 10-15min was found to be adequate to satiate different weights of fish using stomach capacity as the criterion. The effect of pre-freeding deprivation periods of 24h up to 96h on single meal food intake, liver weight and intestine length for different weights of fish showed that food intake increased significantly with starvation period up to 72 hours. However further Increase in starvation period depressed food intake. No significant effect on liver weight was observed, but increasing starvation period decreased the intestine length significantly. Furthermore the study suggested that volume and colour of bile could be used as useful indicators of recent feeding history in fish. Following initial trials to compare techniques, the method of sequential slaughter was used for all digestion studies in 0. niloticus. Quantitative relationships between gastric evacuation coefficient and certain factors affecting gastric evacuation coefficient and time were established. These factors were temperature, fish weight, meal size, food composition and pre-feeding deprivation periods. From the stomach evacuation studies the daily food consumption rates for different weights of fish (based on the assumption that appetite is closely related to stomach evacuation time) were calculated and recommended for feeding of 0. niloticus in intensive culture systems. The recommended daily food consumption was found to vary between 1.1% and 4% b.w. for 200g and 5s fish respectively. Digestive enzyme activities were also investigated. It was shown that activity of pepsin-like, trypsin-like and -amylases could be increased significantly by increasing the dietary level of protein and carbohydrate. Lipases did not show any change in activity with increasing dietary lipid level from 11% to 18%. Pepsin- and trypsin-like enzyme activities were observed to decrease with increasing fish weight, but a-amylase activity increased with fish weight. Overall it was shown that ingested food helps regulate digestive enzyme activity and fish deprived of food for between 24h and 96h showed marked reductions in digestive enzyme activities. Long term trials were also conducted to investigate the effect of feeding frequency, fish weight, feeding rates and food composition on growth, food utilization and carcass composition. The study showed that the optimum feeding frequency for 0. niloticus is twice per day. Increasing the feeding frequency beyond two meals per day results in a significant increase in food intake with no corresponding increase in weight gain or specific growth rate. The relationship between growth and feeding rates showed that specific growth rate increases to various degrees with increasing feeding rate. Comparison between two weights of fish (6.86g and 14.27g) in terms ofi^ food intake for maintenance, optimum and maximum growth showed that smaller fish had higher requirements than larger fish. The maximum feeding rates obtained for both weights of fish (6.86g and 14.27g) were in close agreement with the recommended rates derived from the gastric evacuation studies. Further investigation of the effects on food composition showed a large effect on all nutritional parameters measured. A significant protein sparing effect was observed with increasing dietary level of lipid and carbohydrate. The results of this study are discussed in relation to the data available for a variety of fish species.