Digestive efficiency and nutrient composition gradient in the gut of Oreochromis niloticus L. in Lake Awasa, Ethiopia

Assimilation efficiencies were measured by comparing the amount of nutrient in the food and faeces of fish from the natural environment. The comparison of nutrients was made in relation to reference markers: ash, hydrolysis‐resistant ash (HRA) and hydrolysis‐resistant organic matter (HROM). Assimilation efficiencies of nutrients computed using ash and HRA as reference markers were significantly (P < 0.05) lower than those measured with HROM; digestibility estimates of protein were the least influenced by the type of marker used. Generally, HROM appears to be the most suitable reference marker for a digestibility study in an herbivorous fish such as Oreochromis niloticus. Nutrient composition gradient examined by analysis of stomach, intestinal and rectal contents showed that digestion of all nutrients was completed when the food passed two‐thirds of the way through the length of the intestine. Most of the nutrients were significantly (P < 0.05) affected by the processes of digestion and assimilation in different sections of the gut, and a decline in the quantity of nutrients was obvious towards the rectum.