Gastric digestion and nutrient absorption along the alimentary tract of whiting (Merlangius merlangus L.) fed on natural prey

Studies on the digestion process in fish can elucidate some aspects of nutritional physiology. The movements of food items in the alimentary tract are observed using X‐radiography techniques, and nutrient absorption along the alimentary tracts (expressed as relative percentage absorption gradient) is calculated based on the ash contents of adjacent samples. Apparent digestibility coefficient (ADC) values for protein (80–94%), lipid (90–97%), carbohydrate (80.5%) and energy (85–96%) calculated from laboratory‐fed whiting were comparable with those for other fish fed suitable diets. Considerable proportions of all nutrients were absorbed in the short region (anterior intestine/pyloric caeca) but active absorption continued for all nutrients in transit along the longer middle and posterior intestinal segments. In wild whiting samples, the estimated ADC values were protein (65%), lipid (81%) and energy (65%) in passage from the stomach to the rectal region. It was not possible to ascertain the prey in intestinal samples, which probably contained a mixture of species. Sprats and brown shrimps probably dominated the diet, based on biochemical analysis of stomach contents, but polychaetes and other crustaceans could have made a minor contribution.