Effect of dietary starch on digestion kinetics and nutrient utilization in yellowtail kingfish (Seriola lalandi)

The natural food source of yellowtail kingfish is low in starch, while under commercial farming condition, starch is included at levels >8% to produce extruded diets. In this study, the effect of dietary starch level on digestion kinetics and energy utilization in yellowtail kingfish was investigated. To achieve this, fish were fed equal amounts of crude protein and fat, in combination with either a low amount of starch (LS diet; 0% gelatinized wheat flour) or high amount of starch (HS diet; 20% gelatinized wheat flour). Each diet was tested in triplicate. Six tanks were stocked with 24 fish (mean initial weight 37 g). Fish performance, digestion kinetics, nutrient digestibility along the gastrointestinal tract and in faeces, and nutrient utilization were evaluated. Kinetics of digestion were altered by starch level, such as dry matter content, osmolality and pH. Viscosity along the gastrointestinal tract was not affected by starch level. High starch level negatively affected the nutrient digestibility in chyme (organic matter, crude protein and starch & sugars) and in faeces (organic matter, crude protein, fat, energy, and phosphorus). In the LS diet, the greatest share of organic matter (59.0% absolute digestion) and crude protein (60.8% absolute digestion) was digested already in the proximal intestine, suggesting that the proximal intestine plays the major role in nutrient digestion and absorption yellowtail kingfish are present in the proximal intestine. Despite the fact that fish fed the HS diet had a higher carbohydrate and thus energy intake, a tendency for a lower growth was observed. This may be related to the negative effect of high starch inclusion on nutrient digestibility, an increased energy maintenance requirement, the inability of yellowtail kingfish to utilize starch, or a combination.