Gastric evacuation characteristics, digestive enzyme activity and appetite-related genes expression in juvenile yellowtail kingfish (Seriola aureovittata)

The juvenile yellowtail kingfish (Seriola aureovittata) has strong physiological metabolism and the energy during the growth and development stage, and thus its nutrient requirements are significantly different from those of other carnivorous farmed fish. To identify the digestion and gastric emptying characteristics of S. aureovittata, and determine the optimal feeding interval, in the present study, the changes of contents weight, digestive enzyme activity in liver and intestine and pituitary appetite-related genes expression were evaluated during gastric emptying of the juvenile S. aureovittata (63.96±5.63) g. The degree of fitting of the gastric emptying curve was analyzed using a linear model, square root model, and cubic root model. The results showed that the wet weight of the gastric contents gradually decreased after the feeding and reached 0 at 18 hours, which was a type of linear descending gastric emptying. Hepatic amylase, lipase and chymotrypsin activities increased first and then decreased, followed by a rise and then decreased the "M" type of change trend during gastric emptying. Amylase activity increased significantly from 0 to 6 h after feeding (P<0.05), and lipase and chymotrypsin activity increased significantly from 0 to 9 h (P<0.05). The activities of the three enzymes significantly decreased from 9 to 12 h (P<0.05), and then increased from 15 to 18 h (P<0.05), with the highest value observed at 18 h. Intestinal amylase activity showed the similar results like hepatic amylase, whereas increased at 12 h with the highest value, subsequently significantly decreased until 24 h. The intestinal lipase and chymotrypsin activity increased significantly from 0 to 12 h after feeding (P<0.05), the highest value observed at 12 h, and then gradually decreased. The expression of neuropeptide Y (npy) and orexin (ore) mRNA levels were increased from 12 to 15 h and from 9 to 15 h (P<0.05), respectively. The highest values of npy and ore mRNA was that at 15 h, and then significantly decreased until gastric emptying completed (P<0.05). The residual percentage of gastric contents was negatively correlated with gastric emptying time and the expression of npy mRNA (P<0.05), while npy mRNA was positively correlated with gastric emptying time (P<0.05). In conclusion, the digestive enzyme activity in the liver and intestine of juvenile S. aureovittata was decreased to the lowest from 12 to 15 h after feeding. The best model cube root model for fitting the gastric emptying test data of juvenile S. aureovittata showed 80% gastric emptying time (basic recovery of appetite) and 100% gastric emptying time (complete recovery of appetite) were approximately 14.5 and 19 h, respectively. Meanwhile, the mRNA levels of npy and ore in pituitary of juvenile S. aureovittata obtain the highest value at post-feeding 15 h. Therefore, the optimal interval for feeding is recommended to be fed 14.5 h based on the characteristics of gastric emptying, hepatic and intestinal digestive enzyme activities and pituitary appetite-related gene expression. The relevant results not only help to clarify the optimal feeding interval of S. aureovittata, but also provide theoretical basis and technical support for research on its feeding law.