Effect of water temperature on gut transit time, digestive enzyme activity and nutrient digestibility in yellowtail kingfish (Seriola lalandi)

Yellowtail kingfish (Seriola lalandi) are farmed in sea cages in South Australian waters and exposed to seasonal variations in water temperature. The temperature variations, ranging from 10 to 24 °C, influence feed intake and on-farm feeding practices. Sub-adult yellowtail kingfish are fed twice daily in summer while feeding in winter may be reduced to once every second day. Lower winter water temperatures increase gut transit times which may lead to a condition known as winter syndrome. This syndrome causes an enteritis response in yellowtail kingfish and reduces farm productivity. The impact of water temperature on gut transit time, nutrient digestibility and protease and lipase enzyme activities in yellowtail kingfish (2.12 ±0.03 and 2.77 ± 0.03 kg live weight in summer and winter respectively) was investigated at summer and winter water temperatures; 20.8 ± 0.4 °C and 12.6 ± 0.1 °C respectively, using observations made on dissected fish and detection of the passage of dietary markers. Water temperature had a significant effect on gut transit time, with visual observations demonstrating that at summer water temperatures it takes 12–16 h for all digesta to be voided from the fish compared to 36–48 h at winter temperatures. Temperature did not have a significant effect on the dietary nutrient apparent digestibility coefficients (ADC, %) although there was a numerical tendency for nutrient ADC to be higher in the posterior intestine at summer water temperatures: 46% vs. 35% for dry matter; 70% vs. 67% for protein; 66% vs. 58% for fat; and 70% vs. 63% for energy. Nutrient digestibility did not differ significantly between the anterior and posterior intestinal sections; however, the ADC values were consistently higher in posterior intestine. Protease activity was significantly higher in the posterior intestine and there was also a numerical, but not significant, increase in protease and lipase activities in the anterior section of the intestine during winter. Lipase activity was significantly higher in the anterior intestine during summer, when compared to the posterior section, and numerically higher in this section during winter. In winter it was found that a greater proportion of nutrient digestion occurred in the stomach, pyloric caeca and anterior intestine when compared to summer. This was thought to be due to the slower gut motility and higher enzyme activities observed during winter. Even though gut transit time was slower in winter, the lower water temperature also resulted in nutrient digestibility that was numerically lower.