Due to the importance of major nutrients such as protein and lipid for optimal growth and physiological performances of fishes, many studies have been conducted to determine its requirement for cultured fishes. However, little is known about interactive effects of dietary protein and lipid levels on temperature stress responses in cultured fishes. Northern snakehead is a commercial important fish species in Asia but has faced with continuously increasing water temperature associated with climate change. Thus, in order to ensure the sustainability of northern snakehead aquaculture, it is important to understand how different protein and lipid levels influence physiological performance of this species in response to acute temperature exposure.
A 3 × 3 factorial experiment was designed to evaluate the relationship between interaction of dietary protein and lipid levels and physiological performance of juvenile northern snakehead responding to acute temperature stress. Four hundred five juveniles (initial body weight: 19.3 ± 0.03 g; mean ± SEM) were randomly distributed into each of 27 tanks (15 fish per tank; N = 3 tanks per treatment). Nine diets were prepared to contain three levels of crude protein (41, 44 and 47%) in combination with three levels of crude lipid (6, 9 and 12%). Following the 60-days of feeding trial, the juveniles from each of the nine treatments were abruptly exposed to higher water temperature at 35 ? for 2 h and were recovered at ambient water temperature for 2 h.
Results showed that a significant interactive effect of dietary protein and lipid levels on weight gain, showing the improved values as the protein increased but the lipid level decreased (P < 0.05). There was a main effect of both protein and lipid levels on feed efficiency and condition factor, and the overall trend of changes in those indices was comparable to that shown in weight gain. Plasma metabolites, including total protein and triglyceride levels were also significantly influenced by both dietary protein and lipid levels, whereas plasma total cholesterol and glucose levels were affected by the dietary lipid level only.
Based on results of the three-way (3 protein levels × 3 lipid level × 2 stress condition) ANOVA test, there was the interactive effect of the dietary protein level and stress condition on plasma total protein, triglyceride and total cholesterol levels, whereas neither a significant interactive effect of the two factors (lipid level and stress condition) nor a significant interactive effect of the three factors (protein level, lipid level, and stress condition) was detected.
Expression levels of genes involved in temperature stress responses in various tissues of the juveniles exposed to the acute temperature challenge will be discussed later.