Feeding behavior and stress response explain individual differences in feed efficiency in juveniles of Nile tilapia Oreochromis niloticus

Feed efficiency is a trait of enormous importance in any animal production sector including aquaculture. Individuals that are more feed efficient need to use less feed to achieve similar growth rates as compared to less efficient individuals. Considering that feed represents the largest cost of production and one of the main causes for the ecological footprint of a farm, more knowledge on the extent of individual differences in feed efficiency and underlying physiological processes are needed. This study is the first to investigate individual differences in feed efficiency, measured as residual feed intake and its relationship with feeding behaviour and stress response in one of the most important farmed fish, the Nile tilapia. The individual feed intake of twenty-four juvenile of Nile tilapia was followed during 57 days. Fish were fed until apparent satiation with a commercial feed twice per day. Individual feed behavior was registered once per week and consisted on measuring the latency to start feeding (LAT, min), total feeding time (TFT, min) and the number of feeding acts (NFA, min). Blood samples for plasma cortisol was taken at the end of the experiment (control, indicative of undisturbed levels) and after a stress test (netting), 15 days after the previous sampling. Individual growth rates and residual feed intake were determined at the end of the experiment. Results show pronounced individual differences in residual feed intake as well as a significant correlation with both feeding behavior (feeding latency, total feeding time and number of feeding acts) and stress (cortisol) response. Cortisol levels obtained after the stress test and feeding behavior could explain differences in residual feed intake by 4%. These results show that individuals with higher feeding activity and higher cortisol response are less efficient fish. This suggests that individual differences in feeding activity and stress response explain part of the differences in feed efficiency by explaining variance in maintenance energy expenditure.