Phenotyping individual feed intake through direct measurements remains complex in fish, and one way to overcome this challenge is to find indirect criteria for estimating feed efficiency. This study investigated the correlations between fish residual feed intake (RFI) and fat content or the number of feed demands in ten isogenic lines of rainbow trout. We found positive correlations between RFI and fat content indicating that fatter fish tend to be less feed efficient. However, the association between RFI and fat content was not constant over time and was driven mostly by one replicate from one line. We also found positive correlations between RFI and the total number of feed demands. Our findings highlight the interest of using fat content in selective breeding programs to improve feed efficiency and highlighted the interest in exploring the relationship between feed efficiency and feeding behaviour in fish