Stable isotope profiling for large‐scale evaluation of feed efficiency in Atlantic salmon (Salmo salar)

In growing animals, individual variation in feed efficiency may arise from individual differences in growth rate and protein metabolism. Over a period of time, these factors will affect the ratio between ‘new’ vs. ‘old’ protein, which can be quantified using isotope profiling. The aim of this study was to investigate the relationship between relative weight gain and atom percentage excess 15N in the muscle, liver and mid‐intestine. A 50‐day experiment was conducted with a total of 375 fish initially fed a standard diet, subsequently replaced by one out of five experimental diets, enriched with 15N. In general, fast‐growing fish are expected to have a better feed efficiency, and the results show that this is captured by isotope profiling in liver and muscle tissues. Furthermore, individual variation in isotope content, that is relative fraction of ‘new’ protein, among fish with comparable growth rates was observed, most expressed around ~50% isotope saturation, indicating differences in protein degradation and replacement not attributed to growth. The results suggest that isotope profiles can be used as individual indicator traits for feed efficiency and that inclusion levels of stable isotopes of 1%–2% gave the most reliable results.