Isotopic Analysis as a Potential Tool to Verify Feed Protein Sources for Aquacultured Species

This study identified δ15N stable isotope ratios as a robust tracer for fishmeal inclusion in aquaculture feeds. δ15N and δ13C values from fish muscle samples derived from feeding trials with seven species (n = 3–5 fish per diet group) were evaluated (+/−0.1‰ for both δ15N and δ13C; ~1% relative to % N and % C) to verify whether the presence or absence of fishmeal (FM) in feeds could be detected. C and N isotopic data were also analyzed for feed in two of the trials. δ13C signatures did not differ consistently across diet groups for each species examined, with mean δ13C values for all species investigated being −20.2‰ ± 1.3. In contrast, a strong δ15N distinction was discerned between FM- and non-FM-fed fish for both muscle and feed samples, with FM-fed groups presenting higher values (p < 0.01) than non-FM-fed groups (range 0.8 to 9.5‰). Dietary ingredients other than FM (e.g., fish oil and algal oil) did not impact the δ15N isotopic fingerprint, although the addition of poultry byproduct meal to plant-based salmon diets caused an average 0.3‰ difference in δ15N values. The findings are not absolute as CN isotopes can be used to detect large but not small differences in feed components. Additional research on threshold levels, ingredient sourcing, and species differences is warranted to refine the method to enhance industry transparency and seafood consumer confidence.