Fatty‐acid profiles of juvenile lake trout reflect experimental diets consisting of natural prey

Summary It is relatively well‐known that fatty‐acid profiles of consumers reflect their diets. However, with fish, controlled studies that trace fatty‐acid profiles of natural prey into consumers are lacking. We asked whether lake trout (Salmonidae: Salvelinus namaycush) fatty‐acid profiles reflect diets at 4, 8 or 14 weeks after feeding began. We also evaluated if calibration coefficients were similar for each diet, a key assumption of quantitative fatty‐acid signature analysis (QFASA). In this study, juvenile lake trout were fed commercially available frozen diets of chironomids (Chironomidae: Chironomus spp.), copepods (Cyclopoida spp.), or Mysis (Mysidae: Mysis relicta ) over a 14‐week period. Accurate classification of lake trout into a priori diet groups was attained after 8 weeks of feeding. Calibration coefficients were significantly different among diet groups, especially for lake trout that were fed chironomids, suggesting that diet‐specific modifications to fatty acids occurred. Chironomid‐fed lake trout grew significantly larger than others despite consuming prey that lacked long‐chain essential fatty acids. Furthermore, chironomid‐fed lake trout provide evidence for the conversion of 18:3n‐3 into longer chain n‐3 fatty acids. Our results call for additional studies to better understand how fatty acids reflect dietary origins prior to employing QFASA on wild freshwater fishes. QFASA could provide accurate diet estimates for freshwater fishes with low‐diversity diet compositions, if calibration coefficients for each predator–prey relationship are incorporated.