Interpreting nitrogen stable isotopes in the study of migratory fishes in marine ecosystems

Stable isotopes have proven effective in the ecological study of terrestrial, freshwater, marine and estuarine systems. However, their utility in the study of large migratory fishes in the dynamic ocean may be more difficult to ascertain. Migration across large areas of ocean in short periods of time may result in a large amount of variability in the nitrogen ratios of large pelagic fishes. High variability was found in nitrogen ratios (δ¹⁵N) used to quantify trophic positions of large pelagic fish species and their prey in the southern Gulf of California. Mean δ¹⁵N values for eleven large pelagic predators ranged from 14.8 ‰ for blue marlin to 18.7 ‰ for amberjack. Predators typically present in the Gulf during winter months had a higher mean value (18.5 ± 0.7 ‰) than summer predators (15.8 ± 1.5 ‰). Common prey items, including cephalopods, fish and crustaceans, had mean δ¹⁵N values ranging from 12.2 to 19.1 ‰ (epipelagic fishes = 17.5 ± 0.7 ‰, cephalopods = 15.6 ± 0.9 ‰, inshore fishes = 14.2 ± 1.6 ‰, crustaceans = 12.2 ± 0.4 ‰), in many cases exceeding their predators by more than one full trophic level. We analyzed potential sources of this variability by examining diets, oceanographic conditions and fish migration. Diet shifts with size do not account for the variability, but the likely source appears to be slow turnover rate of body tissues of migratory pelagic fish as they traverse both highly productive and oligotrophic water masses in the eastern Pacific Ocean. From this study, we advise caution when interpreting results of stable isotope ratios in migratory pelagic fishes, and suggest designing studies that account for sources of variability.