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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 (?15N) used to quantify
trophic positions of large pelagic fish species and their prey
in the southern Gulf of California. Mean ?15N 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 ?15N 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 thelikely 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.

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