Stable carbon and nitrogen isotope values of dorsal spine age rings indicate temporal variation in the diet of striped marlin (Kajikia audax) in waters around Cabo San Lucas, Mexico.

Rationale Billfishes, such as marlin, are top pelagic predators that play an important role in maintaining the stability of marine food webs. Notwithstanding the importance of these species, there remain gaps in our knowledge on their movements, foraging, and trophic status in the early stage of life. Methods We measured the δ13C and δ15N values in each annual growth band deposited in the dorsal spine from striped marlin caught off Cabo San Lucas, Mexico, to produce retrospective isotopic profiles that would enable us to detect any significant isotopic changes across development. The samples were analyzed using an elemental analyzer coupled to an isotope ratio mass spectrometer. Results There was no relationship between the size of striped marlin and the δ15N values. Differences in δ15N mean values across different age classes were not significant and the variation in δ15N values through the marlins' life cycle was less than 2‰. However, the mean δ15N values between individuals varied by up to 6‰. The δ13C values increased as a function of age, and the mean δ13C values varied significantly between age classes. Conclusions Fin spines can be used to construct retrospective isotopic histories for the investigation of trophic dynamics and migratory histories in billfishes, for which population dynamics are often poorly known. Copyright © 2015 John Wiley & Sons, Ltd.