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Investigating the feeding ecology and migration of top predators is critical to understand their
role in food webs and monitor the effect of fisheries on food webs dynamics. However, little is
known about the early life history of many migratory species, including striped marlin (Kajikia
audax). Although the striped marlin is a commercially important species, there is currently no
information about food habits throughout their life. To investigate potential ontogenetic changes in diet and/or movement of striped marlin, we compare stable isotope analysis of both muscle and annual growth rings in dorsal fin spines to investigate ontogenetic shifts in diet or movement of striped marlin caught off Cabo San Lucas, Mexico (CSL). We used dorsal spines to determine age and quantified stable isotope ratios of carbon (?13C) and nitrogen (?15N) in each annual growth ring deposited in dorsal fin spines. We constructed retrospective isotopic profiles to detect any significant isotopic changes across ontogeny. We also compared patterns between individuals to investigate the possibility of individuals with different migratory histories aggregate off CSL. We found no relationship between marlin size (lower jaw fork length; LJFL) and ?15N isotopic values in either tissue. Differences in mean ?15N mean values across age classes were not significant and variation of ?15N through life was less than 2‰. However, mean ?15N between individuals varied by up to 6‰. ?13C values increased as a function of age, and mean ?13C significantly varied between age classes. The lack of a trend between ?15N isotopic values and LJFL suggests the possibility of a consistent trophic level for striped marlin throughout ontogeny; this is supported by a lack of significant difference in ?15N values across age classes, as well as no change in food items from juveniles to adult stages, determined in previous stomach content studies. It should however be noted that migrations can mask a trophic shift. However, the difference (up to 6‰) in mean ?15N between individuals suggests that striped marlin at CSL have mixed migratory histories, confirmed by previously satellite tags studies. ?13C in dorsal fin spines increased as a function of age, potentially reflecting an ontogenetic shift in habitat and foraging location; based on known geographical gradients in ?13C, juveniles may reside and feed in offshore oceanic waters while adults spend more
time in inshore waters. The challenge is differentiating between patterns associated with trophic
shifts and migrations. Although, it would be unusual for a fish's diet not to expand with size, some studies using stable isotopes in white shark, also reveal no changes in calculated trophic position of juvenile and adult. Observed differences between isotopic values of muscle and dorsal fin spines are likely a result of the different biochemical composition of each tissue and different isotopic turnover times between tissues. The use of fin spines to construct retrospective isotopic histories can be used to investigate trophic dynamics and migratory histories of other pelagic species, for which population dynamics are often poorly known.

Palabras clave: Enzyme expression; billfish; Stable isotope analysis; Diet history; dorsal fin spine

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