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Mobula thurstoni and Mobula japanica are two of the five mobulid species present in the Gulf of California. They have been an important component of the artisanal elasmobranch fishery in Mexico, but are presently protected as their populations are vulnerable to over-exploitation. Stable isotopes of nitrogen and carbon were used to infer the diet and trophic position of M. thurstoni and M. japanica in the SW of the Gulf of California. A total of 32 M. thurstoni and 6 M. japanica muscle samples were obtained from May 2006 to January 2007. Eleven monthly diurnal and nocturnal plakton tows were carried out from March to November 2006. We analyzed stable isotopos of Nyctiphanes simplex (an euphausid, and the mobulid´s most probable prey) as well as herviborous, carnivorous and omnivorous zooplankton. Based on isotopic fractionation between mobulids and their potential prey, we conlude that both mobulid species fed on Nyctiphanes simplex and not on other zooplankton groups analyzed. We calculated a trophic level of 3.43 for M. thurstoni and 3.48 for M. japanica. The isotopic signal of the zooplankton varied with oceanographic conditions, with lower d15 values in times of higher productivity.

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