Dolphinfish (Mahimahi) are a high-demand resource for sport and coastal fisheries, mainly in the Pacific Ocean. Due to their economic and ecological importance, studies of their biology are very important to understand their function in ecosystems. We used stable isotope and stomach content analyses to determine the most important prey of the common dolphinfish, as well as the trophic level of this species in two areas of the southern Gulf of California. Stomach contents of 445 specimens were analysed. Using both techniques, we found that the most important prey for dolphinfish in the southern Gulf of California were three invertebrate species followed by fish. This contrasts with results from other authors who found that this species was mainly piscivorous in other locations. Stomach content analysis indicated differences in prey biomass by area, season and size class. The isotopic analysis did not show significant differences between seasons or sexes. Both stomach contents and stable isotope analyses showed that although this predator consumed a wide prey spectrum, only a few prey items made up the bulk of the diet, which resulted in a low SD in δ15N values and low Levin's index values. We conclude that this fish is an opportunistic predator that may consume a wide prey spectrum, but that it mainly consumes prey that are abundant in the area, such as crustaceans and cephalopods in the Gulf of California.

Summary The objective of the study was to identify key elements of the feeding ecology of the dolphinfish, which is a high tropic predator important for sport and artisanal fishing in the Mexican Pacific. Feeding habits were investigated during the years 2000–2003. This species is seasonal in the southern Gulf of California and probably remains there because of the abundant prey. The contents of 232 dolphinfish stomachs were analyzed, identifying 98 prey species, although only eight of these were well‐represented in the diet. The most important prey by weight was the fish Hemiramphus saltator, however by number and frequency of occurrence was the crustacean Hemisquilla californiensis. No differences in the diet were found between males and females, although there was an ontogenic diet shift between seasons. There was no relationship between dolphinfish size and prey size, because dolphinfish fed preferentially on prey with an average size of 4.7 cm.

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Accurate knowledge on fish trophic ecology and its modifications is crucial for understanding the impact of global change on ecosystems. In this context, we investigated the value of the δ 13 C and δ 15 N of otolith soluble organic matter (SOM) for identifying temporal diet shifts in American silver perch Bairdiella chrysoura over a 30-yr period characterized by strong changes in its population size and habitats within the Terminos Lagoon (Mexico). We first compared the otolith SOM isotopic signatures from present-day adults to those of muscle and the main local prey. Our results suggest that otolith SOM can be confidently extracted and analyzed for both present and past otoliths of this species. The mean otolith SOM signatures obtained (−15.92 ± 1.35 ‰ for δ 13 C and 9.38 ± 0.93 ‰ for δ 15 N) were consistent with those of the diet as 85% of the individual signatures were included within the prey isotopic niche area. Moreover, this study supports a trophic enrichment factor between diet and otolith (TEF diet-otolith ) close to 0 for δ 15 N, while for δ 13 C, the TEF otolith-muscle of + 0.02 ‰ warrants further investigation. Then, we compared past and contemporary otolith SOM signatures to investigate temporal diet shifts in B. chrysoura. This showed that δ 13 C and δ 15 N differed significantly between the past and present period even if the temporal shift remained relatively small (respectively +1.17 ‰ and -0.55 ‰). The present study substantiates the use of otolith SOM δ 13 C and δ 15 N as a proxy of fish present and past trophic position, opening the possibility for major progress in studies of temporal changes in food web ecology.

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