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Along the Pacific coast of Baja California Sur (PCBCS), the banded guitarfish (Zapteryx exasperata), shovelnose guitarfish (Pseudobatos productus) and bat ray (Myliobatis californica) are highly abundant. Their ecological roles as predators in demersal communities can be key in this ecosystem. To better understand their trophic relationship in the PCBCS, stable isotopes analysis of carbon (ẟ13C) and nitrogen (ẟ15N) were used. Muscle samples (n = 265) were collected from shovelnose guitarfish (n = 94), banded guitarfish (n = 87) and bat ray (n = 84). We observed high variability in stable isotopes values, ẟ13C and ẟ15N of shovelnose guitarfish ranged from −18.53 to −12.85‰ and 15.93‰ to 20.37‰, respectively; banded guitarfish from −18.12‰ to −13.57‰ and 14.41‰ to 19.26‰, respectively; and bat ray from −17.73‰ to −13.98‰ and 13.97‰ to 18.46, respectively. Statistically significant interspecific differences were found (p < 0.05) for ẟ13C and ẟ15N values, as bat ray showed a lower mean ẟ15N value and less negative mean ẟ13C value. Mature male (MM) bat ray) showed significantly higher ẟ15N values and shovelnose guitarfish (MM) significantly lower ẟ13C values compared to other cohorts. Isotopic niche analysis using Bayesian ellipses (SEAc) indicated shovelnose guitarfish occupies the widest isotopic niche compared with bat ray and banded guitarfish. Based on SEAc, the banded guitarfish overlapped 0.46 with the shovelnose guitarfish, while the bat ray overlapped 0.38 and 0.39 with banded and shovelnose guitarfish, respectively. Reported data suggest there is a relative overlap among all species, with probably greater amount of partitioning between the bat ray and the other two species. [ABSTRACT FROM AUTHOR]

The feeding ecology and trophic level of the banded guitarfish, Zapteryx exasperata, from the Gulf of California, were assessed using stable isotope analysis (SIA) of carbon and nitrogen in muscle samples, and stomach contents analysis (SCA). Males and females showed similar diet preferences with a high diet overlap (Cλ = 0.99) and similar values of δC and δN (ANOVA, F = 0.0 p = 0.94 for δC; F = 0.1 p = 0.78 for δN). The main prey groups found were demersal fishes dominated by the daisy midshipman, Porichthys margaritatus, (>90%IRI), however, pelagic fishes were also found ( Engraulis mordax 2%IRI). An ontogenetic diet shift was found, indicated by the importance of crustaceans in the juvenile diet and the significant isotopic differences between juveniles and adults (δC, ANOVA, F = 13.3 p = 0.0004 and δN, ANOVA, F = 4.7 p = 0.03). An overall trend of increase in δN values with increasing body length ( F = 8.15 p = 0.005) was observed. The mean trophic level estimated in this work by the two methods was ~4.1, indicating that this species is a top predator in the Gulf of California. Zapteryx exasperata is a specialist feeder ( Bi = 0.11, H' = 2.32), consuming mainly benthic fishes in coastal areas of the Gulf of California. The importance of this species in the benthic ecosystem equilibrium as a top predator is an important point to keep in mind when developing future management plans of the batoid fishery in the Gulf of California. [ABSTRACT FROM AUTHOR]