Trophic assessment and isotopic niche of three sympatric ray species of western Baja California Sur, Mexico

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 (& x1e9f;C-13) and nitrogen (& x1e9f;N-15) 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, & x1e9f;C-13 and & x1e9f;N-15 of shovelnose guitarfish ranged from -18.53 to -12.85 parts per thousand and 15.93 parts per thousand to 20.37 parts per thousand, respectively; banded guitarfish from -18.12 parts per thousand to -13.57 parts per thousand and 14.41 parts per thousand to 19.26 parts per thousand, respectively; and bat ray from -17.73 parts per thousand to -13.98 parts per thousand and 13.97 parts per thousand to 18.46, respectively. Statistically significant interspecific differences were found (p < 0.05) for & x1e9f;C-13 and & x1e9f;N-15 values, as bat ray showed a lower mean & x1e9f;N-15 value and less negative mean & x1e9f;C-13 value. Mature male (MM) bat ray) showed significantly higher & x1e9f;N-15 values and shovelnose guitarfish (MM) significantly lower & x1e9f;C-13 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.