MEPS 587:31-40 (2018)  -  DOI: https://doi.org/10.3354/meps12419

ABSTRACT: Despite the speculations that jellyfish (hydromedusae, siphonophores, scyphomedusae and ctenophores) may compete with forage fish for prey, there are few direct comparisons of their diets. To determine the dietary overlap between Aurelia sp. (Cnidaria, Scyphozoa) and Brevoortia patronus (Goode, 1878) (Pisces, Clupeidae) in the northern Gulf of Mexico, we collected monthly samples in Louisiana, Mississippi and Alabama coastal waters (USA) during summer and early fall 2009-2010. We determined carbon and nitrogen stable isotope ratios in predators and their potential prey, including small plankton (<200 µm) and mesozooplankton (200-2000 µm), and identified prey in the stomachs of adult Aurelia sp. and B. patronus. Trophic niche overlap was defined using the stable isotope Bayesian ellipses in R (SIBER) procedure and ranged from 0-28% for Aurelia sp. and 0-64% for B. patronus across the 3 sites. While stable isotope values in B. patronus clearly reflected the range of mesozooplankton, those for Aurelia sp. indicated a high individual variability, which likely accounted for the niche separation in Louisiana. Copepods were numerically the most abundant prey in the stomachs of predators at all sites, resulting in a percent similarity index of 93% in Louisiana, 87% in Mississippi and 86% in Alabama. Our results highlight that, despite local and species-specific variability, dietary overlap between Aurelia sp. and B. patronus is high across the northern Gulf of Mexico. Our data contribute to the definition of trophic interactions between jellyfish and forage fish in the Gulf of Mexico region and other ecosystems where they co-occur.

KEY WORDS: Stomach contents · Stable isotopes · Niche overlap · SIBER · MixSIAR · Aurelia sp. · Brevoortia patronus