Little is known about how the establishment of an invasive competitor can affect marine predatory fishes, an important question with strong ecological and commercial implications. The invasion of predatory Indo-Pacific lionfishes (Pterois spp.) throughout the western Atlantic has received considerable attention from the scientific community, resource managers, and the public. Competition between lionfish and native predators is an important potential outcome of the invasion, although supporting empirical evidence for such an interaction has been scarce and equivocal. On coral reefs in south Florida, one of the most likely competitors with lionfish is the Graysby (Cephalopholis cruentata), a similarly sized mesopredator. We measured variation in Graysby diet and condition across a gradient of lionfish biomass on complex, contiguous reefs along the ledge of Biscayne National Park. Graysby and lionfish diet were analyzed using gut contents and bulk stable isotopes (δ13C and δ15N) from muscle tissue. Dispersions of stable isotope values were estimated using SIBER analysis (Stable Isotope Bayesian Ellipses in R) and used to assess lionfish and Graysby “isotopic niche width,” a component of trophic niche. Graysby condition and population status were also quantified via physiological indices and abundance estimates.
Overall, Graysby muscle had similar isotopic composition to that of lionfish, indicating comparable population-scale resource use. Also, δ15N increased with body length in both species at a similar rate (p = 0.001), potentially reflecting mutual ontogenetic diet shifts that could lead to interspecific competition across multiple life stages. The isotopic niche of Graysby contracted by 33% on sites with higher lionfish biomass (Probability = 99.2%), suggesting a specialization of population-level diet. This niche width shift coincided with a 25% decrease in the proportion of Graysby stomachs containing fish (χ2 = 3.96, p = 0.046). Additionally, overlap between Graysby and lionfish isotopic niches decreased by 25% in association with an elevation of lionfish biomass, possibly measuring divergence of resource use. However, Graysby condition did not change across the same lionfish biomass gradient. Relatively low lionfish densities and abundant resources may have limited competition between lionfish and Graysby in Biscayne National Park, or interspecific competition may affect Graysby in ways not measured by this study. Regardless, these results provide empirical evidence for a high competitive potential between lionfish and a native mesopredator, as well as shifts in native fish diet coincident with elevated biomass of this invasive marine fish.