Spatial variation in the diet of generalist predators can have pronounced effects on various processes, including the timing and rate of ontogenetic trophic shifts. Juveniles of the upper-trophic level Gag (Micteroperca microlepsis; Serranidae) are generalist predators that settle to seagrass habitats in seven major estuaries in the eastern Gulf of Mexico. The locations of these estuaries span over 700 km of coastline across warm-temperate and subtropical biomes where their prey communities have been shown to differ. We examined the size-specific (10 cm – 25 cm total lengths) and ontogenetic trends in Gag diet across this relatively large region using a combination of stomach content, bulk stable isotope (δ13C and δ15N), and compound specific amino acid isotope analyses (CS-AA-IA, δ15N glutamic acid – δ15N phenylalanine).
Results/Conclusions
We observed high spatial variability in diet, with the relative importance of major prey groups (e.g., decapods, benthic fishes, midwater fishes) differing by up to two orders of magnitude among estuaries, but without apparent trends (e.g., across latitude, among urbanization levels). Although diets tended to shift from invertebrate to teleost prey with increased size of juvenile Gags, the lengths at which this occurred varied among the seven systems. Bulk isotope analysis reflected linear increases in trophic level with Gag size in five of the seven estuaries, and no change in the other two. Conversely, the CS-AA-IA reflected precise and consistent linear increases in trophic level with size, suggesting conserved trophic growth among estuaries (mean ± se = 0.58 ± 0.05 trophic steps). These results have both methodological and ecological implications. Methodologically, the difference in results between the two isotopic analyses highlights the importance of accounting for baseline isotopic levels, and demonstrates the utility of CS-AA-IA, which required sample sizes of only ~20% of that from bulk analysis. Ecologically, our results suggest that despite geographic heterogeneity in diet, the timing and rate of trophic growth in a generalist predator was consistent across a wide span of estuarine systems.