Spatial and Ontogenetic Trophic Dynamics of Co-occurring Predatory Fishes in a Northern Gulf of Mexico Estuary

Abstract

Co-occurring predators often exhibit ecological niche partitioning, resulting from competition over evolutionary time. However, in productive estuarine ecosystems with high resource availability, predators may occupy similar niches without conflict. Determining the degree of niche partitioning and overlap among co-occurring predators can provide insights into a food web’s function and its potential resiliency to perturbations. This study used stable isotope analysis to assess the trophic ecology of four predators in Galveston Bay, Texas, USA: spotted seatrout, black drum, bull shark, and alligator gar. Spatially distinct primary producer isotopic ratios emerged for both δ13C and δ15N following salinity regimes, which translated to similar patterns in predator tissue. The volume and overlap among species’ trophic niches also varied spatially, with species-specific expansion and contraction of niches across the freshwater-marine continuum. The observed niche patterns were likely related to movements, with implications for trophic coupling across the estuarine landscape. Using regional delineations for baseline values yielded trophic position estimates that were validated by compound-specific stable isotopes and were similar (3.77 to 3.96) for all species but black drum (3.25). Trophic position increased with body length for all species but black drum, and these relationships differed when using estuary-wide versus regionally distinct baselines. Alligator gar gut contents were examined, which primarily aligned with piscivory but also included previously unreported taxa (insect, mammal). Collectively, these results provide evidence for spatial and ontogenetic shifts in trophic ecology within this predator assemblage and highlight the importance of spatial scale when using stable isotopes to examine estuarine food webs.