Ecological Dynamics and Connectivity Within an Assemblage of Predatory Fishes in Coastal Texas

Abstract

Predators shape ecosystem function through direct and indirect effects on prey populations and other predators. Furthermore, co-occurrence of multiple predators is common in most ecosystems and interactions between them can result in far-reaching ecological implications. However, resource partitioning is expected to occur among sympatric predator species to alleviate competitive pressures. This study employed multiple methodologies to examine the habitat use, movements and feeding ecology of an assemblage of estuarine and coastal predators in a Texas estuary and the larger northwestern Gulf of Mexico (nwGOM). Four primary species were chosen to exemplify the diversity of life histories of estuarine predators in this region: Spotted Seatrout (Cynoscion nebulosus), Black Drum (Pogonias cromis), Bull Shark (Carcharhinus leucas), and Alligator Gar (Atractosteus spatula). These species co-occur in estuaries but vary in their degree of freshwater habitat use and differ morphologically and physiologically. In Chapter II, habitat suitability modeling revealed seasonal shifts in the habitat use and spatiotemporal overlap patterns of each predator and four potential prey species, suggesting the potential for seasonality in predator-prey and predator-predator interactions. Acoustic telemetry was employed in Chapter III to further examine habitat use, the results of which primarily aligned with Chapter II but added greater seasonal detail, exposed differences in core use area, and documented long-distance movements outside the estuary. Variation in isotopic niche breadth and trophic position was observed among these predators using stable isotope analysis in Chapter IV, which elucidated the degree of trophic overlap and partitioning between them. Additionally, five shark species were chosen to expand the scope of the study into the coastal and pelagic nwGOM: Bull, Bonnethead (Sphyrna tiburo), Blacktip (C. limbatus), Spinner (C. brevipinna), and Shortfin Mako Sharks (Isurus oxyrinchus). In Chapter V, elemental tracers in each species’ vertebrae cartilage were examined across ontogeny, providing insight into the co-occurrence patterns of these sharks throughout their lifespans. Together, these four chapters described the trophic and habitat overlap and partitioning of estuarine, coastal, and oceanic predators in the nwGOM. Understanding the circumstances in which these predators co-occur with one another allows for inferences into how they coexist in a shared ecosystem.