Factors influencing the nursery dynamics of juvenile bull sharks in two estuaries along the Texas coast

Nursery habitats provide refuge for juvenile organisms to grow and develop, and are utilized by several shark species, including bull sharks (Carcharhinus leucas). Bull sharks utilize estuaries adjoining the Gulf of Mexico as nursery habitat, and are the most abundant shark species found along the Texas coast. Little is known about their nursery dynamics in this region, especially for the young-of-the-year (YOY) age class. This study investigated how predation risk and abiotic factors influenced the occurrence and densities of YOY bull sharks in two Texas estuaries: San Antonio Bay and Sabine Lake using in-situ drumline sampling and historical long-term gillnet monitoring (1985-2018). In San Antonio Bay, the densities of larger sharks posing a threat to YOY bull sharks was highest in the months of May and June, and significantly influenced by location within the estuary. No predatory sharks were sampled in Sabine Lake, suggesting that this entire estuary may serve as important nursery habitat. In both systems, densities of YOY bull sharks were highest in low salinity waters near river mouths, and in San Antonio Bay, lower predation risk was a significant factor predicting densities of YOY bull sharks. YOY bull shark densities were also influenced by temperature, dissolved oxygen, and location within the estuary. Understanding the effects of changing environmental conditions on predation risk and YOY bull shark habitat use allows us to better understand shark nursery dynamics along the Texas coast, and identify important nursery habitats for this estuarine predator.