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

Abstract

Estuarine nurseries are hypothesized to offer juvenile bull sharks Carcharhinus leucas refuge from predators, with reduced mortality compared to adjacent marine habitats. Many estuaries in the Gulf of Mexico serve as nurseries for this species, yet the estuary refuge hypothesis has largely been untested, and it is unclear what factors make estuarine habitats suitable for young-of-the-year (YOY) bull sharks. Using drumline sampling and long-term gillnet monitoring data, this study investigated how predation risk and abiotic factors influenced densities of YOY bull sharks in 2 Texas estuaries. The more saline San Antonio Bay had higher densities of predatory sharks (>150 cm total length) that pose a risk to juvenile sharks, with predatory sharks caught in higher salinity regions proximate to the Gulf of Mexico. Consequently, densities of YOY bull sharks were highest in low-risk habitats proximate to river mouths in San Antonio Bay. In Sabine Lake, both predatory sharks and YOY bull sharks occurred most frequently in less saline habitats. However, only 2 predatory sharks were sampled in this hyposaline estuary, suggesting that encounters with predators are rare. Results support the estuary refuge hypothesis and indicate that YOY bull sharks avoid risky habitats in ecosystems where predators are prevalent (e.g. San Antonio Bay), while some estuaries function as refuges throughout their waters due to hyposaline conditions largely excluding predators (e.g. Sabine Lake). As freshwater inflows into estuaries change in response to anthropogenic and climatic influences, understanding how alterations in salinity affect predation risk and YOY bull shark distributions will aid in predicting the role that estuarine habitats play as shark nurseries.