Using Steroid Hormone Concentrations to Assess the Reproductive Cycle of the Bull shark, Carcharhinus leucas, in Southern Florida

The bull shark, Carcharhinus leucas, is a large species that it is commonly distributed worldwide in tropical and subtropical waters. Despite the bull sharks global distribution, little is known about its life history. In particular, the limited reproductive information suggests the bull shark is placental viviparous, assumed to have a biennial cycle, and that newborn pup nurseries are near the coast. In order to conserve and protect any species, an understanding of the habitats where reproductive events occur is needed. In order to better understand the habitat use in Biscayne bay, Fla, and whether certain areas are critical during the reproductive cycle of bull sharks, I will evaluate circulating levels of the hormones progesterone, estradiol, and testosterone using radioimmunoassay. These samples were collected by the University of Miami opportunistically between 2012-2020 shipped to Arizona State University, where they were analyzed. For my study a total of 73 mature samples, 27 females and 46 males, were collected over the sampling period. The results indicated that Biscayne bay is an important gestation area for bull sharks. The hormonal trends for males and females demonstrated an interesting reproductive cycle, which were further supported through other placental viviparous reproductive patterns. Females had a low level of estradiol throughout most of the year, besides in the summer where there were no sharks in the bay due to movement to estuaries. During their return to the bay, there was a peak in progesterone indicating early pregnancy. Male testosterone levels indicated that there was a production in sperm right before females speculated peak in estradiol.