Characterizing the bull shark Carcharhinus leucas habitat in Fiji by the chemical and isotopic compositions of their teeth

Bull sharks Carcharhinus leucas use estuarine and riverine systems as nursery habitat. The Shark Reef Marine Reserve (SRMR) on the southern coast of Viti Levu, Fiji, is well-known for its adult bull shark population. The species' seasonal departure from the SRMR is related to reproductive activity, but nursery grounds have not yet been identified on the southern coast of Viti Levu. In order to further identify and characterise bull shark habitats in Fiji, 49 teeth were collected from bull sharks encountered at the SRMR and measured for their trace element concentrations, and 22 of them for oxygen isotopic composition in the phosphate group of bioapatite. The trace element analyses yielded relatively high Na, Mg, Sr, and F and low Ba concentrations for all the teeth supporting formation in marine environment. The phosphate oxygen isotope data concur with this result and the data evidently show that these teeth developed under marine condition relating to the temperature and oxygen isotopic composition of Fiji's coastal waters. Therefore, the investigated teeth show no signs of freshwater habitat. Our results do not support the hypothesis that bull sharks enter freshwater habitats, at least not for longer time periods, during their absence from the SRMR. Additionally, the bull shark teeth had unexpectedly high zinc concentration at the very edge of the enameloid. This cannot be explained by environmental factors; therefore the high Zn content is interpreted here as a result of biological process, a reflection of enzyme (i.e., KLK4) related organic matter removal and enhanced crystallization during tooth maturation.