Habitat Use and Population Structure of a Large Pelagic Elasmobranch Based on Vertebral Chemistry

Analyses of the elemental composition of teleost otoliths (and other calcified tissues) are frequently employed to delineate movements and population structuring of wild fish, and are highly informative for fisheries management. Elasmobranch vertebrae provide an analogous, yet largely underutilised structure to better understand patterns of habitat use in cartilaginous fishes. Using laser ablation inductively coupled plasma mass spectrometry, we quantified the vertebral elemental composition of Carcharhinus brachyurus from 5 locations throughout South Australia. Elemental data at the marginal edge was used to derive a multi-element ‘region of capture’ signature and optimized to identify spatial differences among samples. Boosted regression tree analysis indicated that variability in multi-element vertebral signatures were largely due to differences among individuals and size cohorts, with a lesser spatial component. These findings suggest that the two South Australian gulfs are utilized by individuals from different size cohorts, and are reflective of the migratory nature of this cosmopolitan species. These findings will be used as a basis for element profile (transect) analyses to examine age-resolved patterns of movement and connectivity of individuals throughout their lifetimes.