Vertebral chemistry demonstrates movement and population structure of bronze whaler

Abstract

Analysing the elemental composition of calcified structures of fish, such as otoliths, can show movement patterns and population structure — essential data underlying effective fisheries management. Elasmobranch vertebrae provide an analogous, yet largely underexploited component to quantify patterns of population structure in cartilaginous fishes. We quantified multi-element vertebral profiles in bronze whaler sharks Carcharhinus brachyurus from 3 regions in South Australia using laser ablation-inductively coupled plasma mass spectrometry. We used multiple approaches to examine short- (months) and long- (lifetime, years) term perspectives of element incorporation. Boosted regression trees showed that variation in multi-element concentrations at the vertebral edge (representative of the time near capture) was explained primarily by spatial and shark length-based differences. An integrated lifetime elemental signal was similarly influenced by spatial differences and shark length. Cluster analyses of detrended elemental profiles across vertebrae indicated that movement patterns were highly individualistic, reflecting the mobile behaviour of this cosmopolitan species. Differentiation among sampling regions within South Australia suggests that current state-based management appears appropriate. However, when these findings are considered with existing telemetry and molecular genetic data on population structure for the species, it is likely that there is high connectivity throughout the species’ range, and cooperation among management jurisdictions should be explored. More broadly, using multiple approaches to element analysis and the integration of methods (such as telemetry, genetics and chemistry) that vary in their representative temporal and spatial breadths will aid in quantifying the population structure and movement behaviour of sharks.