Evidence for an ecological two-population model for white sharks (Carcharodon carcharias) in Australian waters

Context Our understanding of population- and ecosystem-level processes commonly considers conspecific individuals to be ecologically equivalent. However, individuals of the same species may use resources differently, supporting the prevalence of individual specialisation or ‘apparent specialisation’. Individuals within a geographically defined population may also exhibit complex subpopulation movements, whereby individuals show philopatry to specific regions that further drives individual variation. Aims White sharks (Carcharodon carcharias) are top predators in temperate to tropical ecosystems. In Australia, two discrete subpopulations of white sharks (an east and a southwest subpopulation) have been proposed based on genetics and limited movement across Bass Strait. We aimed to characterise the extent of ontogenetic divergence in resource–habitat behaviour of white sharks from both regions. Methods We used high-resolution retrospective stable isotope profiles (δ15N and δ13C) of 74 white shark vertebral centra to examine ontogenetic trophic–habitat signatures for individuals sampled from both regions. Key results Our results demonstrate isotopic separation between juvenile–subadult sharks sampled east (−13.7 ± 0.72 δ13C; 14.2 ± 0.8 δ15N, n = 47) and southwest (−14.4 ± 0.6 δ13C; 12.5 ± 1.2 δ15N, n = 27) of Bass Strait, but with strong oscillatory trends across both regions, likely related to seasonal movements. Relative individual niche width revealed apparent specialised behaviour of juvenile–subadult sharks within both regions. Conclusions Retrospective ontogenetic isotopic profiles of vertebrae from Australian white sharks provide evidence to support an ecological two-population model for juvenile and subadult life stages. Implications Given many marine top predators are undergoing systematic population declines, understanding individual variation in diet and movement in the context of population structure and true or apparent specialisation is central to elucidating their ecological roles.