Decoding fingerprints: elemental composition of vertebrae correlates to age-related habitat use in two morphologically similar sharks

We compare vertebral microchemistry with previously described age-related movement patterns of bull sharks Carcharhinus leucas and pig-eye sharks C. amboinensis within coastal waters of north Australia. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) quantified the chemical signatures of nursery habitats within the vertebrae of juvenile and adult sharks. We examined evidence for adults returning to these habitats by applying LA-ICP-MS along a growth axis of their vertebrae. We transposed chemical signatures with growth increments in adult vertebrae to correlate with age estimates. Unique elemental signatures were identified in each of the fresh water nurseries, but we did not find them in adult vertebrae. Age-specific changes in vertebral microchemistry in mature female bull sharks correlate with periodic returns every 1 to 2 yr to less saline environments to pup. We were unable to discriminate among natal habitats of pig-eye sharks using elemental fingerprints, and age-specific changes in vertebral microchemistry were also absent. We conclude that changes in vertebral microchemistry correlate with known habitat use patterns of the bull and pig-eye sharks, showing the potential of vertebral microchemistry to discern movement patterns in sharks.