Plasticity in the diel vertical movement of two pelagic predators (Prionace glauca and Alopias vulpinus ) in the southeastern Indian Ocean

Management and conservation of marine predator species relies on a fundamental knowledge of their movements and behaviour. Pop-up satellite archival tags were used to investigate the vertical movement patterns of five blue sharks (Prionace glauca) and one thresher shark (Alopias vulpinus) within the southeastern Indian Ocean. Sections of similar depth distribution, identified using a split moving window analysis, were investigated in relation to the thermal structure of the water column and activity rates. Minimum horizontal displacement of between 66 and 5,187km for blue sharks and 16km for the thresher shark were recorded over 863 tracking days. Maximum depths ranged from 540 to 807m for blue sharks and 144m for the thresher shark. All sharks displayed plasticity in their depth distribution, with diel vertical movements and surface-oriented movements the two most common patterns. Diel movement of prey is the most likely explanation for diel vertical movements of thresher and blue sharks. This study has improved our understanding of the vertical movement patterns of these pelagic predators and the relationship between their depth distribution, temperature, and activity