Vertical habitat and behaviour of the bluntnose sixgill shark in Hawaii

Bluntnose sixgill sharks are apex predators and scavengers that are near-globally distributed in slope and shelf habitats, but many aspects of their behaviour and ecology are poorly understood. A better understanding of how oceanographic variables influence sixgill shark behaviour may help predict their distribution, response to increasing anthropogenic stressors including climate change, and role in ecosystems throughout their geographic range. We used satellite telemetry to observe the vertical behaviour of four bluntnose sixgill sharks in the subtropical oligotrophic waters of Hawaii. A strong diel vertical movement cycle was observed, with sharks spending nighttime in thermocline waters and descending into the oxygen minimum zone in daytime. Depth changes generally occurred between nautical twilight and sunrise/sunset. Dive initiation and dive completion were significantly correlated with nautical dawn and sunrise, respectively. A stepwise generalised estimating equations model was used to investigate vertical speeds in the daytime and nighttime depth habitats, and this analysis revealed that photic zone light level was the primary factor correlated with vertical speed. Outside of the depth transitions, higher vertical speeds were observed when photic zone light was low, suggesting more active foraging in the shallow nighttime habitat than the deep daytime habitat.