Behavioural responses of white and bull sharks to static counterilluminated decoys

Abstract

Sensory information drives animal behaviour and thus understanding perception is integral to species conservation and management, including the mitigation of human-shark interactions. A recent study on white sharks ( Carcharodon carcharias ) showed that manipulating the visual cues of objects that resemble natural prey reduces the likelihood of sharks biting the objects, potentially offering new technology to reduce shark-bites. These experiments used artificial counterilluminated prey (seal-shaped decoys) towed behind a boat. However, white sharks also target static objects. Moreover, there is a risk that bright lights may be attractive to other potentially dangerous shark species. In this study, we evaluated the potential of counterillumination for shark-bite mitigation by comparing interactions of white and bull sharks ( Carcharhinus leucas ) with static counterilluminated decoys against non-counterilluminated control decoys. Lighting of different intensities arranged as horizontal stripes on the underside of oval-shaped decoys were tested. White sharks interacted less with counterilluminated decoys compared to control decoys, at the medium and high light intensities. However, they interacted more when the stripes were dimmest or the lights were on but covered. Whereas bull sharks were unaffected. Counterillumination technology shows promise for reducing human-shark encounters with white sharks when sufficiently bright, while posing minimal risk of attracting bull sharks.