Cyclone Ilsa in April 2023 led to significant seabird mortality on Bedout Island

Abstract

On 13 April 2023, Cyclone Ilsa crossed Bedout Island in the Timor Sea off Western Australia’s remote north coast. Extensive damage was done to the entire island, with all coastal vegetation stripped clear from the island’s surface. Here we use repeated aerial and ground-based transect surveys of Bedout’s important seabird populations during 17 April – 21 July 2023 and Monte Carlo modelling to estimate total mortality for three species. We estimate that 80–90% of the populations of three seabird species (Masked Booby Sula dactylatra bedouti, Brown Booby S. leucogaster, and Lesser Frigatebird Fregata ariel) were killed in the storm, with only 40 breeding Masked Booby (a putative endemic subspecies) recorded on the island 15 weeks later. The frequency and intensity of such storms is likely approaching a threshold beyond which Bedout’s seabirds cannot readily recover, with cyclones hitting the island, on average, every seven years in recent decades. We discuss the implications for Bedout Island, and for wildlife conservation more broadly in the face of increasing, climate-driven storms and impacts documented worldwide.