Return of the lamingtons–boom-and-bust populations of the barrens forming sea urchin Tripneustes australiae in Lord Howe Island Marine Park

Abstract Sea urchins in the genus Tripneustes have fast growth and boom-and-bust population dynamics. Tripneustes australiae is endemic to southeast Australia and northwest New Zealand and in 2024 exhibited a population increase (‘boom’ phase) in Lord Howe Island Marine Park (LHIMP). Grazing pressure in population outbreaks of this species creates barrens-type habitat. We used Reef Life Survey data to document the density of Tripneustes species spanning 18 years (2006–2024). Two outbreaks were evident, the first in 2008 followed by a marked decline and the second in 2024. There was no difference in the densities of T. australiae in no-take sanctuary zones and habitat protection zones of LHIMP over most years, except for higher densities in sanctuary zones in the boom year 2024. The Admiralty Islands appear to be a recruitment hotspot for this species. Visual surveys undertaken for the 2024 outbreak showed that most T. australiae occurred at 6–12 m depth and were < 5 cm test diameter indicating recent recruitment. The presence of juveniles was confirmed by the absence of gonads. The drivers of the boom-and-bust population dynamics of T. australiae are likely related to inherent demographic traits of the genus associated with fast growth to reproductive maturity and a short life span. Recruitment anomalies of T. australiae to LHIMP may be related to climate warming and altered current connectivity from the Australian continent. The population dynamics and ecological impacts of T. australiae are important to understand in the high conservation value Lord Howe Island World Heritage Area ecosystems.