Rosemary Kate Steinberg , John Turnbull, Tracy Ainsworth, Alistair Poore, Emma L Johnston

Context: Diseases have impacted coral populations worldwide, leading to population declines and requiring active restoration efforts.

Aims: Describe population and individual impacts of necrotising disease in the Endangered octocoral Dendronephthya australis.

Methods: We quantified population loss and recruitment using reference photos, survey, and GPS mapping and described disease lesions using histopathology.

Key results: From December 2019 to January 2020, we observed polyp loss, necrotic lesions, and loss of large colonies of D. australis at Botany Bay, NSW, Aus. By September 2020 only a few scattered recruits remained, and all large colonies were lost. Histopathology of colonies sampled in January 2020 confirmed that the disease had resulted in necrosis, gastrovascular canal collapse, and internal colony integrity loss, leading to mortality. New recruits were recorded within 10 months of disease onset, and large colonies within 18 months.

Conclusions: While the necrotising disease had significant impacts on both the individual and population level, natural recruitment began quickly. As such, unlike in other populations, restoration is not currently required in the Bare Island D. australis population.

Implications: The extent of disease impact at the individual and population levels suggests that monitoring for lesions should be undertaken before developing conservation and restoration strategies for this species.