Trends in seabird breeding populations across the Great Barrier Reef

The Great Barrier Reef is an iconic ecosystem, known globally for its rich marine biodiversity that includes many thousands of tropical breeding seabirds. Despite indications of localised declines in some seabird species from as early as the mid‐1990s, trends in seabird populations across the Reef have never been quantified. With a long history of human impact and ongoing environmental change, seabirds are likely sentinels in this important ecosystem. Using four decades of monitoring data, we estimated site‐specific trends for nine seabird species from 32 islands and cays across the Reef. Trends varied markedly among species and sites, but probable declines occurred at 45% of the 86 species‐by‐site combinations analysed compared to increases at 14%. For five species we combined site‐specific trends into a multi‐site trend in scaled abundance, which revealed probable declines of common noddy (Anous stolidus), sooty tern (Onychoprion fuscatus), and masked booby (Sula dactylatra), but no long‐term changes in the two most widely distributed species, greater crested tern (Thalasseus bergii) and brown booby (Sula leucogaster). For brown booby, long‐term stability largely resulted from increases at a single large colony on East Fairfax Island that offset declines at a majority of other sites. While growth of the brown booby population at East Fairfax points to the likely success of habitat restoration on the island, it also highlights a general vulnerability wherein large numbers of some species are concentrated at a small number of key sites. Identifying drivers of variation in population change across species and sites while ensuring long‐term protection of key sites will be essential to securing the future of seabirds on the Reef