Tracking seabird migration in the tropical Indian Ocean reveals basin-scale conservation need

Understanding marine predator distributions is an essential component of arresting their catastrophic declines.^1,2,3,4 In temperate, polar, and upwelling seas, predictable oceanographic features can aggregate migratory predators, which benefit from site-based protection.^5,6,7,8 In more oligotrophic tropical waters, however, it is unclear whether environmental conditions create similar multi-species hotspots. We track the non-breeding movements and habitat preferences of a tropical seabird assemblage (n = 348 individuals, 9 species, and 10 colonies in the western Indian Ocean), which supports globally important biodiversity.^9,10,11,12 We mapped species richness from tracked populations and then predicted the same diversity measure for all known Indian Ocean colonies. Most species had large non-breeding ranges, low or variable residency patterns, and specific habitat preferences. This in turn revealed that maximum species richness covered >3.9 million km², with no focused aggregations, in stark contrast to large-scale tracking studies in all other ocean basins.^5,6,7,13,14 High species richness was captured by existing marine protected areas (MPAs) in the region; however, most occurred in the unprotected high seas beyond national jurisdictions. Seabirds experience cumulative anthropogenic impacts¹³ and high mortality^15,16 during non-breeding. Therefore, our results suggest that seabird conservation in the tropical Indian Ocean requires an ocean-wide perspective, including high seas legislation.¹⁷ As restoration actions improve the outlook for tropical seabirds on land^{18,19,20,21,22} and environmental change reshapes the habitats that support them at sea,^15,16 appropriate marine conservation will be crucial for their long-term recovery and whole ecosystem restoration.