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 km(2), 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 impacts(13) 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.