Combining monitoring data from multiple sites to assess population status and trends of White-tailed Tropicbirds (Phaethon lepturus) in the Seychelles

Seabirds, being long-lived top-level marine predators, are often considered to be valuable environmental indicators. With growing evidence of seabird declines worldwide, it is essential to monitor changes in populations and determine drivers of change. The Seychelles Archipelago supports the greatest abundance of seabirds in the tropical Indian Ocean, but regional seabird population breeding success and trend data are not readily available despite multi-year data collection at several locations. Here we use monitoring data on White-tailed Tropicbirds (Phaethon lepturus) in the Seychelles to present the first nation-wide assessment of the status and trends of breeding success at five key sites. Mean annual breeding success was 15% for Aldabra Atoll in the southern Seychelles and 33-55% in the inner islands (Aride, Cousine, Denis, and Cousin). In the inner islands there are indications of potential population declines (inferred from declining nesting activity rates at Aride), despite stable and comparatively high breeding success. This contrasts with Aldabra where nesting activity has been relatively stable, but a decline in breeding success is causing concern. Our study highlights clear priorities for management of P. lepturus populations in the Seychelles; the causes of the declines in P. lepturus breeding population at Aride and breeding success at Aldabra should be a focus of future research and monitoring. We propose the adoption of standardized monitoring programs and increased site coverage across the Seychelles and the broader Western Indian Ocean. This, coupled with increased pooling of data and regular analysis, will strengthen the collective effort toward the conservation management of this and other seabird species within this global seabird hotspot.