Oiseaux marins des atolls d’Entrecasteaux (parc naturel de la mer de Corail) : bilan des observations des deux dernières décennies | Seabirds of the d’Entrecasteaux atolls (Natural Park of the Coral Sea): a review of observations for the last two decades

The Coral Sea, considered to be the last wide region in the tropical ocean moderately impacted by human activities, is a hotspot of diversity and abundance for tropical seabirds. The d’Entrecasteaux atolls north of New Caledonia are listed as UNESCO World Heritage site since 2008 as they are an important area for the conservation of these birds. Regular and long-term monitoring of seabird colonies can point to possible demographic changes and help identify their causes. The objective of this work is to update our knowledge of the marine avifauna of d’Entrecasteaux atolls by integrating new census data since the review published by Robinet et al. [Colon. Waterbirds, 20 (1997) 282-290]. Robinet et al.’s (1997) review gathered count and phenology data acquired during visits in 1995 and 1996 on the Huon, Fabre and Surprise islets, supplemented by observations made from 1986 to 1993 by local naturalists. Concretely, we extracted from the reports of ornithological expeditions to the d’Entrecasteaux atolls in the last two decades: (i) the list of seabirds breeding there; (ii) the abundances by species; and (iii) the possible demographic trends over the past two decades. We paid particular attention to the census of wedge-tailed shearwater populations, because of the high abundance of this species in the Coral Sea. The original reports from 15 expeditions between 2002 and 2018 were examined, compiled and analyzed. All expeditions took place during the warm season. For each expedition, the list of species observed breeding and colony-size estimates were recorded. For each species on each islet, the highest estimate over the period was retained. Some time series on Surprise islet were sufficiently long, and the sampling method (transect counts) sufficiently explicit, to be retained for the present work; these concerned four species: wedge-tailed shearwater, brown booby, red-footed booby and black noddy. We also examined the time series of masked booby counts on Huon and Surprise islets. The null hypothesis of no trend in abundance over years was tested using the Mann-Kendall nonparametric trend test. During the period, 13 species were recorded as regular breeders on the islets and cays of the d’Entrecasteaux atolls: wedgetailed shearwater Ardenna pacifica, red-tailed tropicbird Phaethon rubricauda, masked, brown and red-footed boobies Sula dactylatra, S. leucogaster and S. sula, lesser and great frigatebirds Fregata ariel and F. minor, crested tern Thalasseus bergii, black-naped tern Sterna sumatrana, bridled and sooty terns Onychoprion anaethetus and O. fuscatus, and black and brown noddies Anous minutus and A. stolidus. Six species (wedge-tailed shearwater, red-tailed tropicbird, masked booby, brown booby, crested tern, brown noddy) were breeding on all four islets. The fairy tern Sternula nereis was also observed regularly and is a likely breeder. The white-tailed tropicbird P. lepturus, the roseate tern Sterna dougallii, the little tern Sternula albifrons and the white tern Gygis alba were observed occasionally. There was no indication of the presence of an unidentified petrel or of the once-abundant silver gull Chroicocephalus novaehollandiae on these islets, both mentioned in the 19th-century literature. The wedge-tailed shearwater was the most abundant seabird with a total estimate of over 55,000 breeding pairs, or ≥ 2.1% of the global population of the species. The population size of the sooty tern reached several tens of thousands of pairs; those of the brown booby, the red-footed booby, the lesser frigatebird, and the two noddies were over a thousand to several thousand; those of the great frigatebird, the masked booby and the crested tern reached a few hundred; those of the black-naped tern and the bridled tern, a few dozen; the total number of the red-tailed tropicbird hardly exceeded ten pairs. The wedge-tailed shearwater appears to have declined substantially over the last two decades. A negative trend was also observed for the three booby species and for the black noddy. Species richness and abundance at the d’Entrecasteaux atolls confirm the importance of the latter for conservation at the regional scale. The population sizes of wedge-tailed shearwater, brown booby, lesser frigatebird and black noddy had global significance. The present data also confirm the long-lasting impacts of the alteration of the breeding habitat of seabirds on their colonies. The rarity of the red-tailed tropicbird and the absence of the silver gull contrast with 19th-century descriptions when the four main islets were still covered with forest and highlight the persistent alteration of plant and animal biodiversity on these islets. The negative population trends observed for the last two decades could reflect a general problem linked, for example, to ocean warming, or to pollution by plastics, or to a decline in ocean productivity, or to poorer accessibility of prey to seabirds, this at the regional scale or beyond. Alternatively, local factors could be responsible, such as insufficient protection of breeding seabirds from disturbance. The present survey enabled us to update our knowledge of the seabird communities of the d’Entrecasteaux atolls. The current level of protection of seabird colonies there is insufficient not only with regard to stated management objectives, but also according to internationally recognized criteria. An active restoration of the original tree habitat on the three islets now deforested (Huon, Le Leizour, Fabre) deserves consideration. Also, "monitoring" and research protocols could be improved to minimize disturbance to breeding seabirds.

Les atolls d’Entrecasteaux en mer de Corail forment une zone importante pour la conservation des oiseaux marins tropicaux. Le suivi régulier et sur le long terme de leurs colonies permet de repérer d’éventuels changements démographiques et peut aider à en identifier les causes. Les informations pertinentes à la démographie des colonies d’oiseaux marins des atolls d’Entrecasteaux, extraites de 14 rapports de missions ornithologiques faites entre 2002 et 2018, ont été compilées et analysées. Durant la période, 13 espèces se sont reproduites sur les îlots et cayes de ces atolls : puffin du Pacifique Ardenna pacifica, phaéton à brins rouges Phaethon rubricauda, fous Sula dactylatra, S. leucogaster et S. sula, frégates Fregata ariel et F. minor, sterne huppée Thalasseus bergii, sterne à nuque noire Sterna sumatrana, sternes bridée et fuligineuse Onychoprion anaethetus et O. fuscatus, et noddis noir et brun Anous minutus et A. stolidus. La sterne néréis Sternula nereis, régulièrement observée, est un nicheur probable. Aucune mention n’est faite d’un pétrel non identifié ni de la mouette argentée Chroicocephalus novaehollandiae jadis abondante sur ces îlots, tous deux mentionnés dans des notes ornithologiques du XIXe siècle. La rareté du phaéton à brins rouges (onze couples reproducteurs) contraste avec les descriptions du XIXe siècle lorsque les quatre îlots étaient encore couverts de forêt. Le puffin du Pacifique est l’espèce la plus abondante avec plus de ≥ 2.1 % de la population mondiale de l’espèce. Les atolls d’Entrecasteaux hébergent aussi ≥ 1 % de la population mondiale du fou brun, de la frégate ariel et du noddi noir. Une diminution de la population est observée pour le puffin du Pacifique, les trois espèces de fous et le noddi noir. Différentes hypothèses explicatives sont discutées. Eu égard aux enjeux, le niveau actuel de protection des colonies d’oiseaux marins de ces atolls paraît insuffisant.