Oeno Island


Site description (2006 baseline):

Site location and context
Oeno is a low oceanic atoll to the North of Pitcairn. The fringing reef is approximately circular and about 4 km in diameter. It is breached by a single passage, on the northern margin. Through this passage, shallow-draft boats (e.g. the Pitcairners' longboats) may enter the lagoon, which is uniformly shallow, less than 3 m deep, with scattered reefs. There is a single motu, about 80 ha in extent, south-west of the centre of the lagoon. North of the motu and intermittently connected to it, a sandspit runs towards the reef.Oeno is a remarkably undisturbed atoll. The margin comprises either sandy beach or reef flat. Behind this lies shoreline scrub, often dominated by Suriana maritima. At the south end of the motu is closed forest of Argusia argentea and Pisonia grandis, the canopy reaching 7-9 m. Most of the rest of motu is clothed in more or less open Argusia scrub. In total there are only 16 vascular plant species present, but this includes the endemic Bidens hendersonensis var. oenoensis. There is also a stand of coconut trees, introduced by the Pitcairners, at the north of the island.Because the entrance to the lagoon is too shallow to allow the passage of ocean-going yachts and because there is no secure anchorage outside the reef, Oeno is rarely visited. Once or twice a year, the Pitcairners visit the island to camp in the vicinity of the coconut grove. The visits, typically in the southern summer, normally last around a week and are viewed as holidays.



Key biodiversity
Oeno's chief ornithological importance is the Murphy's Petrel colony, estimated in 1991 at 12,500 pairs and therefore the second largest in the world. It would be worthwhile to assess whether the 1997 eradication of Pacific rats Rattus exulans from Oeno has been followed by an increase in the population of Murphy's Petrels. Similarly, in the light of likely decreases in the breeding populations of Herald and Kermadec Petrels between 1922, the year of the Whitney Expedition visit, and 1991/92, checking for signs of recovery of these populations would be valuable. Unfortunately, monitoring this situation will be difficult because of the intrinsic difficulty of reaching the island and because the Pitcairners' holiday visits to the island are usually in the austral summer rather than during the Murphy's Petrel winter nesting season. Phoenix Petrels, apparently disappeared from Oeno between the Whitney visit in 1922 and the 1991/1992 Expedition, but perhaps 12-20 pairs were present in 1997 and 1998. Oeno supports populations of other seabirds, notably Red-tailed Tropicbird (500-1000 pairs), Masked Booby (250-300), Red-footed Booby (25), Greater Frigatebird (100) and Fairy Tern (< 1000).Oeno is also important as a wintering site for the Bristle-thighed Curlew Numenius tahitiensis. In January 1990 about 100 were recorded.It is possible that the widespread Spotless Crake Porzana tabuensis breeds on Oeno for it is seen regularly. Otherwise there are no landbirds.

Non-bird biodiversity: The flora of Oeno is poor in species. Only the local variety of Bidens hendersonensis is endemic; this was not seen in 1991/92, but was relocated in 1997. On the other hand, the marine mollusc fauna is much richer. Around 240 taxa are known, of which about two percent are endemic to the Pitcairn group. One species, a liotinine gastropod, belongs to a genus (and therefore species) known only from Oeno.



Pressure/threats to key biodiversity
The vegetation of Oeno could alter drastically, with consequences for birdlife, if certain plant species (e.g. Lantana camara) reached the island. Great care should taken to avoid the accidental or deliberate introduction of exotic species. For example, soil, seeds or seedlings should not be taken to Oeno from Pitcairn. If newly-arrived weedy species are noticed, they should be uprooted and destroyed immediately. All food waste should be properly buried.Rats were eradicated from the island in 1997. The project was carried out by Wildlife Management International with funding from the British Department for International Development. As discussed above, logistic difficulties have precluded any monitoring of the impact of this on seabird nesting success or numbers. The most likely route of re-invasion is if rats were transported from Pitcairn, where they are still present, to Oeno in a Pitcairn longboat. This is a real risk since the longboats are loaded with provisions alongside the Pitcairn jetty and then unloaded while at anchor only some 10 metres from Oeno's beaches. When loading for a trip to Oeno, the Pitcairners should take every precaution to ensure the longboats are rat-free.Tourist visits to Oeno are being actively planned. If such visits are well-managed, they need not harm the island's wildlife. It is, however, essential that the operators undertake the strictest precautions to ensure that neither animals nor plants are inadvertently introduced. The risk of such introductions would be greatly increased if ever tourists stayed overnight on Oeno. Equally important, any developments that depend on dredging or blasting a deeper channel through the reef passage and across the lagoon to the motu should be absolutely prohibited. The risk of altering the sedimentation pattern of the entire lagoon is high, with potentially severe consequences for the marine molluscs. At worst, the ensuing change in water flow could lead to the disappearance of most of the island.Oeno appears to qualify as a Ramsar site and therefore its designation should be promoted after full consultation. Ramsar designation does not preclude ecologically sensitive visits or sustainable local exploitation of reef fish, but would recognize and help perpetuate Oeno's importance as a haven for birds and as a rare example of a pristine Polynesian lagoon.



Acknowledgements
Site-account by M. de L. Brooke.


Recommended citation
BirdLife International (2024) Important Bird Area factsheet: Oeno Island (Pitcairn Islands (to UK)). Downloaded from https://datazone.birdlife.org/site/factsheet/oeno-island-iba-pitcairn-islands-(to-uk) on 23/12/2024.