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This island has a significant number of breeding green turtles, and up to 40 females come ashore to lay eggs each night. Turtle harvest and egg collection were recently stopped by the local authorities and national and international NGOs, which led to the complete protection of turtles on Sangalaki and in a 3 km zone around it. Additionally, a ranger station has been built and the permanent presence of patrolling personnel and police and military was established.

While the problem of egg collection and adult harvest has been solved there, another threat appeared: the black or roof rat (Rattus rattus). This invasive alien species was unintentionally introduced to the island at an unknown date. Although not rigorously quantified, large numbers of the eggs and hatchlings (possibly the majority in many nests) were being destroyed or hunted and killed by these exotic rodents. Photos are available from the authors.

It became necessary to find a swift but comprehensive solution to the problem on Sangalaki. It was decided to tackle the problem by a full elimination of the island’s rat population. Searching the available literature and catalogues of major European Natural History Museums suggested that no native rodents were present on the island. This was augmented by direct observations and live trapping: indeed there was no native ground-living mammal. A plan was proposed and agreed upon by the local authorities. All materials for the operation were bought locally and as no salaries were charged by the executing personnel, the financial demands of the eradication attempt were kept to a minimum.

The eradication work started with the arrival of the team on the 18 June 2003 on Sangalaki. After inspection of the island the preparation of the poisoning campaign began with the establishment of a 25 m grid containing 222 bait points across the island. An additional line of 22 points encircled the islands shoreline where the rats were known to occur. Sangalaki has no permanent settlement, but a dive resort covers much of the eastern part of the small islet. This area was also divided into squares like the forested western part of Sangalaki. In addition to the 25 m grid points extra points were established at the dive lodge houses to account for the large concentration of rats in this area.

The poison used on Sangalaki was the anticoagulant brodifacoum, which is lethal to rats even if consumed in small quantities. Anticoagulant rodenticides act by disrupting Vitamin K production, which is needed for blood clotting. In combination with the increase of blood pressure and the weakening of the body’s cell-membranes the consumption of this poison leads to the death of the rats within three to five days. To make it attractive for consumption, the poison is stored in rice grains, which – for better palatability under wet or humid conditions – are covered with wax and pressed into small, blue cubes. Although this poison is formulated as a rodenticide, it also has the potential to harm other mammals and birds. This can happen by secondary poisoning (through eating contaminated rats) or by accidental consumption of bait. To reduce this risk, the poison contains the bitter substance “bitrex” which prevents accidental consumption of the bait by most ‘non-target-species’. This does not limit successful bait take by rodents, as they are unable to detect this particular compound. To further exclude any risk to native animals on the island, poison bait was placed in specially made bait stations, produced locally from empty plastic water bottles, cut open and modified with wire. Like other bait-containers, these bottles served the multiple purposes of preventing bait take by other animals and protecting the bait against rain.

After all the preparations were finalized (which included the elimination of alternative food sources and the improvement of hygienic conditions on the island) the poisoning was carried out as planned. Bait was placed across the whole island of Sangalaki, with highly lethal doses for rats. The points were checked every morning for the next three weeks. Each day, missing poison was restocked and data about feeding signs and number of blocks gone noted carefully. After day four an initial crash of the islands rat population set in, with around 50 dead animals found across the whole island in the following three days. All of the dead rats located were collected and incinerated. The tropical climate, action of invertebrates and the scavenging by monitor lizards (Varanus salvator) ensured the rapid removal of rat carcasses, the majority of which are thought to die underground. There were no apparent side effects on the environment or the green turtle population on Sangalaki.

Toward the end of the six week project all remaining bait and all markers and bottles were taken in and dangerous materials disposed of safely. Trapping and tracking for potentially surviving rats revealed no rodent or its signs. There was no evidence of turtle hatchlings that had been injured or killed by rats. Following the eradication a re-infestation prevention scheme was set up on the island. It consists of 40 permanent bait stations, each one stocked with a lethal doses of rodenticide for any newly colonising rat. After this important measure, the island was declared “presumably rat free” on the 28 July 2003, when with the first author finally all eradication staff departed from Sangalaki.

Six months later, in January 2004, a visit of the first author to Sangalaki showed that no rat had survived the control operation. As no further losses of turtle eggs or hatchlings due to rat predation were recorded, the eradication of this invasive alien species on the island can be considered a complete success and contributed to the ongoing marine turtle protection work on the island. Plans for related work on neighboring islets are currently under discussion. A full report on the project is available upon request from InGrip-Consulting.

Acknowledgements: Numerous local people supported our work and we share with them our final success. Turtle-Foundation’s board members H. Cordes and E. Meyer always gave the project always the highest priority and arranged whatever was needed for its smooth implementation. Jack Frazier kindly provided background on the importance of the local turtle population in the Derawan-Islands. Finally, thanks to Susann Buechner for her patience during the preparation and execution of this work.