Individual study: Eradication of black rats Rattus rattus to enhance breeding success of green turtles Chelonia mydas and other fauna on Sangalaki Island, Kalimantan, Indonesia
Meier G. (2003) InGrip-Report No.1, prepared for Turtle Foundation by InGrip-Consulting & Animal Control. Hauptstr. 1 - 82541 Ammerland, Germany (added by: Showler D. 2005).
Sangalaki Island, a small island off the east coast of Kalimantan, received official protection as a conservation area in January 2002, primarily due to the presence of a small population of nesting green turtles Chelonia mydas. Subsequently, it was found that a significant population of introduced black rats Rattus rattus was present and that these were predating on turtle eggs and hatchlings, as well as other native fauna and flora. It was estimated that the proportion of turtle nests surviving to hatching was at best 10% and that failure was primarily due to rat predation. It was therefore decided to attempt to eradicate the rats from the island.
The low-lying island of Sangalaki (13.25 ha) is situated off the east coast of Kalimantan. About two thirds of the island are wooded, mostly with secondary growth. In February 2003 the task of attempting to eradicate rats from the island was given to InGrip-Consultancy. The decision to try a complete eradication was made because of the island's small size, and poison-baiting using rodenticide was decided upon as the best method due to the absence of any endemic, native or threatened rodent species. Apart from black rats, fruit bats (probably Pteropus sp.) which roost on the island in small numbers, were the only other mammal present. There was concern for the omnivorous monitor lizards Varanus salvator (that might prey on dying or dead rats and eat the bait itself) and megapodes (prob. Megapodius cumingii), which might be attracted to some baits. Measures were undertaken to reduce potential poisoning of non-target species (see below) and it was considered that the loss of a few non-target individuals was acceptable given the highly probable detrimental impact of rat predation also on the eggs and young of these two species. All operational costs were covered by the Turtle Foundation.
Rodenticide: It was decided to use a commercially available rodenticide thus reducing the risk of incidental poisoning of non-rodent species. A 'second generation anticoagulant' (poisons which interrupt the animals Vitamin K production thus hindering the process of natural blood clotting), was selected. The poison, trade name 'Klerat' (available from the production company, Syngenta) is produced as impregnated rice grains mantled by a wax layer for better weather protection and pressed into cube form, was used on Sangalaki. The toxin in Klerat is brodifacoum at a concentration of 0.005 mg, which allows a rat to ingest a lethal dose at a single feeding. In Indonesia, Klearat came in 5 kg bags each containing 1,000 5 g blocks. 'Bitrex' (a bitter substance) had also been added to put of other animals from eating the bait – rats are unable to detect Bitrex.
Bait stations: In February 2003, a grid was established over the whole island with points every 25 m (forming squares of 25 m²) at which bait stations were laid. These were marked by tying a piece of brightly colored cloth on a branch above each. To keep costs down, bait stations were made from locally available, emptied 1.5 l plastic water bottles. These were cut at top and bottom to produce a 22 cm long (10 cm diameter) tube. Two pieces of wire were then inserted through the middle of each tube and the bait (which proved rather friable) placed between the wires. To restrict access by other animals e.g. monitor lizards and young megapodes, at either end of some of the tubes a metal sheet was placed but leaving a gap allowing enough space for a rat to enter. Certain bait stations were frequented by 'unknown bait-spreading organisms'. At these the tube was extended by a tunnel-like construction made out of 30 x 30 cm pieces of cut wire-mesh.
Effectiveness of poison-baiting: The last living rats were observed on the 14 July and all bait and bait station were withdrawn on the 26 July 2003). The total amount of bait used was around 55 kg, (equivalent to 4.15 kg/ha). Live trapping with Tomahawk collapsible live-traps (Type TLT no. 202 - squirrel size) was undertaken for several days towards the end of the eradication period to detect any potentially surviving rats. Highly attractive bait (fish, shrimps, chicken, fruits, peanut butter, toasted bread and cookies) was used in traps set at localities, spread over the whole island. This resulted in the catch of eight animals (including one female with juvenile) in seven trapping hours. The same trapping procedure was undertaken also on 19 and 26 July 2003 (total of 444 trapping hours) and no rats were caught. Sangalaki Island was therefore declared temporarily rat free on the departure of Guntram Meier (overseeing the eradication) on 28 July 2003.
Impact on non-target species: No negative impact (sick or dead individuals) was recorded on monitor lizards or megapodes. Other species present during the period of poison-baiting (which might eat dead or dying rats) were also carefully monitored e.g. birds including brahminy kite Haliastus indicus, osprey Pandion haliaetus, white-bellied sea-eagle Haliaetus leucogaster and Pacific reef egret Egretta sacra. No mortalities were recorded. Many rats are thought to have perished in underground nests and those observed dead were quickly consumed by ants and crabs. These two factors, as well as the other precautionary measures, may have futher helped to keep potential incidental poisoning to a minimum.
Ongoing monitoring: Forty permanent bait stations have been established across the island to prevent re-infestation by rats. It is hoped to initiate studies to investigate the effects of the rat eradication on populations of native fauna and flora.
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