Successful eradication of invasive rodents from a small island through pulsed baiting inside covered stations

  • Published source details Orueta J.F., Aranda Y., Gomez T., Tapia G.G. & anchez-Mármol L. (2005) Successful eradication of invasive rodents from a small island through pulsed baiting inside covered stations. Biological Invasions, 7, 141-147.


Islas Chafarinas (a Spanish ruled territory) are a group of three islets located in the southwest Mediterranean Sea off the coast of northeast Morocco. The smallest, Ilsa del Rey (11.6  ha) (35˚20'N, 2˚25'W) supports the second most important Audouin's gull Larus audouinii breeding colony in the world. To reduce the predation threat to this rare gull, other nesting seabirds and other native fauna, it was decided to try and eradicate invasive black rats Rattus rattus.

Population size estimation: In February 1992 (two months prior to the first laying of poison bait), three trapping grids (traps 10 m apart) were installed: 5 x 10 traps in the northern part of the island (covering 0.36 ha), 5 x 8 traps in a central area (0.28 ha) and 10 x 10 in a southern area (0.81 ha). Traps (tied to a wooden peg with string; baited with a fish oil, sugar and flour mix, soaked on a piece of lamp wick) were set at dusk and checked at dawn; any set trap was sprung to avoid harming birds or reptiles.

In 1996, 1997 and 1999, during the last week of September, three lines of 10 snap stations (two traps at each) were used to estimate population size. Trapping effort was 950 trap-nights in 1992, 300 trap-nights in 1996 and 1999 and 240 trap-nights in 1997.

1992 poisoning: In 1992, 148 baiting stations (120 g pelleted brodifacoum 50 ppm in a plastic prism-shaped container) were installed on the islet, approx. 25-30 m apart. After six nights, any remaining bait was measured and removed. Three 'pulses'were indertaken in April, August and October. After this, bait consumption reached zero and there were no signs of rats for over 2 years, until 1995. In 1995, rat scats were observed, and the population increased rapidly.

1999-2000 eradication campaign: When snap trapping success reached 37 captures/100 trap-nights, a new campaign was started in late 1999. Flocoumafen 50 ppm (in16 g wax-cereal blocks) was dispensed inside 180 baiting stations (plastic prism containers, and also plastic boxes with a 5 cm diameter access hole on one side) laid 25 m apart. Flocoumafen was selected, as the bait used in 1992 was no longer available. Eleven consecutive pulses were maintained between November 1999 and February 2000; less than 1 kg/ha of bait was used on each occasion.

Capture rates/100 trap-nights were 9.6% (1992), 31.3% (1996), 27.5% (1997) and 37.0% (1999). Populations densities in 1992 prior to poison baiting were estimated at between 93 individuals/ha in the north and 51/ha in the south parts of the islet.

Between 2000 and spring 2003 (i.e. after the second eradication campaign), there were no rat captures or signs of rats detected.

In 1999, the greater density of baiting stations and the proximity of pulses (without letting the rat population recover) was considered key in contributing to the eventual eradication success. Monitoring, using snap traps and baiting (66 stations) is ongoing to guard against re-invasion.

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