A rat free island, but how do you detect the last rat?

By Nik Cole, Durrell Mauritius Programme

It is day 16 of the rat eradication on the island nature reserve, Ile aux Aigrettes. We have not found any rat chew marks in the bait, no rat faeces or encounters with live individuals during our daily searches across the entire island for the past six days. We are starting to feel rather positive that our hard work has paid off and to top it all off it has stopped raining!

However, can we be certain that we have got all the rats? The truth is, we simply do not know, at least not yet. It could be that one or two rats still survive in the “hole-y land” where the rocky topography is so complex that rats could remain undetected and where there is sufficient food for them to not bother with our bait. Also, how do you detect one or two rats on an island of 26ha? We only need to miss one pregnant female or a pair of rats and we could have a booming population within a couple of months.

When we started the baiting, we considered alternative ways that we could detect/trap rats if they did not all take the poison. The typical snap traps were definitely not an option because of the risks of catching and maiming the endangered wildlife on the island. The island is a sanctuary for the endemic Telfair’s skink, ornate day gecko, the Pink Pigeon, Olive White-eye and Mauritius Fody. The introduced Aldabra Giant Tortoises that act as analogues for the now extinct Mauritian tortoises also inhabits Ile aux Aigrettes. All of these animals, particularly their eggs and young, are at risk from rat predation, but they would also be at risk from certain rat trapping methods.

However, the use of live traps has been an option from the start and before the baiting regime we ordered as many as we could afford and obtain within a short timescale. Twenty six traps arrived on day 7 and were deployed as we exchanged our stations for plastic bottles. Baited with very stinky salted fish (supposedly irresistible to a rat!), the traps have been moved and re-baited every few days to strategic points along the poison grid. Trap locations were chosen based upon previous rat activity, particularly rough areas in the “hole-y land” and where bait has been consistently consumed or damaged by crabs and snails.

In such areas we also tried to limit non-target bait consumption by placing the stations in areas where only rats could easily access them, such as along sturdy and low branches of ebony trees. Each live trap has been opened every evening, then first thing in the morning before we walk our lines the traps are checked and closed. We only conduct the live trapping at night when the rats are mostly active, but also to reduce the chances of trapping the very curious, endemic birds and reptiles, which are usually active during the day.

Apart from smelly fish, rats also love peanuts, chocolate, raisins… basically anything sweet and calorific. We considered placing a peanut, chocolate and raisin mix in the live traps, but apart from the cost and the chocolate melting everywhere in the heat, we knew that we would only capture each other eating the bait before it became available for rats. From the outset we considered using wooden chew-sticks soaked in peanut oil as a tool to detect the presence of rats. Rats attracted to the smell of the peanut oil would chew into the stick leaving their distinctive bite marks behind. However, it became apparent that wooden lollipop sticks and chop-sticks, which are ideal for the job, are a rare commodity in Mauritius. After much searching we found a medical supplier that stocked wooden tongue depressors, which suited our needs perfectly.

After soaking the tongue depressors in a mixture of peanut butter and vegetable oil for the past two days the chew-sticks were ready to be positioned across the island. This morning whilst we walked our lines, we nailed or wedged a baited chew-stick into place at each of the 394 stations across the island. We also reduced the amount of poison bait at the stations by half to further limit the amount available for non-target animals.

Day 20 and ten days without rat sign. We feel confident enough to remove the poison from the stations, but leave the plastic bottles in place just in case we trap a rat or detect their gnawing marks on the chew-sticks We have found that we need to re-apply peanut oil to the chew-sticks every two days. It certainly feels strange to be wandering around the forest with a bottle of oil and a tiny paint brush, only to feed the ants, crabs and occasional house gecko that suck the sticks dry.

Another day or two and we will be able to release the tortoises that we penned to prevent them eating the poison. A positive discovery that we have made whilst searching the island intensively has been 17 baby tortoises that have hatched on the island. These were collected and sent to the Gerald Durrell Endemic Wildlife Sanctuary on the mainland where they will be fed and watered for the first few precarious years of life.