The RaCE for Aldabra’s future (Part VI) |29 September 2025

SIF staff checking bait bola for any sign of rat activity © Annie Simmons

The Aldabra rat and cat eradication (RaCE) feasibility project aims to assess whether a large-scale eradication of introduced rats and cats on Aldabra is feasible. With major advances in eradication science, strengthened biosecurity on Aldabra, and clear evidence that removing invasive alien species is the single most impactful action to improve island native biodiversity, there’s never been a better time to assess feasibility for an eradication on the atoll.

Aldabra Rat and Cat Eradication trials make promising progress

How many feral cats are there on Aldabra, what habitats do they prefer and what are their main food sources?

A network of camera traps was set up across two areas on Grande Terre as part of the RaCE feasibility project. Most cat detections were in beach habitats, where cats feed on turtle hatchlings, followed by mixed scrub and Pemphis habitat, with the lowest detection probabilities in mangroves. The team are collecting data that will provide an estimate of the total population and are also GPS tracking individual cats to better understand their behaviour and home ranges.

When is peak breeding activity for rats and do they stop breeding in the dry season?

When planning a rat eradication, it is vital to understand whether there are times of year when rats do not breed. This allows the eradication to be planned when there is the lowest number of rat nests and therefore the highest chance of success. The team has been catching and dissecting rats year-round to record signs of breeding. So far, the team has found that rats on Aldabra stop breeding in most habitats towards the end of the dry season in August-September. However, rats in mangroves and on Grande Terre continue to breed during this period. This will be accounted for if an eradication goes ahead and, in the meantime, breeding monitoring continues to capture breeding patterns across seasons, with results feeding into the eventual timing of the eradication.

How much bait would be needed to ensure 100% of rats are exposed?

One of the key questions for an eradication is what the bait application rate would be, and this is very specific due to a number of factors. Too little bait and the eradication risks being unsuccessful as there needs to be enough bait for all individual rats across the atoll to consume a lethal dose. On the other hand, too much bait increases the risk of consumption by non-target species and potential environmental impacts. So far the team is seeing high or 100% consumption rates in trapped rats, which indicates the application rate is likely sufficient for full exposure. A second trial is scheduled in the late dry season.

How do rats move around in mangroves?

To understand how the rats are using the canopy the team have been trialling an innovative method called ‘spool and line’ monitoring, which has never been carried out in the Indian Ocean before. A spool of thread is attached to the rat, which then unravels as the rat moves, leaving behind a 3D map of the routes the rat takes. It can take the team hours to track the journey of a single rat, trying to spot the fine thread in a maze of 6m high canopies!

How can large areas of mangroves be baited to ensure bait remains available long enough for all rats to consume it?

One of the biggest challenges for an eradication on Aldabra is the 1720 hectares of mangroves that fringe the lagoon. The traditional eradication method, aerial broadcasting of bait pellets, is not feasible in areas that are tidally inundated twice a day. The team has trialled the ‘bola’ method: bait blocks threaded on string and thrown by hand into canopies. Over 5500 bolas were deployed this way, and early results show that rats can reach and consume bait suspended above ground, with availability lasting beyond the critical three-day window.

What is the density of rats in each habitat type?

Live-trapping and ear tagging of rats over seven-night sessions are being used to estimate rat density and understand territory size and movement patterns. This data is essential for designing an effective eradication strategy tailored to specific habitats. The data is still being collected and we will have estimates of densities in each habitat by the end of the feasibility project.

Which bait brand is most palatable to rats?
The team set up bait choice experiments to watch to see which of the bait brands are more attractive/appealing to rats and also non-target species. The team placed equal amounts of two types of bait at 20 bait stations per grid. Camera traps then record which species consume the bait and each morning the bait is weighed to determine consumption of the different types of bait. This work is ongoing and the results will help us choose the most effective bait type for the Aldabra eradication.

Which other species on Aldabra might be impacted by bait?

Camera trap footage has shown that crabs, rails, tortoises, fish, and shrimp may consume bait. A full risk analysis is underway to assess risks to native species and guide mitigation strategies. Lessons from other eradications will also be used to inform species-specific safeguarding.

How often do rats move between the islands of Aldabra?

For the purpose of future research into the population structure of rats on Aldabra and how much rats gene flow there is between the islands, the team have been collecting rat DNA samples on each of the four main islands of Aldabra. SIF are actively seeking a research collaboration to analyse these DNA samples, specifically to help inform the eradication plan.

Contributed

Quote 1:

“Aldabra is a place I hold very close to my heart; there is truly nowhere else like it. Its biodiversity is extraordinary, but also so fragile that nothing can be allowed to destroy it. No monetary value can ever be placed on such a treasure. Eradicating invasive species is essential to protecting this natural heritage, so that future generations may inherit Aldabra as rich, pure, and beautiful as it was always meant to be.”

Jeanette Larue – Previous SIF board member

Quote 2:

“Aldabra’s future depends on its ecological integrity. Invasive alien species, such as rats and cats, have significant negative impacts on island ecosystems. Species eradication is not easy, and sometimes tough decisions have to be made, but for the love of biodiversity, so unique and irreplaceable as that of Aldabra, we must take decisive action to maintain this delicate ecosystem. I strongly believe in SIF’s efforts and have no doubt the team will get the job done.”

Ashley Dias – Small Grants Programme Committee Chairperson