Protecting threatened Over-seas biodiversity

Positive results on mangrove rat control!

As part of the LIFE BIODIV’OM project, GEPOMAY has implemented actions aimed at controlling populations of black rats in the mangroves in which the Madagascar pond-heron nests. After several years of action, the first results are coming in and are very encouraging!

Installation of A24 traps in mangroves – GEPOMAY

One aim : controlling Black rat population

The black rat is native to tropical Asia. The species has colonized the whole world by accompanying navigators through the boats. The species is thus considered an invasive alien species (IAS) because it has a high rate of colonization through significant reproduction. It has an opportunistic omnivorous diet and competes with naturally occurring species. Finally, it becomes an important predator for local species. The species is recognized as a major threat to many species of birds, weakened by predation of eggs, juveniles and adults.

In Mayotte, the black rat is present throughout the territory. GEPOMAY demonstrated that rats were also present in the mangroves and close to Madagascar pond-heron nests. In addition to a disturbance during nesting, the Black Rat is suspected of predating the eggs of the species, as is the case for many other species of herons in the world. Given the threat status of the Madagascar pond-heron in Mayotte and around the world, it is therefore essential to control Black Rat populations in heronries to protect the species and thus ensure its reproduction.

How to control the Black rat?

In Mayotte, GEPOMAY uses A24 Goodnature© mechanical traps, autonomous traps that can trigger 24 times. A bait at the bottom of the trap attracts the rats which will be killed instantly by a compressed air piston. The traps are deployed at least during the reproduction period of the Madagascar pond-heron, period of presence of eggs and young of the year that must be protected.

How to assess the impact on rat populations?

Capture of a rat in a cage for the CMR protocol – GEPOMAY

To monitor rat populations, GEPOMAY uses two methods implemented in three different mangroves:

  • Chewing cards (or Goodnature© chewcards): small cards which, once chewed by rats, leave a characteristic imprint. The less the footprint and the less the rats on the are

Figure 1: Appearances of Goodnature chewcards, from left to right: instantly attached with visible bait; attacked but not consumed; eaten by rats.

  • The Capture-Marking-Recapture (CMR) protocol: the objective is to capture rats in cages to band them and then release them. Thanks to the number of individuals newly captured or recaptured over the sessions, it is possible to estimate the density of rats present in the area.

Encouraging results!

The two graphs below demonstrate that the control via the A24 traps reduced the density of rats on the target areas within the three mangroves, thus resulting in a significantly lower footprint proportion on the chewing cards. This result is also observed on the CMR protocol to a lesser extent.

Chewing cards

Figure 2: This graph shows the proportions of chewing cards consumed by rats in each mangrove during a period of effective control (red) and a period without control (green). The differences are statistically significant.

The CMR protocol

Figure 3: This graph shows the evolution of rat density estimates – calculated using the CMR – in each mangrove before and after the effective control period. The differences are not statistically significant.

The studies carried out by GEPOMAY provide a better understanding of the effect of A24 Goodnature© lethal traps on rats in the mangroves. Rigorous monitoring of black rat populations on Madagascar pond-heron breeding sites is necessary and optimization of the control by the association for the protection of biodiversity is underway.

The Group for the Study and Protection of Birds of Mayotte would like to thank the volunteers who worked in the field to support the team fighting against the Black Rat in the mangroves. If you also wish to participate, do not hesitate to contact them at