The Greater Wellington Regional Council (GWRC) has the task of controlling rat numbers and has set up a grid of rat bait stations 150m by 100m inside the Mainland Island. Contractors are used to apply bait, particularly during the breeding season for birds. MIRO members are involved in monitoring the numbers of rodents and mustelids (stoats and weasels) in the area.
To ascertain levels of rodents and mustelids in the Mainland Island and the effectiveness of the application of rat poison, surveys are carried out quarterly by volunteers. 100 tracking tunnels are used according to the Department of Conservation protocols with half of the tunnels in the Mainland Island, and half outside but it in the control area. Results give the percentage of tunnels that show the presence of rodents.
The efficacy of the application of the rat bait can be clearly seen with much lower levels of rats inside the Mainland Island. Periods of high rodent tracking are associated with the occurrence of beech mast years in the Park. Ideally, tracking levels for rats will be maintained below 5% during the breeding season for birds.