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Hirakimata Rat Monitoring - phase 1

by John Ogden

The first phase of the Great Barrier Island Environmental Trust pilot rat monitoring program on Hirakimata is now completed. Another phase is currently underway, when rat numbers are expected to be higher.

Rats on Hirakimata are a serious threat to the small populations of tomtit, robins and kakariki on the mountain, and also to nesting Cook’s and black petrels. The results (Figure 1) establish clearly that ship rats are just as abundant on Hirakimata summit – in the black petrel nesting area – as they are at Windy Canyon and elsewhere in the bush on Great Barrier Island. There is no significant difference.

Figure 1: Overall results – numbers of rats killed over seven weeks using 10 traps at each location. The columns are actual numbers and the error bars are approximate standard errors.

Nearly all the rats were adults, with a slight preponderance of males. Interestingly, the rats at Windy Canyon mostly belong to the dark blackish morph of ship rat, while those on the summit are predominantly the white- bellied form, but in all other respects the populations show no differences. No kiore were caught at either location.

Unfortunately, the set-off counters purchased from Goodnature with the traps proved unreliable, necessitating more days climbing to the summit than anticipated. The addition of peanut butter seems to make the traps more attractive to rats than the long-lasting chocolate lures supplied.

As killing devices the A24 traps are effective (Figure 2) and we are working with Goodnature on the counter problem for Phase 2 of the program.

Figure 2: The A24 is a self-resetting multi-species kill trap targeting rats and stoats. Attracted in by a long-life lure, once triggered, the killed pests fall from the trap which resets up to 24 times for each gas (CO2 ) canister. Source: Goodnature

 If you’d like to be involved please call Alison Walker, John Ogden or Kay Stowell or email(contact.gbiet@gmail.com)