2.2.4 Case study - Rako / Buller’s Shearwater, Poor Knights Islands, Aotearoa New Zealand

Megan Friesen, St Martins University / University of Auckland

Rako, or Buller’s Shearwaters (Ardenna bulleri) (Vulnerable – IUCN Red List), breed only on the Poor Knights Islands in the northern Hauraki Gulf, 22 km offshore from the North Island of New Zealand. These islands are strictly protected as nature reserves and are predator-free. The island group is comprised of two main islands, Tawhiti Rahi (163 ha) and Aorangi (110 ha), as well as several small islets. This survey covered the two main islands, with visits to each in December and April during the incubation and chick provisioning periods respectively.59

The aim of the survey was to estimate the breeding population size of Buller’s Shearwater using a repeatable method that accounted for habitat preference and incorporated baseline information on breeding activity. The results would provide a baseline estimate of pairs in their core breeding habitat, allowing future population trends to be determined and inferences made on marine ecosystem health.

The survey’s results revealed that the breeding population of the Buller’s Shearwater was substantially lower than some earlier estimates (which did not incorporate habitat preferences). The number of active burrows of Buller’s Shearwater estimated in this study – considered to be approximate to the breeding population – is within the range of estimates made in previous, recent rapid land-based surveys (for Aorangi only) but much lower than the coarse estimates made in the 1980s (which presumably included non-breeding individuals). It is important to note that the population estimate did not include difficult-to-access sites on steep, fragile slopes, which were not included in the survey. However, unlike previous studies, the survey’s estimate was based on both habitat modelling and randomised quantitative sampling and represents a repeatable assessment of the breeding population for Buller’s Shearwaters.

Figure 82. Predicted number of Buller’s Shearwater burrows per 25 m2 cell based on habitat modelling for both Tawhiti Rahi (a) and Aorangi (b) with topography (grey).59