The re-establishment of a customary harvest of kuia (grey-faced petrels) by Ngāti Awa, Bay of Plenty
Ngāti Awa from the Bay of Plenty have applied a rāhui (temporary ban) on the customary harvest of fledgling kuia (grey-faced petrel; called oi by neighbouring iwi) chicks on Moutohorā (Whale Island) since the late 1950s because of concerns over declining numbers.
Although the exact cause of the decline in kuia isn’t known, rats and rabbits on the island are believed to have had such significant impacts that few chicks fledged between 1972 and 1977. Both pests were eradicated from Moutohorā by 1987, and subsequently the birds’ breeding success increased. However, the rāhui has remained because of uncertainties about what constitutes a ‘safe’ level of harvest. The island is managed by Te Tapatoru a Toi, a committee consisting of representatives from Te Rūnanga o Ngāti Awa, Department of Conservation (DOC) and the general public. Scientists from Maanaki Whenua, in collaboration with Te Rūnanga o Ngāti Awa are studying kuia to determine the population size, adult survival and breeding rates on Moutohorā, and what an annual customary harvest would mean for the population.
Phil Lyver says that the first goal was to estimate the current size of the kuia breeding population. The researchers carried out breeding burrow occupancy surveys during peak incubation and late chick-rearing periods over three breeding seasons and also investigated how burrow entrance densities varied with predictors such as soil, topography and vegetation. These data were analysed to estimate breeding success and the total number of breeding pairs on the island.
Researcher Amy Whitehead estimated burrow densities based on habitat characteristics. Of these burrows, 55% were occupied by breeding kuia and 46% of these pairs successfully hatched chicks. Burrow densities across the island were predicted most strongly by soil type, altitude, topography, vegetation canopy and ground cover. When these finer-scale habitat-linked variations in burrow density were scaled up using a GIS-based habitat model for the whole island, the data indicated that there are likely to be approximately 84,000 breeding pairs on the island (Fig. 1).
The second goal of the study was to determine what would be a safe customary harvest, and to compare the relative effects of a range of harvest rates and strategies. A mathematical population modelling approach was used to assess the impacts of removing from 5% to 60% of pre-fledging chicks each year.
Researcher Chris Jones says that without any harvesting, the kuia population probably grows at just over 2% per year, which is within the range of published estimates for long lived, slow-reproducing petrel species. With harvest intensities of up to 30% of chicks, the population is likely to continue growing, albeit at a reduced rate (Fig. 2).
The study found that, in general, harvesting a fixed proportion of chicks is ‘safer’ for sustaining the population than a fixed quota strategy, which is based on taking a set number of chicks each year. This is because with a proportional harvest strategy, if the population declines for some reason, the number of chicks harvested is reduced accordingly. However, in practical terms, a fixed quota system is easier to manage because it is easier to count the number of chicks harvested than to estimate the total population size every year (as would be required to guide a fixed proportion harvest).
Chris and Phil therefore proposed two options for managing the harvest of kuia chicks on Moutohorā:
- Set a very conservative fixed quota to limit the harvest to what would be sustainable under most circumstances outside of some unpredictable catastrophic impact on the breeding population.
- Develop an index of population size (such as a ’harvest rate’ or burrow entrance counts) and use this to detect changes in the breeding population over time. This would then allow a proportional harvest strategy to be used.
This research was funded by Ministry of Science and Innovation through the C09X0509 (Mauriora ki nga Oi) and C09X0908 (Te Hiringa Tangata) grants.