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POP2020-04: Grey petrel population estimate methodology, Antipodes Island (PDF, 1,876K)

Summary

Antipodes Island is thought to have the largest population globally of grey petrels Procellaria cinerea, but the trend in population size over time remains unknown. This work focuses on planning an updated estimate of population size and trend. We collate and assess resources from previous work, using these to develop recommendations for field work that will yield a robust population estimate.

We first collated resources, then compared and contrasted methods and findings from previous work. Requirements for a robust, repeatable population size estimate and best-practise approaches are discussed. Taken together, previous work and requirements inform a range of options for population size estimation, with key pros and cons noted for each field strategy.

Balancing effort, flexibility and precision of the population size estimate, the recommended field strategy is spatial coverage distance sampling. This approach uses distance sampling following a simple-random design that maximises spatial coverage. Several other good options suggest variations but with key things in common: timing (occupancy sampling should occur in second half April), accounting for habitat lost to landslips, and using true surface areas of grey petrel habitat in calculations. With broad sampling across the grey petrel range, an accurate, robust, repeatable population size estimate can be produced.