White-chinned petrel distribution, abundance and connectivity have circumpolar conservation implications

Albatrosses and petrels are a group of oceanic seabirds that spend most of their lives at sea. The Southern Ocean, which rings Antarctica in a continuous belt of wind and currents, supports most of the world’s albatrosses and petrels. The conservation status of many oceanic seabirds has deteriorated dramatically over the last two decades, due to mortality from incidental bycatch in fisheries and depredation by introduced mammals at breeding sites. Globally, seabird bycatch is highest in Southern Ocean waters and introduced mammals occur on a third of sub-polar and high-latitude seabird islands. The seabird species most frequently killed in Southern Hemisphere fisheries bycatch is the white-chinned petrel Procellaria aequinoctialis. Almost three decades after substantial white-chinned petrel mortality in fisheries was first recorded, capture rates remain high despite substantial global efforts to reduce bycatch rates. Population impacts are exacerbated by introduced mammals at some breeding sites, yet some island populations are still virtually unstudied. White-chinned petrels breed on eight subantarctic island groups around the Southern Ocean. Key steps toward targeted conservation are obtaining robust estimates of abundance and at-sea distribution, and defining the scale of genetic conservation units within the species. Population-level questions in these key areas limit the ability to gauge the impact of current threats locally and around the Southern Ocean, and hinder informed conservation, management action and policy development. This thesis broadly asks at what scale(s) processes affect species distribution, abundance and connectivity in the Southern Ocean ecosystem, with the white-chinned petrel as the focal species. It evaluates status and connectedness of white-chinned petrels breeding on subantarctic islands around the Southern Ocean via three broad approaches: - Population size estimates for the Auckland Island and Campbell Island breeding populations, the last two island groups lacking estimates of white-chinned petrel numbers (Chapter 2); - Tracking devices follow the at-sea movements and distribution of 150 white-chinned petrels year-round, from all major breeding islands (Chapter 3); and - Molecular genetics tests connectedness within the white-chinned petrel metapopulation, using sequences from mitochondrial and nuclear genes as well as genomic data from every island population (Chapter 4). This thesis provides the first robust population size estimates for white-chinned petrels at the Auckland Island and Campbell Island groups, including 11 islands (Chapter 2). Burrow numbers were sampled widely to capture spatial variability (33–241 randomised sampling sites per island). Estimated burrow numbers were corrected with detection rates and occupancy rates to estimate numbers of breeding birds. The Auckland Island group has an estimated 186,000 (95% CI: 136,000–237,000) white-chinned petrel breeding pairs, and the breeding population of the Campbell group is estimated ~ 22,000 (15,000–29,000) pairs. The New Zealand region supports almost a third of white-chinned petrels globally, substantially more than suspected. Importantly, the estimates establish repeatable population baselines. Tracking data from all major island populations except Campbell Island were analysed together, giving the first metapopulation-scale picture of the at-sea distribution of adult white-chinned petrels (Chapter 3). The movements of 150 adult petrels (9–33 petrels per island group) were tracked for an average of 369 days with light-level geolocation GLS loggers. Quantitative density estimates for white-chinned petrels show key global density hotspots (off South America, New Zealand, and southern Africa). Island population-specific distributions highlight areas used only by adults from a given island population. Island-specific distributions also show spatial segregation between island populations varying across the year to an extent unusual for seabirds, so the implications for resource partitioning are explored (Chapter 3). Using comprehensive sampling from every island population, high-resolution genomic data (60,709 genotyping-by-sequencing loci) was compared with data from widely-used mitochondrial genes (entire cytochrome b gene and the highly variable 1st domain of control region) (Chapter 4). Genomic data revealed genetic structure in white-chinned petrels at very fine scale (among islands) and at broad oceanic scales (between Atlantic and Indian Ocean regions) that was not detected in analyses of single genes. Three ocean-basin scale evolutionarily significant units, ESUs, were identified. There is promise that some island populations are sufficiently unique to link mortality in a specific fishery to a given island (Chapter 4). The results of the thesis are synthesised (Chapter 5) to explore the implications for conservation and the broader biogeographic context

Low burrow occupancy and breeding success of burrowing petrels at Gough Island: a consequence of mouse predation

The predatory behaviour of introduced house mice Mus musculus at Gough Island is known to impact on albatross and petrels, resulting in the Tristan Albatross Diomedea dabbenena and Atlantic Petrel Pterodroma incerta being listed as “Critically Endangered” and “Endangered”, respectively. Although predation has been documented for two burrowing petrels and one albatross species, the impact of house mice on other burrowing petrels on Gough Island is unknown. We report burrow occupancy and breeding success of Atlantic Petrels, Soft-plumaged Petrels Pterodroma mollis, Broadbilled Prions Pachyptila vittata, Grey Petrels Procellaria cinerea and Great Shearwaters Puffinus gravis. With the exception of the Great Shearwater, breeding parameters of burrowing petrels at Gough Island were very poor, with low burrow occupancy (range 4–42%) and low breeding success (0–44%) for four species, and high rates of chick mortality in Atlantic Petrel burrows. Breeding success decreased with mass, suggesting that smaller species are hardest hit, and winter-breeding species had lower breeding success than summer breeders. The results indicate that introduced house mice are having a detrimental impact on a wider range of species than previously recorded and are likely to be causing population declines among most burrowing petrels on Gough Island. The very low values of burrow occupancy recorded for Soft-plumaged Petrels and Broad-billed Prions and greatly reduced abundance of burrowing petrels in comparison to earlier decades indicate that Gough Island’s formerly abundant petrel populations are greatly threatened by the impact of predatory house mice which can only be halted by the eradication of this species from the island

Genomics detects population structure within and between ocean basins in a circumpolar seabird: the white‐chinned petrel

The Southern Ocean represents a continuous stretch of circumpolar marine habitat, but the potential physical and ecological drivers of evolutionary genetic differentiation across this vast ecosystem remain unclear. We tested for genetic structure across the full circumpolar range of the white‐chinned petrel (Procellaria aequinoctialis) to unravel the potential drivers of population differentiation and test alternative population differentiation hypotheses. Following range‐wide comprehensive sampling, we applied genomic (genotyping‐by‐sequencing or GBS; 60,709 loci) and standard mitochondrial‐marker approaches (cytochrome b and 1st domain of control region) to quantify genetic diversity within and among island populations, test for isolation by distance, and quantify the number of genetic clusters using neutral and outlier (non‐neutral) loci. Our results supported the multi‐region hypothesis, with a range of analyses showing clear three‐region genetic population structure, split by ocean basin, within two evolutionary units. The most significant differentiation between these regions confirmed previous work distinguishing New Zealand and nominate subspecies. Although there was little evidence of structure within the island groups of the Indian or Atlantic oceans, a small set of highly‐discriminatory outlier loci could assign petrels to ocean basin and potentially to island group, though the latter needs further verification. Genomic data hold the key to revealing substantial regional genetic structure within wide‐ranging circumpolar species previously assumed to be panmictic