Antipodean albatross multi-threat risk assessment

Antipodean albatross (Diomedea antipodensis antipodensis) is one of two subspecies of wandering albatross Diomedea antipodensis, with Gibson’s albatross D. antipodensis gibsoni forming the other subspecies. The species is endemic to New Zealand, with Antipodean albatross nesting primarily on Antipodes Island (and a few breeding pairs on Chatham and Campbell islands), whereas Gibson’s albatross breeds at the Auckland Islands.

The Antipodean albatross population has undergone a considerable decline since 2005, but the underlying causes of this decline remain unknown. The at-sea distribution of Antipodean albatross covers an extensive area from Australia to South America and there are a number of potential threats that may impact the population. The goal of this study was to investigate the causes of the Antipodean albatross decline. The investigation included a literature review to collate information of the different threats to Antipodean albatross and similar species, followed by an analysis of data and modelling to assess the impacts of different threats.

The analysis included the development of a Bayesian model to estimate the principal demographic parameters of the population and how they varied over time. The estimated parameters confirmed previous results that the population decline was a result of a decrease in survival rate, mostly of adult females, and in the breeding probability and breeding success.

The threat of fisheries was assessed by estimating the annual number of captures in New Zealand trawl, bottom-longline, and surface-longline fisheries. The estimation highlighted a peak in captures prior to 2005, and around 22 estimated captures per year across these fisheries thereafter. The overlap in the spatial distribution between Antipodean albatross and international surface-longline fisheries showed an overall decrease around 2005, due to an overall decrease in fishing effort. Nevertheless, the overlap with fleets flagged to China and minor countries increased after 2007, and was negatively correlated with female annual survival. This relationship was not evident with other fleets. In addition, the differences between male and female survival, and between the periods pre- and post-2006, in overlaps with the fleets flagged to China, Vanuatu, Fiji, and Solomon Islands, were also the most consistent with differences in survival rates.

A simulation tool based on the population model was developed and made publicly available. The development included an expert workshop that allowed participants to test different management scenarios in view of the potential impacts of different threats. In this workshop, the fisheries operating in the Tasman Sea were found to be the most likely cause of the observed population decline. Varying the impact of different international fisheries individually, the fleet flagged to China was the only fleet that could potentially explain both the change after 2006 and the female-biased impact. When the impact factor of this fleet was set to lead to around 1450 hypothetical deaths annually across the entire distributional range of the species, the simulation showed a stabilisation of the population, and an even survival rate between both sexes, at around 97%. Changes in oceanographic and climate parameters since 1995, and the overlap with plastic pollution in the Pacific Ocean, did not explain the patterns in demographic parameters.

These results suggest that further research on fisheries in the Tasman Sea would support an increased understanding of population declines of Antipodean albatross.