Assessing inter-annual variability in Antipodean albatross distribution

This study assessed the temporal variability in seabird distributions estimated from tracking data and its impact on a key derived output, the spatial overlap with fisheries. It used Antipodean albatross (subspecies Diomedea antipodensis antipodensis) as a case study, owing to the availability of long-term tracking data for this subspecies, and a recent increase in tracking effort across life stages. The analysis confirmed key spatial differences in the distribution of Antipodean albatross by breeding status, age, and sex. Considering these differences led to an improved distribution map of all life stages combined, which integrated all available tracking data with weights by life stage from a recently updated population model. The distribution map was compared with surface-longline fishing effort in the Southern Hemisphere to assess variability in particular areas of overlap with fisheries (“overlap hotspots”); this temporal assessment included the application of an overlap statistic. Although there was variability in the location of overlap hotspots through time, there were distinct areas that were consistently classified as overlap hotspots in the time period from 1997 to 2019. These areas included the Tasman Sea, an area eastward and northward of New Zealand’s North Island, and off the coast of Chile.

The development of the distribution maps included the testing of a resampling-based approach to assess the differences in distribution overlap across time periods via randomisation. This testing found that other track features, such as the track length (measured in the number of observations) also needed to be accounted for when comparing distributions across time periods. This aspect is particularly relevant to seabird species for which multiple tag types have been used throughout the time series of tracking data. In the current analysis, there was some support for a change in distribution for non-breeding females over time. Nevertheless, the differences was not significant, possibly owing to the small number of tracks in early years.

The approach used here, of applying the overlap statistic to assess overlap hotspots and their consistency through time, is broadly applicable to other seabird species with tracking data. The findings from this research were presented at the 14th meeting of the Ecologically Related Species Working Group (ERSWG14) of the Commission for the Conservation of Southern Bluefin Tuna in March 2022. Since then, discrepancies in the tracking data have been corrected, leading to updated distribution maps of Antipodean albatross.