Calls reveal population structure of the blue whale across the southwest Pacific Ocean and the southeast Indian Ocean
Our inability to directly observe animals in marine environments has limited our understanding of certain species. Passive acoustic monitoring is a cost-effective method to monitor vocal marine mammals and has proven a powerful tool in revealing species occurrence and distribution. The blue whale, although the largest animal alive can be difficult to study because of their pelagic habitat, wide dispersal and low population densities. Within the Southern Hemisphere the blue whale complex is diverse, to date there are seven blue whale ‘acoustic populations’ producing regionally-specific calls: the Antarctic, Sri Lankan, Madagascan, Australian, Solomon, New Zealand and Chilean blue whale call types. While the blue whale complex is well studied in the southeast Indian Ocean, the population structure of blue whales in the southwest Pacific Ocean remains unclear. Understanding species population structure is necessary for wildlife management and has great conservation implications. We examined the distribution of Australian and New Zealand blue whale call types at five sites spanning over 7,370 km across the southwest Pacific Ocean and southeast Indian Ocean. Acoustic data were collected from 2009–2012. Using automated detector methods, to interrogate over 50,400 hours of underwater recordings, we identified the presence of Australia and New Zealand blue whale calls types and clarify population structure of blue whales, at the junction of the Pacific and Indian Ocean.
We found that the Australian continent acts as a geographic boundary, separating New Zealand and Australia blue whale acoustic populations at the junction of the Pacific and Indian Ocean basins. We located blue whales in previously undocumented locations, including the southwest Pacific Ocean, in the Tasman Sea and the Lau Basin. Based on the number and seasonality of calls detected at different latitudes and ocean basins, we infer differences in area use. We suggest long calling periods may indicate aggregation sites, whereas short calling periods may indicate a migratory corridor. In the absence of genetic resolution these acoustic populations offer a unique window into the blue whale population complex. Our understanding of population complex across this broad scale has significant implications to recovery and conservation management for this species, at a regional and global scale. Clarifying the occurrence, distribution and behaviour of these acoustic populations can allow for specific objective driven management actions.