Aerial survey of the southern right whale ‘western’ sub-population off southern Australia

The Southern Right Whale Conservation Management Plan 2011–2021 outlines the current status of, and threats to, the southern right whale in Australian waters and prioritises recovery actions during this period. The long-term vision for the recovery of the southern right whales in Australian waters is that the population has increased in size to a level that the conservation status has improved, and the species no longer qualifies for listing as threatened under any of the EPBC Act listing criteria. The plan must be periodically updated to reflect new knowledge and prioritise the research needed to monitor population recovery and predict the impacts of threats such as climate change.

Aerial surveys of southern right whales have been conducted across the southern Australian coast from Perth (WA) to Ceduna (SA) since 1993, as part of a long-term program to monitor their recovery. The surveys data provide a long-term population trend for this ‘western’ population, and provide an understanding of connectivity with the ‘eastern’ population as part of a national population assessment. The NESP Marine Biodiversity Hub (the predecessor to the NESP Marine and Coastal Hub) funded the aerial surveys in 2015–2020.

In Australia’s south-east (Victoria, Tasmania and New South Wales), there has been little sign of recovery in southern right whale numbers following intense commercial whaling. A working hypothesis assumes separation between the ‘western’ and ‘eastern’ populations, largely due to loss of ‘cultural memory’ of whales migrating to the eastern range breeding areas. Given the relative paucity of animals that visit the southern Australian coast in areas other than south-west Australia, the western population is currently considered to represent the majority of the ‘Australian’ southern right whale population. The count data from these aerial surveys provide data on population trend and estimates of population size for the ‘western’ population, and hence majority of the Australian southern right whales. The associated photo-identification data provides life history information (such as calving intervals) and connectivity between the ‘western’ and ‘eastern’ populations and contributes to the national southern right whale photo-id database: the Australasian Right Whale Photo-Identification Catalogue (ARWPIC).

Continuing monitoring of the population is needed to establish if periodic low whale counts in recent years represent anomalous years, or are an indicator of some longer term and continuous change to recent recovery rates. It also ensures that the long-term program for the region maintains an uninterrupted time series, which is particularly important due to the species non-annual breeding cycle, in which females have a calving rate of one calf every three to four years. Annual surveys are critical to maintain an acceptable level of precision in estimating population trends and key life history parameters to track the recovery of the species.

Approach and findings

This project continued the ongoing annual aerial survey of southern right whales to inform the long-term population trend and was undertaken in August 2022. The survey recorded a total 526 southern right whales, consisting of 247 cow-calf pairs, 31 unaccompanied adults and one yearling. This resulted in a population estimate of 2675 whales for the ‘western’ population, increasing in size at ~5.3% per year based on female/calf pairs and the long-term population trend data.

While there is an increasing trend in population abundance, there is also evidence of a potential slowing down in the rate of increase of cow/calf pairs over the past 13 years (7.5% in 2009 compared with 5.3% in 2022) and there continues to be highly fluctuating annual variation in abundance and associated fluctuations in cohort structure. The inter-annual variation in sightings of cow/calf pairs is related to their breeding cycle (typically a three-year cycle), which can also be affected by environmental factors on their foraging grounds.

The 2022 surveys recorded the lowest number of unaccompanied animals (males and females without a calf) ever throughout the time-series of the annual aerial surveys since 1993 when survey coverage between Cape Leeuwin and Ceduna began. Across this long-term time series, there is a particularly notable decline in sightings of unaccompanied animals in the past five years. It is unclear what factors account for the decline in these sightings or may influence the variation in numbers of unaccompanied animals on the southern Australian coast. Lower than expected counts in the long-term data may provide evidence of a slowing population growth rate, which can only be assessed by continued annual population surveys to assess population trend data.

To evaluate the recovery of the southern right whale population, it will remains critical to collect long-term data on the population and monitor annual variability in whale numbers related to the non-annual female breeding cycle. National and international collaborations are essential to better understand whether variability in abundance and key life history parameters may be due to short-term climate dynamics, longer-term climate change, and/or anthropogenic threats. Annual surveys will reliably detect changes in whale abundance from one year to the next, and inform longer time periods as part of the long-term dataset. However, considerable inter-annual variation in whale numbers makes it difficult to predict patterns in population trends. Continued annual surveys of the population are critical to be better able to identify immediate threats to this population of endangered whales.

Outcomes

The findings of this project contribute to informing the Australian Government’s Southern Right Whale Conservation Management Plan 2011–2021, and in particular provide further information on population trends and the extent to which 2021 might be considered an anomalous year. These outputs will be important for identifying priority areas of research for the species, in light of a review of the management plan and development of future management plans.

Data from this project also contribute to the International Whaling Commission Southern Ocean Research Partnership (IWC-SORP) funded project “Multi-ocean assessment of southern right whale demographic parameters and environmental correlates” through the Australian Southern Right Whale Consortium. This is a collaboration between countries from the South West Atlantic (Brazil/Argentina), South East Atlantic (South Africa), Australia and New Zealand, with the aim to compare population demographics across the main Southern Hemisphere wintering grounds.

The data also make an essential contribution to a larger dataset aimed at determining absolute abundance, spatial connectivity, changes in life history parameters across the population and environmental influences on these, however it is beyond the scope of this project to carry out such analyses.

Project location

Across the southern Australian coast from Perth, WA, to Ceduna, SA