Relative abundance of the ‘western’ population of southern right whales from an aerial survey 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 survey data provide a long-term population trend for this ‘western’ population, and 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 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 the majority of the Australian southern right whales. The associated photo-identification data provide life history information (such as calving intervals) and connectivity between the ‘western’ and ‘eastern’ populations and contribute 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 whether 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 with two aerial observer teams completing the survey during August 2021. The survey added to time-series data used to determine the long-term population trend. The long-term trend in abundance for the ‘western’ population based on cow/calf pairs is an annual increase of 5.4%, which is below the maximum biological rate of increase of 6–7%, although still suggests a healthy population increase. The subsequent population estimate for the Australian ‘south-western’ population is 2549 whales. This represents the majority of the Australian population, given the relatively low numbers in the ‘eastern’ population of 268 breeding females.

Unusually low numbers of whales have been recorded during surveys in 2007, 2015 and most recently in 2020, with these years identified as anomalous years. These low numbers may reflect some disruption to the approximate three-year female breeding cycle. Unpredictable fluctuations in relative overall numbers and associated fluctuations in cohort structure have been reported in other southern right whale populations in the Southern Hemisphere, such as South Africa. These have been suggested to reflect fluctuations in food availability on feeding grounds in the Southern Ocean and near the Antarctic continent, and can affect the reproductive success of females. Calving rates of southern sight whales have consequently been suggested as a potential indicator of climate change impacts in the Southern Ocean.

Inter-annual variation in whale counts has increased since 2007, according to the population long-term trend data. Anomalous years of pronounced low whale numbers are potentially becoming more frequent, with more dynamic cyclical patterns in whale numbers. Given the non-annual breeding cycle of southern right whales, however, it is difficult to determine the reason for the anomalous years. They may reflect some form of a cycle that females are conforming to as the population recovers, or environmental changes that are affecting their breeding cycle.

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. This supports continued annual surveys of the population to be better able to identify immediate threats to this population of endangered whales.

Project location

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