5 November 2023
Southern right whale aerial surveys undertaken in 2022 between Cape Leeuwin, WA, and Ceduna, SA, recorded the lowest number of unaccompanied animals since 1993. The reasons for the low numbers are unclear.
Leader of the hub project, Dr Joshua Smith of Murdoch University, says preliminary results of the 2023 annual survey also show very low sightings of southern right whales, with the numbers substantially down from the previous several years.
“While 2022 had the lowest number of unaccompanied animals (whales without calves) since the aerial surveys began in 1993, this year there were extremely low numbers of females with calves,” he says.
“In 2023, while the number of females with calves was similar to 2015, it was the lowest recorded since 2007, which also resulted in the lowest overall count of animals since 2007. There are indications in the long term trend data of a potential slowing in the rate of annual increase in the numbers of southern right whales in the western population.”
Extreme whale watching
Southern right whale surveys were initiated by John Bannister of the Western Australian Museum in 1976, when the whales were beginning to be seen more frequently in parts of the southern Australian coast. The aerial surveys from Perth to Ceduna have been conducted since 1993 to monitor the long term population trend for the ‘western’ population. They contribute to understanding how the ‘eastern’ and western populations are connected, as part of a national population assessment to monitor the recovery of this Endangered species. The National Environmental Science Program has funded the surveys since 2015 (previously through the Marine Biodiversity Hub).
The 2022 aerial survey was undertaken over eight days and 36 hours of flying, with the pilot and photographer onboard the Cessna 172 at an altitude of 300 metres. Dr Smith says the greatest challenge is always to get the best possible weather window of light winds and no rain during the six to eight day survey period.
“Every year it requires waiting and watching the weather to determine when to start the surveys,” Dr Smith says. “This can be stressful when everyone and the plane is on standby to go. Flying in the small Cessna aircraft to undertake the survey is a lot of fun though, but when the aircraft needs to bank to circle the whales for photo-identification of the callosity patterns on their head, it is not for the faint hearted.”
By the end of the 2022 survey the team had collected almost 4500 images and recorded 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, and a ~5.3% annual increase based on female/calf pairs and the long-term population trend data.
The survey whale count is a relative abundance of the population, which given the consistency with which the surveys are undertaken provides a very powerful long term dataset. While there are often highly fluctuating annual variation in abundance and cohort structure, the long term dataset provides indications there may be a potential slowing down in the rate of increase of cow/calf pairs since 2009 (7.5% in 2009 compared with 5.3% in 2022). 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.
Decline in unaccompanied animals
The 2022 survey also recorded the lowest number of unaccompanied animals (males and females without a calf) since the survey began, with a particularly notable decline in the past five years. It is unclear what factors account for the decline, or may influence the variation in numbers of unaccompanied animals on the southern Australian coast. The lower than expected counts may be evidence of a slowing population growth rate.
To evaluate the recovery of the southern right whale population, it 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.