A whale species that visits Australia's coastline each year is showing troubling signs, as its once-steady population growth has stalled.
Southern right whale numbers, which have been increasing for decades, have recently slowed and are beginning to show slight signs of decline, according to research released on Thursday.
The findings raise concerns about the species' recovery, though the reasons why it is occurring remain unclear.
Researchers analysed 49 years of aerial survey data from 1976 to 2024, stretching across more than 2000 kilometres of coastline.
Anne Grundlehner, a postgraduate researcher at the University of Tasmania's Institute for Marine and Antarctic Studies, said they were concerned to discover a halt in the population's growth.
"The population has long been assumed to be doing quite well in their recovery, and this is the first time we have found evidence that this has changed," Ms Grundlehner told AAP.
"We found that annual births began to stagnate around 2016 and observed declining trends in the number of animals visiting the coast."
Southern right whales visit shallow coastal waters of Australia, New Zealand, South America and South Africa between mid-May to mid-November.
Sheltered waters in these locations provide a safe haven for females to give birth and nurse their calves.
The species was preferred by whalers from the 1800s, with only few hundred animals remaining by the time whaling was banned.
The study, published in the journal Global Change Biology, found while the number of mothers and calves that visit the coast has only shown a slight decline, the number of other individuals dropped by 66 per cent between 2011 and 2024.
Ms Grundlehner said there were many potential drivers behind the halted growth, but believed additional research was needed.
"In the worst-case scenario, it is really driven by anthropogenic factors (human activity) or climate change," she said.
"But in the best-case scenario, it could just be that this population is finding a new balance with present ecosystem conditions, which are likely different from the marine ecosystems before whaling.
"Perhaps the population is finding a new balance with their abundance in the present ecosystem conditions."
