Southern right whales, a species rebounding from near extinction after centuries of commercial whaling, are now exhibiting a concerning decline in breeding rates linked to climate change, scientists have warned. The trend, observed in populations off the coast of Australia, signals a broader vulnerability of marine ecosystems to warming oceans and shifting prey availability.
For decades, the southern right whale population in the Great Australian Bight has been closely monitored through photo-identification, a technique that allows researchers to track individual whales by their unique callosities – distinctive patterns of rough skin on their heads. This long-term data set, spanning over 35 years, has revealed a significant shift in calving intervals since . Where whales previously gave birth every three years, they are now exhibiting cycles of four to five years between calves, according to Dr. Claire Charlton, a marine biologist and director of Current Environmental.
“They feed in Antarctic and sub-Antarctic waters during our summer time and then migrate up to our coasts during winter,” Dr. Charlton explained. “The whales come every year to breed, mate, rest and socialise.”
The research, published in Scientific Reports, establishes a correlation between these declining reproductive rates and climate-driven changes in the whales’ foraging grounds. Specifically, the study points to sea ice extent, the frequency of marine heatwaves and alterations in prey availability as key factors impacting the whales’ ability to successfully reproduce.
“We know that the ocean is warming, the sea ice is melting, that causes other environmental changes,” Dr. Charlton said. The findings add to a growing body of evidence suggesting that krill-dependent predators, including seabirds and other whale species, are increasingly vulnerable to the effects of climate change in the Southern Ocean.
Southern right whales undertake an annual migration, spending the summer months feeding on zooplankton and krill in the cold waters of the Southern Ocean, often near Antarctica. During the winter, they travel north to warmer coastal waters off Argentina, Australia, Brazil, South Africa, and New Zealand to breed and calve. This reliance on specific feeding grounds makes them particularly sensitive to changes in ocean conditions.
The decline in breeding rates is not isolated to Australia. Similar trends have been observed in southern right whale populations off the coasts of South America and South Africa, indicating a widespread phenomenon. This suggests that the underlying drivers – changes in the Antarctic ecosystem – are impacting the species across its entire range.
The species was hunted to near extinction in the 19th and 20th centuries by commercial whaling operations. While populations have recovered significantly since the implementation of international whaling bans in the late 1980s – with current estimates ranging between 2,346 and 3,940 individuals in Australia, representing 16 to 26 percent of pre-whaling levels – the recent slowdown in reproductive rates raises concerns about the long-term sustainability of the recovery.
Dr. Peter Corkeron, a marine ecologist and adjunct senior research fellow at Griffith University, described southern right whales as “tractors of the ocean,” highlighting their feeding behavior of systematically scouring dense patches of zooplankton. He emphasized that the observed changes in calving intervals are a direct consequence of deteriorating conditions in their Antarctic and sub-Antarctic feeding grounds.
“As mammals, the choice to have a baby is demanding,” Dr. Corkeron explained. “If a female wants to maximise her lifetime reproductive output, she has to balance having babies and living a long time. When conditions are getting worse, you pull back on having as many babies.”
The findings are being viewed as a “warning signal” about the broader impacts of climate change on marine life. Scientists stress the urgent need for coordinated conservation efforts to mitigate the threats facing southern right whales and other vulnerable species in the Southern Ocean. Long-term monitoring programs, like the one conducted in the Great Australian Bight, are crucial for understanding how these animals are responding to a rapidly changing environment.
Whale scientist Vanessa Pirotta, who was not involved in the study, underscored the importance of continued data collection. “We need to continue to learn more about [southern right whales] given that we were responsible for so much of their loss and where their populations are right now,” she said. The future of these magnificent creatures, once brought to the brink of extinction, now hinges on addressing the escalating challenges posed by a warming planet.
