Since whaling was widely banned in the 1930s, the number of southern right whales has risen, but it is unclear how climate change will impact their recovery in the future.
El Niño is a weather event caused by winds that push warm surface water from around Indonesia and Australia across the Pacific Ocean to South America.
Climate change means that El Niño events are likely to occur more often and with higher intensity, says Macarena Agrelo at the Federal University of Santa Catarina, Brazil.
This leads to the melting of ice shelves in West Antarctica, which can reduce the abundance of krill, a major food source for southern right whales.
Agrelo and her colleagues analysed data from the Southern Right Whale Program, which has tracked 1380 female whales within a larger population living around the Valdes peninsula off the south-east coast of Argentina since the 1970s, to predict how more frequent and severe El Niño events will affect whale survival.
“I was modelling the female whale numbers over time when I noticed that, although they were generally high, there were years that the population dipped,” says Agrelo. “Then looking at the patterns, I realised they were El Niño years.”
The researchers speculate that reduced krill abundance during El Niño years leads to the death of female whales that have recently given birth.
“After giving birth and the lactation period, female whales are thin and need a year to recover. If an El Niño event reduces the krill available, the females may die from lack of food,” says Agrelo.
The team predicted whale population growth under different scenarios of climate change.
Under the same frequency and intensity of El Niño events as the past 50 years, the population has around a 90 per cent chance of reaching 85 per cent of its estimated pre-whaling size, of 35,000 whales, over the next 100 years.
In the worst-case scenario of a 4.4°C rise in average global temperature above pre-industrial levels by the end of the century, the most pessimistic predictions give the population no chance of reaching 85 per cent of the pre-exploitation population.
Under the best conditions of a more-optimistic scenario in which global temperature rise is unlikely to exceed 2°C, the whales could reach up to 90 per cent of the pre-whaling population size.
“The study provides pretty compelling evidence that the recovery of southern right whales could be impacted by climate change,” says Anthony Richardson at the University of Queensland, Australia.
These findings highlight the need for climate action as whales are key players in marine ecosystems, locking carbon in their bodies for decades until their carcasses sink and support biodiversity on the sea floor. “Whale faeces also fertilise ocean waters with nutrients such as iron, which in turn support krill numbers,” says Agrelo.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abh2823
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