Climate change may threaten the survival of marine mammals such as dolphins and have more far-reaching consequences for their conservation than previously thought, according to a study. In early 2011, a heatwave caused the water temperatures to rise to more than four degrees above the annual average, said researchers from the University of Zurich (UZH) in Switzerland.
The extended period caused a substantial loss of seagrass, which drives the Shark Bay ecosystem, a UNESCO world heritage site in Western Australia.
Researchers from UZH have now investigated how this environmental damage has affected survival and reproduction of dolphins.
They used long-term data on hundreds of animals collected over a ten-year period from 2007 to 2017.
Their analyses revealed that the dolphins’ survival rate had fallen by 12 per cent following the heatwave of 2011.
Moreover, female dolphins were giving birth to fewer calves—a phenomenon that lasted at least until 2017, researchers said.
“The extent of the negative influence of the heatwave surprised us,” said Sonja Wild, a former PhD candidate at the University of Leeds in the UK.
“It is particularly unusual that the reproductive success of females appears to have not returned to normal levels, even after six years,” Wild said in a statement.
There are several possible explanations for this phenomenon, for instance neglect of calves, increased newborn mortality, delayed sexual maturity or a combination thereof, but researchers have not yet been able to investigate them in detail.
Interestingly, the heatwave did not have the same effect on all dolphin groups, researchers said.
Dolphins that use sponges as tools—a socially learned foraging technique that helps dolphins to locate food in deep water—were not as badly affected as those that do not use this technique.
“Nevertheless, our work raises concerns that such sudden events might have quite negative long-term effects even in groups of marine mammals that are known to adapt usually well to novel environmental conditions,” said Wild.
The study shows for the first time that marine heatwaves not only affect organisms at lower levels of the food chain, but also might have considerable long-term consequences for the animals at the top, such as dolphins.
“Marine heatwaves are likely to occur more frequently in the future due to climate change,” said study leader Michael Krutzen, professor at the Department of Anthropology at UZH.
“This is worrying not only for the long-term prospects of marine mammal populations but also for the entire oceanic ecosystems," Krutzen said.