In early 2011 at Shark Bay, a heatwave caused the water temperature to rise more than four degrees above the annual average. This period caused a substantial loss of seagrass – which drives the Shark Bay ecosystem – in this Australian coastal area, which is also a UNESCO world heritage site.
Researchers from UZH – collaborating with researchers from the University of Bristol – have now investigated the environmental impact of the 2011 heatwave, and how it has affected the survival and reproduction of dolphins, using long-term data on hundreds of animals – over 5,000 dolphin encounters – collected over a ten-year period from 2007 to 2017.
Analysis of the data revealed that the dolphins' survival rate has fallen by 12 per cent following the heatwave of 2011, indicating long-lasting negative impacts on both the survival and birth rates on the iconic Indo-Pacific bottlenose dolphin population in the area. Furthermore, female dolphins were giving birth to fewer calves, which lasted until 2017.
"The extent of the negative influence of the heatwave surprised us," said Sonja Wild, former PhD candidate at the University of Leeds and first author of the study. "It is particularly unusual that the reproductive success of females appears to have not returned to normal levels, even after six years."
There are several possible explanations for this phenomenon. The researchers suggest the loss of food availability and habitat has prevented fish stocks from recovering to pre-heatwave levels forcing the dolphins to spend more time searching for food leading to reduced vigilance, and ultimately, more shark predation on calves.
Reduced food availability may also have led to increased mortality of young calves when both the mother's and the offspring's nutritional needs could not be sustained. However, the researchers have not yet been able to investigate these in detail.
The heatwave did not have the same effect on all dolphin groups within the Shark Bay area, however, with the researchers finding that dolphins that use sponges with tools – a socially learned foraging technique that helps the species to locate food in deep water – were not as badly affected as the dolphins who did not.
Dr Simon Allen, senior research associated at Bristol's School of Biological Sciences said: "Our findings suggest that extreme weather events may be too sudden or disruptive for even highly adaptable animals to respond, leading to negative impacts at the population level. We are yet to see recovery of seagrass meadows or fish stocks and now we find negative impacts on dolphins from just one event.
"Given that marine heatwaves are occurring more often in association with climate change this raises serious concerns over the long-term prospects for the dolphin population, commercial fisheries and the ecosystem as a whole."
Wild also added: "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."
The UZH researchers also show in their study for the first time that marine heatwaves not only affect organisms at lower levels of the food chain, but also may 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 Krützen, 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."
