Without the underwater listening devices set up to monitor nuclear bomb tests, the population of blue pygmy whales “Chagos” discovered only last year in the Indian Ocean would still be unknown to us.
And now researchers from UNSW Sydney have analyzed nearly two decades of recordings to conclude that the distinctive songs of these pygmy blue whales have increased in frequency over this time.
Given that actual sightings of pygmy blue whales are still extremely rare – despite their length of 24m – the researchers cautiously suggest that the results show stocks of the subspecies could rebound from being hunted to almost -extinction in the 20th century.
But while this may be good news for scientists and conservationists, researchers are concerned about the effects of climate change on rising sea temperatures, with a potential ripple effect on their main source. of food, krill.
In a recent study published in the journal Marine Science FrontiersUNSW scientists analyzed up to 18 years of continuous underwater acoustic recordings from devices known as hydrophones stationed on either side of the island of Diego Garcia, a militarized atoll ruled by the United Kingdom and forming part of the Chagos Archipelago.
An interactive Google map frame showing the location of Diego Garcia Island. Zooming out will reveal that it is right in the middle of the Indian Ocean.
Sing the blues
They wanted to find correlations between the presence of Chagos whales and other factors such as sea surface temperature, food source and changing weather conditions, including the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD).
By comparing the frequency of whale song recorded by the two sets of hydrophones placed northwest of Diego Garcia and southeast of the island, they reconstructed a pattern of behavior and migration correlated with different levels of everyone’s abundance of food and sea temperature. area about 200 km away.
Audio recording of Chagos whale song captured by CTBTO hydrophones in the Indian Ocean.
“We used up to 18 years of acoustic recordings of Chagos whales from data we got from the United Nations, CTBTOsaid lead author Lyra Huang, referring to the Comprehensive Nuclear Test Ban Treaty Organization which installed the devices in 2002.
“And we found different seasonal patterns of the whales on the west and east of the island. We found that first of all the amount of whale songs increases over the study period on the two sites.Secondly, they are mainly on the southeast side from September to November, then they migrate to the northwest side and stay there until February.
“Our analysis shows that these movements are linked to environmental factors such as sea surface temperature and food availability.”
View of a krill
Specifically, the researchers found a positive correlation between the amount of chlorophyll-a in the southeast of the island and the increased presence of Chagos whales, as well as lower sea surface temperatures when they seem favor this area.
According Professor Tracey Rogers who is co-author and expert on how mammals overcome the challenges of changing environments, the presence of lots of chlorophyll-a is a potential indicator that there is more food at these times for whales. Indeed, high concentrations of chlorophyll-a show that there is an abundance of phytoplankton. And since krill feed on phytoplankton, this makes it an attractive feeding site for whales.
“We see high concentrations of chlorohpylla-a on the southeast side of the island, consistent with times when there are more recorded whale songs there, but no correlation on the northwest side,” said Professor Rogers.
“This tells us that the whales are most likely feeding on this southeast side and then migrating to the western side of the island.
“Our previous research picked up these same whale songs as far away as Sri Lanka and the west coast of Australia. This means that these whales migrate across the Indian Ocean which is really unusual for blue whales as they generally move north and south to and from Antarctica but these guys go east to west, and vice versa.
Changes in sea temperature at both sites are also significant, the researchers said. Prof Rogers said more research is needed to understand what effect warming sea surface temperatures might have on the Chago whale population.
“This region of the world, in the middle of the Indian Ocean, is experiencing very significant changes in water temperature,” she said.
“So it would be really interesting to see how this newly discovered population is going to behave in the future – is it going to be better for them or it’s not going to be as good? But at the moment things seem to be going very well. for them.
The team also studied the effects of cyclic weather patterns like El Niño/La Niña and the Indian Ocean Dipole. While the ENSO patterns did not seem to be strongly correlated, the IOD phases – where the western part of the Indian Ocean becomes alternately warmer (positive phase) and colder (negative phase) than the eastern part – seemed to have a relationship on whale song frequency.
“During the positive IOD phase, it’s cooler in the southeast site with potentially more nutrients for blue whales, and we detected more whale songs here,” Ms Huang said.
Professor Rogers said the CTBTO will be an invaluable resource when assessing the health of Chagos blue whale populations in the future.
“Ocean conditions in the Indian Ocean are projected to change under future climate scenarios,” she said.
“As the CTBT Organization continues to monitor our oceans, listening for unapproved nuclear tests, it continues to collect data scientists need to understand how warming central Indian Ocean waters are affecting whales. Chagos and other marine animals of the region.”